T here has been intense interest in the coordination compounds of

unsaturated sulphur donor chelating ligands, dithiocarbamates, and their

related molecules from chemists, physicists, biologists and theoreticians

alike owing to their interesting chemical properties and possible wide

applications." Interest in molecular structural investigations and chemical

studies of these metal chelates covers a full gamut of areas ranging from

general considerations of metal-sulphur bonding and the formation of four-

membered chelate rings to the employment of these ligands in inorganic

qualitative analysis,j their practical application in organic synthe~is,~

medi~ine,~ and b io l~gy ,~ and their uses as vulcanisatiorl accelerators,'

floatation agents,"' fungicides, pesticides" radiation protectors,12

antioxidantslbd photostabilisers of polymers.'-' Their role in material

science has also been quite significant. The interesting low spin + high spin

cross-over phenomenon was first reported in an iron(1II)dithiocarbamate

complex.l5 There are several metal dithiocarbamate complexes with bridging

sulphur centres whch are known to participate actively in super exchange

phenomenon imparting novel magnetic properties to these systems.16 In this

chapter, the interesting ligation characteristics of dithiocarbamates and

structural features of their various transition metal complexes in general and

the coordination chemistry and stereochemistry of copper complexes in

particular, along with the scope of the present work which covers the

relatively unattended aspect of the primary amine derived dithiocarbamates

are discussed.

1.1 Dithiocarbamates as ligands

The ligand system such as dithiocarbamates, xanthates,

dithiophosphates, dithiophosphinates, and dithiocarbimates are all referred

to as 1,l-dithiolates. Various nucleophiles (Z- or 212-) are capable of attacking

carbondisulphide to form 1,l-dithiolates (la) and (lb).

Metal ions can react readily with (la) and (lb) to yield complexes with the

possibility of the two sulphur atoms getting bound to the same metal,

forming a four-membered chelate ring. A wide variety of ligands can be

made available by merely varying Z as shown in Table 1.1.

Nucleophilic attack of secondary or primary amines on CS2 in alkaline

medium is known to generate R2N-CS2-M+, which can be considered as salt of

carbamodithioic acid.'

RzNH + CS2 + MOH -+ RzNCSSM + H20

The free dithiocarbamic acids are relatively unstable and only a very few

have been isolated.'7 The simplest member of the series, I-IlNCiSH can be

obtained as an unstable crystalline solid by acidification of a concentrated

solution of the ammonium salt.1"

Table 1.1. Major types of 1,l-dithioluto ligands

=Composition / Structure I Name 1

RzNCSi

S /- RS-C<t Q Thioxanthate

S

S /-

R--C<. @ Dithiocarboxy late S

ROCSz

S /-

R2NZC<. 0 S

The disubstituted dihocarbamates are more stable, although they

decompose under acidic conditions according to the equation,

Dithiocarbamate

S /-

KO-C;. o S

CS2-

R2PS2

- -

(R0)2152

-

R~AsSZ~

(R0)2AsS<

Xanthate

/ so S=C\

so R S \ /F

P I 0 R' \-i

RO S \ /-

P I 0 RO' \k R S

\ /C

As;. 0 R' S

RO, /.? A?. O

RO' S

Trithiocarbonate

Dithiophosphinate

Dithiophosphate

Dithioarsinate

Dithioarsenate

R ~ N C S ~ ~ carbamodithioate anion popularly known as dithiocarbamate ions

(Dtc), have function which does not, however, behave quite like the

sulphur analogue of carboxylate (COO-) moiety. The difference is brought

about mainly by the N atom which is directly connected to the carbon of CS2.

In a detailed IR study, Chatt et al. showed that a significant contribution from

structure (2c) was necessary in order to describe the electronic structure of

dithiocarbamate~.'~

Their conclusion is based on the presence of a strong absorption peak,

the 'thioureide ion' band, in the 1542-1480 cm-' region of the IR spectrum

which is observed for all of the dithiorarbarnic acid drrivativc.~. The

contribution of resonance form (2c) to the structure of Dtc ligands and

complexes was offered as a possible explanation for the varying antifungal

activities of these compounds.20 What becomes evident from the contribution

from these various resonance structures is that there is an extended

conjugation encompassing at least four atoms. This imparts special property

to the molecule including stability. Evidence for delocalisation over the four-

atom skeleton S2CN of the dithiocarbamate system was observed by

Colapictro et al. in the short C-S and C-N distances of 1.720 and 1.344 A respectively, in the structure of N ~ ( S ~ C N ( C Z & ) ~ ] . ~ H ~ O . ~ ~ The effects of the

conjugated n system are also seen in the hyperfi~w interactions observed in

the NMR spectra of certain alkyl and aryl dithiocarbamate complexes of

iron(II1). The double bond character of the C-N bond in dithiocarbamate

complexes should result in hindered rotation of the NR2 group. This effect

was observed in the Mo(RzDtc).l(NO) complex. 'I'he NMR spectrum of t h s

compound is consistent with non-equivalent alkyl groups on the NR2 moiety

as a result of hindered rotation.22

Dith~ocarbamates are versatile ligands capable of coordinating in a

variety of forms.'' The diverse nature of its ligation characteristics is

presented in (3) where the ligand moiety can act as a monodentate, chelating

bidentate or as a multidentate to two or even three metal centres.

\ s 'i-M \ /S-M \ />\ \ /* \ \ /> N ; e C 4S N-C, M NI;-;C, M / \;/ / \-/ / \- / N-C' / \ S, S , S-M /N -C\ S-M

M M

Almost all transition and non-transition metal ions exhibit strong

coordinating affinity to the dithiocarbamate moiety and there does not seem

to be any ring-strain instability for four-membered cyclic structure of the

resulting complexes. Excellent reviews ar- available on ligation

characteristics of dithiocarbamates and covering structural aspects of the

wide variety of their metal ~omplexes .~ -~ Unlike many other donor atoms the

sulphur centres in Dtc stabilise polynuclear complexes with mixed valence

state. Another important characteristic of the Dtc ligand is its ability to

accommodate metal ions in unusual oxidation stc>tes, especially remarkable

being its potentiality to stabilise transition metal ions in higher oxidation

states.2,' The stability of complexes with unusually high or low oxidation

I

states of the central metal atom depends largely OIL the possibility for charge

levelling by o-bonding and x-back bonding. While in the case of normal

1,l-dithiolates these two electronic affects of the sulphur atoms are

considered to be of the same order of magnitude, in the dithiocarbamato

ligands an additional x-electron flow from the nitrogen atom to the sulphur

atom via a planar delocalised n-orbital system would make them strong

electron donors able enough to accommodate metal ions at higher oxidation

states. In contrast to the transition elements, the main group element

dithiocarbamates often have asymmetrical metal-sulphur bonds due to the

lack of p.-d, interaction. In these compounds the a-bonds are responsible for

metal-sulphur interaction. High oxidation states for these dithiocarbamates

are only found when high electron density is brought upon to the metal by

o-donating groups. For instance, MeSb(R2Dtc)z exists whereas Me2ISb(R2Dtc)z

does n0t.I'

It is not surprising that dithiocarbamato compounds with copper in the

oxidation state +3 are stable; instead it must be regarded as unexpected that

Cu(I) dithiocarbamato complexes exist The latter complexes are not simply

monomeric, but they are tetrameric or polymeric metal cluster compounds.

Obviously, the stability must be attributed to the metal-metal bond rather

than the stabilising effect of the ligand. The same holds good for the

hexameric Ag(1) and dimeric Au(I) dithiocarbamates. In all other

dithiocarbamate complexes in which the metal has a low oxidation state the

existence of this type of compounds is due to other, low-oxidation-number

stabilising ligands. Examples NO+ in Fe(EtzDtc)z(NO)z and CO in

Fe(E tzDtc)(CO)r .>+

Table 1.2. Stable uridcrflcrflon states of some metal ions in their dithiocarbamate cc~npleues

Metal dithiocarbamate species

The strong donating abilities of the Dtc ligands are lost when the

nitrogen is bound to aryl groups as in an aromatic system like diphenyl

dithiocarbamate. The C-N stretching frequency which is located around

1500 cm-1 in the al~phatic dithiocarbamates is seen lowered in the aromatic

dithiocarbamates." It is found that the lone pair of the nitrogen atom in

dithiocarbamate complex becomes progressively more important for the

donation of electrons, the higher the oxidation state of the metal."

The metal-sulphur distances are consistently longer in the

1,l-dithiolato systems than in the bis 1,Zdithiolene (substituted and

unsubstituted ethene-1,2-dithiolates (4a) and benzene-1,2-dithiolates (4b)

complexes.

The reason for this difference is directly related to the electronic

structures of the ligand systems and also the way the ligand molecular

orbitals interact with the metal valence orbitals upon complexation. In

Figure 1.1, simplified pictures of the molecular orbitals of the two basic

ligand systems, viz., ethene-1,2-dithiolate a ~ d dithiocarbamate are

pre~ented.~ The 3n, function in the dithiolene ligands possesses correct

symmetry and appropriate energy to interact strongly with the metal d.

function to produce more stable, extensively delocalised molecular orbitals in

the metal complexes. Moreover, whereas the 3x, functions are filled when

the 1,Zdithiolene ligands are in their classical dianion formulation, the

orbitals are empty when the ligands are in their htghly oxidised dithione

formulation. It is possible, for these x functions to, therefore, serve as

acceptor orbitals, thus giving these ligand systems the x-acid character. This

type of interaction is not possible in complexes of the 1,l-dithiolato ligand

system such as dithiocarbamate because of the change in symmetry of 3n, of

SzCNHi. It is important to note that the x-acid character results primarily

from the 3n, function delocalised over the S-C-C-S backbone and not from

the vacant d orbitals of sulphur. If the use of the sulphur d orbitals were of

greater importance one would expect to see little change in metal-ligand n

bonding in going from the 1,2-dithiolene complexes to the 1,l-dithiolato

system. Thus the dithiocarbamate ligands exhibit little of the x acidity which

would serve to enhance the electrophilicity of, for example, the d9 Cu(Q ions

in their complexes. As a result, while the Cu(I1) dithiocarbamates are subject

to considerable axial ligation with coordinating Lewis bases, the adducts

formed by them are relatively unstable and cannot be isolated.2

a C-N

a a O ' C - N

Energy

Figure 1.1. A simplified representation of the zv nnzolecular orbitals of the hvo basic ligand systems ethene-1,2-dithiolate, s~c&?-, and dithiwarbamate, SzCNHi.

Normal coordinate analysis of some transition metal dithiocarbamates

have been reported.26-z Absorptions of diagnc~stic value occur in three

regions in the IR spectra. The 1450-1550 cm-1 region is associated primarily

with the thioureide vibration and is attributed largely to the v(CN) vibration

of SzC-NR2 bond. An increase in the double bond character of the C-N bond

(2) results in higher frequencies for this vibration.' A nearly linear correlation

is found in a plot of the v(CN) versus the methylene proton resonance from

lH-NMR data (6, ppm) on symmetric Et2Dtc complexes. An increase of the

partial positive charge on the nitrogen results in a desheilding of the

N-bonded methylene proton. But 6(CH2) and v(CN) are not interdependent

in complexes with asymmetric Et2Dtc bonding.2y An observed decrease in

v(CN) in the sequence R = Me > Et > Pr - Bu, is generally paralleled by the

calculated sequence obtained by increasing the po nt mass of the alkyl group

only. To reproduce the observed sequence, however, it was found necessary

to decrease the C-N force constant (f) in the seqLence Me > Et > Pr - Bu.

These results indicate that both electronic and kinematic effects are important

in determining V(CN).'~ According to Chatt et nl. the energy of the v(CN)

band falls roughly into groups according to the probable arrangements of

sulphur atoms around the central metal atom, the order of decreasing

frequency being planar > tetrahedral > octahedral > distorted octahedral >

pyramidal.19h The v(CN) band is known to undergo a blue shift in the

dithiocarbamato complexes with bidentate or multidentate bonding mode,

while for unidentate coordination this stretching is seen to be shifted towards

lower wavenumbers or remains unchanged at the value of the free

dithiocarbamate ammonium salt30

A second region between 950 and 1050 cm-I is associated with v(CS)

vibration and has been used effectively in differentiating between

monodentate and bidentate R2Dtc ligands. Two absorptions in the region of

1050-950 cm-1 is a diagnostic criterion for asymmetrically bound RzDtc

group.3' This criterion that distinguishes mo.~odentate from bidentate

bonding is shown to be valid, provided comparison is made between

complexes containing the same alkyl groups. It appears that the splitting of

the v(CS) vibration also should occur with unsymmetric bidentate bonding.

Monodentate bonding should be assumed only if the splitting exceeds

20 cm-l.Z8 Small splittings (-15 cm-1) of the v(CN) vibration also should occur

for unsymmetrical bidentate or monodentate binding of the RzDtc ligands.

Because of the ~nherently large width of the v(CIQ)-Emd W m a y not be,

however, observed." The absorptions in 300-400 cm-1 region is associated

with M-S vibrations.

By analysing the IR spectra of a numtc?r of bis(dithiocarbamat0)

palladium(II) complexes Sceney and Magee concluded33 that bands appear

and disappear in the region of v(CS) absorption in a completely random

fashion. Their intensity also vary without any apparent order. It appears

therefore, that v(CS) vibrational modes must be highly coupled with other

modes and are very sensitive to environmental changes.

1.2 Structural features of dithiocarbamate complexes

With 'pure' dithiocarbamate complexes in which no other coordinating

ligands are present, four definite structural types are obser~ed .~

(c) (4 (5)

(The four basic structure types observed for 'pure' dithiocarbamate complexes (a) the square planar coordination geometly, (b) the five coordinate dimer (c) the four coordinate dimer (d) the octahedral coordination geometty: (0) metal, (0) sulphur, ( 0 ) carbon, (0) nitrogen)

The first type (5a) is that of an essentially planar coordination

geometry which is found for all the structurally known bis complexes of

Ni(I1). In the complexes Ni(SzCNHz)z, Ni(S2CNEt;)z and Ni(SCN(n-GH7)z)z

the SC and C-N distances average 1.70 and 1.34 A respectively confirming

that the all the canonical forms (2a-2c) contribute to the electronic structures

of the ligands."~s The average SNi-S bond angle of 79" indicates the

magnitude of deviation from perfect square coordination. The average

intraligand S S distance is a very short 2.85 A and the average interligand S S

distance is 3.41 A. These values contrast sharply with the near equality of the

corresponding intra and interligands values in the monomeric bis

1,Zditluolene structures in which the planar arrangement is essentially

square.

Blauuw et RI. synthesised complexes of stoichiometry

AuX(SCN(~-(;HY)~) by the reaction of [Au(SzCN(n-GHY)~)] with various

halogens.17 The structure determination of this complex showed that it

consists of planar Au(LU) cations of [Au(SzCN(nGHr)2)2]+ and linear Au(1)

anion [AuBrz]( ).jX The structure of the interesting monodithiocarbamate

Cu(III) complex [CuBr2(SCN(nGHs)z)] reveals a strictly planar coordination

about the central metal.3y

Two basic types of dimeric structures have been observed for the bis

complexes containing dithiocarbamate ligands systems. The first structure

type is that of the five coordinate dimer observed in dithiocarbamate

complexes of copper(I1). The coordination in both the Cu(I1) dimers,

CUZ[SZCN(C~&)CI)Z]~ and C U ~ [ S Z C N ( ~ - G H ~ ) ~ ] ~ is best described as square

pyramidal.'"*l One of the Cu-S basal distances is slightly but significantly

larger than 2.312 A (of the other three) by 0.03 A and involves the sulphur

which serves as the apical atom to the centrosymmetrically related copper

atom. In the half dimer unit, the Cu atom is displaced out of the plane of the

four basal sulphur atoms by 0.26 A. The dimer linhagcs of 2.81 A (Cu-S) are

very weak and the [Cu(SzCNRz)z]z complex is found lo have a normal

monomeric molecular weight in such non polar solvents as benzene and

The dimeric arrangement found with such Cu(II)(RzDtc)2

complexes is pre-empted in Cu(MePhDtc)z, because of the orientation of the

phenyl rings. These rings are nearly normal to tl e plane of the rest of the

molecule, an orientation that appears to be dictated by the steric interactions

of the adjacent methyl s~bst i tuents .~~

The bridging dimer linkage has shrunk in the zinc complexes

Znz[S2CN(CH?)z)4 and Z~~[SZCN(CZ&)~]~ to a value of 2.383 A while the one

Zn-S basal distarre involving the bridging sulphur has increased frorn

2.342A to a value of 2.851 A.44.45 The primary function of one of the

dithiocarbarnate ligands has clearly changed from that of a chelating agent to

that of a bridging group. The coordination geometry about the Zn atom is

severely distorted and the four shortest Zn-S distances are directed to the

corners of a severely distorted tetrahedron (6).

(6)

[Idealised shucture of the Zn(Me2Dtc)z complex: (0) zinc, (0) sulphur, ( 0 )

carbon, (0) nitrogen]

The apparent strengthening of the dimer linkage is still not great

enough to maintain i t in solution. The molecular weight data indicate the

presence of monomers in non polar solvents such as benzene and

chlorofo~m.'~ The structure of Cdz(SzCNEtz)4 is also seen similar to that for

the zinc dimer.ib The two seemingly different structure types of the dimers

are still closely related to one another. Further, sbuctures in the intermediate

region between the two limiting geometries have also been obse r~ed .~

Structural studies on tris-dithiocarbamate complexes like

Ru(SCNMe2)3, Cr(SzCNMe& and Fe(SzCN(nE&,)& show the different

transition metal ions to possess significantly distorted octahedral

coordination geometries, the distortion being in the direction of triogonal

p r i ~ m . ~ ~ , ~ V o r example in the iron complex the sulphur donors are arranged

in two parallel equilateral triangles, one of which is rotated 32" (twist angle)

relative to the other (As shown in (7) the twist angle 0 behveen the upper and

lower triangles is 60" for octahedron and 0" for triogonal prism)

[Projection of the upper and lower hiangles of a geometry between an octahedron and Lriogonal prism. Definition of the twist angle O]

The distraction can be considered to be resulting from the strain of the

four-membered chelate rings and the relatively small 'bite angle' of the

1,l-dithiolato ligand system.

Metals like titanium, vanadium, thorium and niobium form complexes

of the compositions M(RzDtc)t. The crystal structure of Ti(Et2Dtc)r has been

determi11ed.4~ The complex contains eight coordinate Ti(1V) and chelating

Et~Dtc ligands. The coordination geometry of TiSs core is very close to

dodecahedral.

The monovalent metal ions Cu(I), Ag(I) and Au(1) are known to form

an interesting series of polymeric compounds with the general formula

[M(S2CNR2)In where n is the degree of polymerity. When M is Au(I), n equals

2; when M is Cu(I), n equals 4; and when M is Ag(I), n is found to be 6. In all

these metal clusters the oxidation state of the metal atom is +1.50.51 Cotton has

pointed out the necessity of low formal oxidation states for the metal ion in

order to achieve the formation of the M-M bond.52 Many sulphur containing

ligands favour cluster formation because these ligands can very effectively

delocalise and distribute the charges within the molecules. Along with this

levelling of charges the possibility of inter ligand S-S bonding may also play

an important role. Because of the great interest in cluster compounds and

metal-metal bonding in general, the structural studies of a large number of

complex compounds containing Dtc ligand systems have been made in detail.

The interesting and pertinent features in discussing these structures concern

the number and arrangement of metal atoms in the cluster, the metal-metal

distances, and the coordination of the sulphur atom around each metal atom.

In [Au(S2CNR2)]2 complex, the two Au(1) ions, which are separated by

only 2.76 A, are each coordinated in a linear arr.~ngement by two sulphur

atoms from different Dtc ligands. The two Dtc ligands can be considered to

be serving as bridging ligands between the Au(1) ions. The structure is such

that a twofold axis of symmetry passes through the two Au(1) ions and the

linear AuSz coordination is perpendicular to the metal-metal axis of

symmetry. Another twofold axis passes through the C-N bonds of both of the

Dtc ligands and the overall symmetry for the [AU~(S~CN(~€~H,)~] complex

can be taken as D2 or 222.s3,X The Au-Au distance is shorter than the

corresponding value found in the structure of the metal. Raman studies

indicate a contribution of resonance structures involving the entities shown

in (8).s5

Introduction 16

In the tetrameric Cu(1) compound of molecular formula

[CU~(SZCN(CZH~)Z)~] the Cu(1) ions are located at the corners of a slightly

distorted tetrahedron with Cu-Cu distances ranging from 2.6 to 2.7 A.51 AS

shown in (9) each of the Cu(1) ions is coordinated to sulphur atoms in a nearly

planar triangular arrangement and each sulphur atom coordinates to either

one or two Cu(1) ions.

Hesse also examined the structure of the hexameric

[Ag6(SzcN(C~tt)~)h] c o m p l e ~ . ~ The metal atoms I :Irm a somewhat distorted

octahedron with six comparatively short and six longer edges. The short

edges correspond to metal-metal distances, r hich are comparable or

somewhat longer than those in the metallic phase of silver. The long edges

form two centrosymmetrically related triangles in the silver octahedron.

Outside each of the other six faces of the octahedron one Dtc ligand is

situated, linked to the silver atoms of the face by silver-sulphur coordination,

one of the sulphur atoms is linked to one and the other to two silver atoms.

The silver coordination is threefold but not planar, the metal atoms being

situated 'inside' the plane of the coordinating sulphur atoms.

When the N-substituted alkyl groups are small, the hexameric Ag(I)

complex is very insoluble in organic solvents and appears to have some

polymeric properties. However, when more bulky alkyl substituents are

employed in the Dtc ligands the hexanuclear compounds show little tendency

to form higher polymers.

13 Copper--coordination chemistry

Copper occurs in a range of oxidation states 0 to 4. The Cu(0) and

Cu(1V) states are extremely limited. The Cu(II1) oxidation state is less

uncommon, and has been characterised for a few compounds including

copper dithiocarbamate ~omplexes.l,~,57,58 The Cu(I) and Cu(I1) oxidation

states are the most abundant for the metal. Cu(I1) is the more stable of the two

under normal conditions and form a wide variety of simple compounds and

complexes. The chemistry of Cu(1) is very much less extensive than that of

Cu(I1) and a number of accounts occur which describe the chemistry of

simple compounds of Cu(I) with less emphasis on the formation of its

cornple~es.~~~5~-61 The realisation that a copper(I) species may be involved as

the precursor of the silent partner in the type 111 copper p r ~ t e i n s ~ ~ ~ ~ has

resulted in a renaissance in the coordination chemistry of copper(1)

compounds which is reflected in the amount of space given to the chemistry

of copper(I) and (11) in the 'Advanced Inorganic Chemistry' of Cotton and

W i l k i n s ~ n . ~ ~ In the first edition of the book in 1952, more space was devoted

to copper(II) than to copper(I), while in the fourth edition the space allocation

is reversed.

1.3.1 Copper(11)compleres

Some of the salient aspects of Cu(I1) complexes are worth mentioning

here.

Copper(I1) forms complexes with coordination numbers four, five and

six, the latter being predominant A significant number of 7 and 8 coordinate

geometries also occur. Unlike other first row transition metal ions, the

copper(1I) complexes are characterised by a variety of distortions.65,66

Majority of six coordinate copper(II) complexes involve an elongated

tetragonal or rhombic octahedral structure, with a few involving a

compressed tetragonal structure. The tetrahedral geometry of Cu(I1) ion

always involves a significant compression along the S4 symmetry axis. Only

the square planar geometry is regular for Cu(II) ions, but even here it

involves a slight tetrahedral distortion. Copper(I1) ions with five

coordination rarely possess a regular square pyramidal geometry; it generally

undergoes both an elongation and a triogonal in-plane distortion," or less

frequently, a tetrahedral distortion. The triogonal bipyramidal geometry of

Cu(II) may be regular, but is frequently distorted towards a square pyramidal

stereochemistry.

Generally, Cu(L1) complexes are blue or green due to d-d electronic

transition causing absorption in the 600-900 nm regions. If there is a strong

charge transfer band spreading to the visible region the' complex appears red

or brown. Since copper(I1) ion is subjected to Jahn-Teller distortion and a

regular octahedral complex is not formed, the formal E, and Tzg terms get

splitted. The spectra do not usually correspond to the simple ZE, -+ 2T2g excitation67 but rather to one based on altered multiple states as shown

in (10).

(10)

[Splitting of 2Eg and 2T~g states in Cu(II)]

Tetragonal copper(Il) complexes are expected to show the transitions

~BI, + 2A~, 2B~, + 2B2, and ~BI, 2E, but bands due to these transitions

usually overlap to give often one broad absorption band.m.~~~

Four<oordinate Cu(I1) complexes are common, but the strict

tetrahedral or square planar stereochemistries are rare. Intermediate

stereochemistry of approximate D z ~ symmetry is more usual and four

transitions (between the d-orbitals) may be observed.b7b,7u The spectra of such

complexes often show two or three more or less resolved bands below about

20000 cm I. The polar~sahon properties of these bands have been studied in

some detail in certa~n cases assisting in the assignments of the transitions

1nvolved.67a "

1.3.2 Copper(1~0mplwolion characteristics

The Cu(1) and Cu(II) ions can readily form complexes in which the

cations act as Lewis acids and the ligands as Lewis bases. While Cu(I1) is

generally considered as borderline based acid, Cu(1) clearly behaves as a soft

acid and the nature of stability of the ligand to Cu(I) is that of a soft base with

class (b) behaviour (Table 1.3).72 In general the halides can form a wide range

of complexes, with the C1, Br and I ions predominant, but with very few

examples of the F ion acting as a ligand.73 Among the 0, S, N and P donor

ligands the 0 and N ligands dominate the chemistry of Cu(1I) while S and P

ligands are more frequent in Cu(I) chemistry.74~7~ 'This reversal of ligand role

is also influenced by the reducing properties of many S, I' and I ligands and

the ready reduction of the Cu(I1) ion (Equation 1.2) to the stable Cu(1) species

with these ligands.

Table 1.3. Hard and sop acid-base classification of copper ion and ligands

The hard-soft concept accounts for the following reactions and product

formations."

(a) CuCl~.H20 + KC1 So2(aq'q) ) K[CuC12]

Hard

(b) CuBrz + KBr -% K[CuBrz] lxrll

Border line

C ~ P P ~ ~ ( I I ) (a) Acid

(b) Base

(d) K[Cu(CN)z] Cuz+(aq) + KCN -t Cuz(CN)l excess

son C ~ P P ~ ~ ( I )

F- < C I ~ < B C ~ < l

O < < S = Se = Te

N < < P > A s > S b > B i

F~ > CI > Br- > I

0 ;.> S > Se > Te

N >> P > As> Sb> Bi

The electrode potentials of the reactions (1.1) and (1.2) readily lead to

the disproportionation as in equation (1.3) shown ldow.57

C U ~ + ~ , ) + ~ -+ CU+(,+ Eo = 0.15 V

2Cu+(,,, 4 Cu" + CU~+(.,), EQ = 0.37 V

Consequently the concentration of the Cu+ ion in aqueous solution is

extremely low (K for equation (1.3) being of the order of lo6) compared with

the very hgh stability of the Cu2+(aq) cation. For this reason water is rarely

found as a ligand to Cu(I), but is a common ligand in copper(1I) chemistry.74

The electrode potentials of reactions (1.1-1.3) get readily modified by

complex formation with appropriate ligands (Table 1.4).75

Table 1.4. (a) Reduction potentials of some Cu(II)/Cu(I) couples und (b) [Cu (Il)]/llcu (I)? ratios

(a) -

Cu(ll)lCu(l) Couple

C N ~ -

r c1- In laccasse

In ceruloplasmin

Loxalate < -0.2 I I 'en - HzN(CHz)zNH2; ' M e e n - HzN(CHz)3NHz; *Mesen - HzN(CH&NHz

bipy I:':: Glvcinate

0.15

0.12

-0.01

-0.16

-

-

In this way the concentration of Cu(I) in aqueous solution can be

significantly increased by complex formation such as in [CuC12]- and

[Cu(CN)2] anion by addition of an excess of tht. appropriate ligand. This

strategy can be made use of the preparation of various Cu(1) complexes.

Alternatively, Cu(I) can be brought into nonaqueous solution in which the

solvent is a good ligand for Cu(1) and forms complex. Such complex solution,

(for example, [Cul(NC-Me)r]X), can be used for the preparation of other Cu(1)

complexes by reacting with suitable ligands.

1.3.3 Copper(1) complexes-stereochemistry

Cu(1) can exist as mononuclear, bi-nuclear, tri-nuclear, tetra, penta and

exanuclear complexes. Even octa and decanucl~ar complexes have been

Many of the Cu(1) complexes are polymeric in nature and

possess chain and ribbon structures and infinite three dimensional lattices. In

all these complexes copper has the relatively low coordination numbers of

two, three and four and very rarely five.

In the solid state the stereochemish-y of Cu(1) in its mononuclear

complexes as determined by X-ray crystallography is dominated by four

coordination. The four- coordinate Cu(1) complexes are generally tetrahedral.

A significant number of three and two coordinate complexes are also known;

very few five coordinate complexes exist and six coordination or above is

unknown. This contrasts with the predominance of six coordination in the

chemistry of Cu(I1) and the absence of two or three coordination in the solid

state and with the formation of a significant number of seven and eight

coordinate geometries.65,m

Binuclear complexes form a significant class of Cu(I) complexes

involving bridgmg by one or two, but not three Iigand atoms or groups. In

practice the bridging role is most common for halide ions, especially iodides.

The resulting structures mainly involve a symmetrical arrangement of two

single atom bridges with either trigonal (llb), tetrahedral ( l la) and mixed

trigonal-tetrahedral (llc) copper stere~chemistries."~

(a) (b) (c) Tetrahedral Trigonal Mixed trigonal tetrahedral

[X = Brrdg~rig lrganJ whrc h may be (I) slngle anron, e.g., C1; (11) polyatomrc anlon, e.g., NCS; (111) organlc link]

(11)

[Stereochemishy of binuclear Cu(1) complexes]

Trinuclear complexes of Cu(I) are much less common than either

mononuclear or dinuclear complexes and occur ns a linear arrangement of

bridged Cu(1) atoms, as a triangular arrangement ar as a planar or puckered

six numbered ring with bridging X groups (12).W-%

x, lX\ /x\ f X Cu Cu , \ , \

X X X X

(a) (b) (c)

Linear Triangular Planar hexagorlal ring Puckered hexagonal ri

(12)

[Stereochemistry of hinuclear Cu(1) complexes)

Tetranuclear complexes of Cu(1) occur nearly as frequently as the

dinuclear complexes, especially when iodide or si~lphur ligands are present.

No strictly linear tetranuclear C u A structures are known; square planar or

rectangular Cu4 un~ts exist but these are much less common than the regular

and distorted tetrahedral units (13).

PLANAR

( i ) (4 (i)

TETRAHEDRAL 0 /

s*\

X flLt7\

1 ,x+.cu\L /+q->':\ Cu- X cu-

L ' / (ii)

[CUX]4L4 closed cubane [Cu(S2CNEt2)I4 closed cubane

(iii) . .

[ cuJ I~ ]~ - open cubane [C4X4] stepped cubane

(13)

[Stereochemistry of tetranuclear Cu(1) complexes]

Introduction 25

The tetrahedral Cu,X4 is the most common tetranuclear Cu(1) species

and primarily occurs as the 'cubane' structure C U J X L where X is C1-, Br or

r.",m A relatively complex cubane type C U ~ S ~ units occurs in [Cu(S2CNEtz)]4

in which one of the S atoms constitutes the Cu& and the second S of each

S2CNEt2 group symmetrically bridges two copper atoms on the four side-face,

Cu-Cu pairs.5' Pentanuclear and hexanuclear conlplexes are only of limited

occurrence.

The Cu(1) complexes so far described are characteristic with their

relatively low coordination numbers of two, three and four. Such low

coordination numbers are conducive57 to bridging ligand functions and hence

to infinite lattices involving chain (or ribbon), sheets and three dimensional

lattices.&Y In (14) are shown the varieties of structures possible for polymeric

copper(I) complexes.

One of the most structurally intriguing complexes of Cu(1) is one with

the CuOzCMe composition. Its structure retains the unique acetate ligand

bridging role of the,Cu(II) complex, but this is now restricted to planar

dimer~c un~ts, llnked by further oxygen br~dges into a staggered linear

polymer whlch 1s planar overall (16a).W If Cu-Cu distance of 2.556(2) A is

considered non-bonding, the Cu03 chromophore is unusual as it has a T

shaped structure that involves two angles much less than 120°, namely 80.4

and 110.5", with a t h ~ r d 0-Cu-0 angle nearly linear at 170.1". The electron

diffraction structure of anhydrous cuprous acetate has been reported to

~nvolve a drmeric planar sbucture related to the above solid state structure

with relatively shorter Cu-Cu distance of 2.491 A (Figure 1.12b).y1

CllAlN AND RIBBON STRUC'I'URI:~

S~ngk churn

(1) (h) Two-coordinate Three-cwrdnate--pla~lar or stepped

\ ,Cu, ,Cu, ,Cu

C u \ ,X\ ,X, / Cu, ,Cu, ,Cu

x ' x "(' x \ % (") (\I) (w)

Four-coordhate Stepped straight 'Twisted step

(U) (W I h u h k nbbon Pkuuir

INFINITE. TWO-DIMLXSIONAL SkEETS

( 1) (u) INFlNl fE TIIREE-DIMENSIONAL IATTICHS

X X X I I I

,cu-cu*cu, X X X X X X

(14) [Stereochemistry of polymeric copper (I) complexes]

TH3 YH3 '9-c\9 7"' 'cu ---- u. A,

9'c'~ I T P P 0 \ 0 'cu...-- 7"---"7" 7 I F \ CHi 0,

O'F'O

C C H3

C H3

(a) (b)

(16)

[(a) Polymeric Cu(1) acetate and (b) Dimeric Cu(1) acetate]

1.3.4 Redox properties of copper(l)/(l) systems

Redox propert~es of Copper(I)/(II) systems are associated with three

p r o c e s s e ~ : ~ , ~ ~ , ~ ~ (a) electrolytic oxidation or reductions (Eqn. 1.4); (b)

disproportionation (Eqn. 1.5); (c) the oxidation of copper(1) with molecular

ultimately yielding water (Eqn. 1.6).

Electrolytic reduction of Cu(1) to copper metal is readily carried out as the

second step in the electrolytic refining of copper metal and in the copper

plating process. Although the disproportion reaction is favourable in aqueous

solution, the low solubility of the Cu(1) species in water renders this reaction

less important than the oxidation to Cu(I1). Complex formation in solution

notably increases the solubility but also increases the stability of the Cu(1)

species to the disproportionation process. Due to this the coordination

chemistry of the copprr(I)/(II) redox process is primarily concerned with the

reduction of copper(I1) to copper(I), a process that has been already discussed

(Table 1.4).

There are a number of reports of many organic molecules being

reduced by cupric ions to form copper(I) complexes.% Trimethyl phosphine

sulphide, for example, reacts with Cu(I1) salts in acetone medium forming

Cu(I) complexes with compositions [Cu(Me3PS)CI], and [Cu(Me3PS)3] X(X- =

CIOI- or BF4). X-ray crystal studies show the complex to contain three

t,oordinate ~ o p p e r ( I ) . ~ , ~ ~

CuClz.2HzO is known to get reduced by dithiobiuret (17) in a 95%

ether-ethanol solution to give, as one of the products, an olive green material

with structure (IS).*

This is an unusual example of a complex bearing a positively charged

ligand moiety.

Interesting copper(1) complexes have also been reported to form on

addition of copper(I1) salt to a refluxing solution of excess ligand 1-

methylimidazoline-2(3H)-thione (meimtH) in acetone-acetonitrile. The

complexes species formed are [Cu(meimtH)j]N03 and [Cu:(meimtH)6](BF~)z

as well as [Cuz(meimeH),-1%. 3H20 from aqueous acetone.lw"J2

The oxidation of copper(I) with molecular oxygen ultimately yielding

water (Eqn. 1.6), a process associated with the redox properties of copper(Q

system, merits attention. This is the process that is responsible for the air

sensitivity of copper(1) compounds, and equally for the catalytic role of

copper(I) in the oxidation of organic mole~ules . '~~~~* Copper(I), for example,

is the catalyst in the oxidation of dimethyl sulphoxide to dimethyl sulphone

using Cu(NCMe)dBF, and in the oxidative coupling of phenols to

q u i n ~ n e s . ' ~ ~ , ' ~

1.3.5 Biological copper

In biological copper systems the copper is involved in three basic

p r o ~ e s s e s : ~ Z - ~ , ~ . ~ ~ - ~ ~ ~ (1) electrolytic redox processes, involving copper(III)/(I),

(2) oxygen atom processes, involving the absorption of oxygen molecules and

their ultimate reduction to water,'"lW (Cu @)/(I) systems) and (3) transport

processes,* through which copper is absorbed and rejected by the body. In

general, copper(1) acts as the electron donorl'u for processes (1) and (2)

involving copper proteins, while in process (3) copper(I1) is known to be

primarily involved. As for the nature of interaction of copper(I) with protein

there is, however, surprisingly little information available. The copper(I) site

is generally considered to be a hydrophobic area in the protein and the

characteristically positive reduction potentials of the copper biological

systems suggest that the reduced state of the copper is compatible with stable

copper(1) stereochemistry. This is generally assumed to involve a distorted

tetrahedral geometry. The more positive reduct i~n potentials may also be

associated with the 'similarity' of stereochemistry in both the copper(I) and

(11) oxidation states and in this respect a common tetrahedral stereochemistry

might well be appropriate as this geometry is known to be stable for both the

oxidation states.

1.4 Copper complexes of dithiocarbamates-a review

The d~thiocarbamates derived from both primary and secondary

amines show comparable ligation characteristic^',^ with almost all metal ions.

It is worthwhile to have an overview on copper complexes of

dithlocarbarnales, whlch could throw light on the ligation characteristics and

nature of interachon of both N-monosubstituted and N-disubstituted

dithiocarbamates with copper ions.

The chemistry of univalent copper dithiocarbamates has been studied

in considerable detail. The Cu(I) complexes of the disubstituted

dithiocarbamates (derived from secondary amine) Cu(I) complexes are not

much stable and their solutions oxidise to the corresponding cupric

complex."' l'he complexes of monovalent copper can be obtained by the

following methods: (a) treatment of Cu(1) oxide with the sodium salts of the

ligands in an inert atmosphere, (b) oxidation of copper, bronze or metallic

copper with tetra alky l thiuram disulphides in chloroform or benzene, and (c)

reaction of alkali metal Dtc with a cuprous halide in aqueous c o n d i t i o n . ~ ~ ~ ~ ~ 7

Kkerstrom conducted detailed molecular weight studies on these complexes

and showed that Cu(1) dithiocarbamates usually exist as tetramers in CSz or

benzene solution. It appears that the size and structural characteristics of the

alkyl chains on the ligands play an important role in determining the degree

of p ~ l y m e r i t y . ~ ~ ~ ~ " ' ~ The crystal structure of (CuEkDtc), complex has been

determined."l The disubstituted dithiocarbamates of Cu(l1) are stable, watrr-

-- --

I I . : . , ';. insoluble compounds.~~5 'The these species along with

other similar complexes are already discussed in Section 1.2. Cu(II) reacts

quantitatively with Pb(RzDtc)z, Bi(R~Dtc)3 and Tl(RzDtc)? complexes to form

the deep brown-red Cu(R2Dtc)z complex. These re rctions have been used for

the indirect determination of Pb(II), Bi(II1) and TI(II1) ions.l16 Crystal structure

determination of the Cu(Et2Dtc)z has shown that it is a dimer with five-

coordinate Cu(I1) i o n ~ . ~ ' ~ , ~ l W e w r n a n et al. have shown that Cu(pyrrole Dtc)~

is isomorphous to the square planar Ni(pyrrole Dtc )~ and that the former is

monomeric with a square planar CuS4 chromophore."Y The tendency of the

pyrrole ring to preserve aromaticity makes the resonance form RZN+=CS,~-

quite unimportant in the structure of the ligand. Complexes of this ligand

including Cu(I1) complex, show the C-N vibration between 1250 and

1350 ~ m ~ ~ . l L "

The EPR spectra of Cu(R2Dtc)z complexes have been investigated in

considerable detnil.ljll2' Viinngard and Akerstrom examined the room

temperature EPK spectra of the Cu(Et2 Dtc)z, Cu(i-pr Dtc)2 and Cu(MePhDtc)z

complexes in benzene solution and found them 1.2 be almost ide1itical.'2~ A

typical spectrum consists of a quartet with g = 2.046 and A = (0.74 f 0.02)

cm-I. Patterson and Viinngard obtained the EPR spectra of polycrystalline

samples and calculated the parameters and evaluated the degree of covalency

of the Cu-S bonding in them.122 They concluded that Cu-S o-bond is

appreciably covalent. The EPK studies show that, in general, covalency in the

M-S bonds increases strongly in the order: Cu < Ag < Au and that the metal

3d character of the MO of the unpaired electron decreases from 50% in

Cu(EtzDtc)z to 26% in Ag(HzDtc)z and 15% in the homologous Au(II)

c o m p l e ~ . ~ ~ , ~ ~ ~

Interaction of Cu(Bu2Dtc)z with pip, py and n-hexylamine has been

studied by variable temperature EPR measurements. Evidence for the

formation of 1: I adducts was presented by Gorden and Rieger.'26 In a similar

study of EPR and absorption spectra of the Cu(pipDtc)? and Cu(EtBzDtc)~

complexes, Yordonov and Shopov observed the foi.mation of 1:2 adducts with

py having no evidence for the 1:l a d d ~ c t l ~ ~ This shows the influence of R2

group on the ability of the Cu(R2Dtc)z complexes to form base adducts of

varying compositions. Yordanov et al. studied thc interaction of Cu(1I) ions

with Ni(Et2Dtc)~ and Cu(Et2Dtc)z and detected the formation of CuX(Et2Dtc)'

and Cu(Et2Dtc)' spe~ies.lzJ2~

A detailed electrochemical study on sixteen different Cu(R2Dtc)z

complexes has been reported."' The oxidation of the Cu(R2Dtc) complex is

unambiguously defined as the one-electron reversible process.

The reduction of the Cu(R2Dtc)z complexes often appears quasi

reversible, presumably owing to a structural reorganisation of CuSd core of

the [Cu(RzDtc)?] product.

Although the monoanion could not be isolated the solutions containing

this species show reversible two one-electron oxidation step by cyclic

voltammetry. In general, for the reduction step given above the potentials are

more positive by about 1 volt than the corresponding potentials in the

reduction of Ni(II2Dtc)z complexes and illustrate convincingly the

~nvolvements of the metal ion in the reduction proc-ess.

Oxidation of the Cu(Bu2Dtc)z or [Cu(Buz Dtc)]4 by halogens results in

the formation of the diamagnetic square planar monomeric [Cu(BuzDtc)Xz]

complex (X = C1, Br).I3l Brinkhoff obtained [Cu(R2Dtc)2]+ cation by the

oxidation of the Cu(lI) complexes with iodine.132 The increase in the Cu-S

stretching frequency observed in the Cu(R2Dtc); cations (385, 410 cm-')

compared to that in the Cu(R2Dtc)z complexes (- 345, 370 cm-l) has been

attributed to the strengthening of the Cu-S bond as a result of removing an

electron from an antibonding orbital composed primarily of metal and

sulphur functions. The structure of the [Cu(Bu~Dtc)z]G complex shows a

Cu(II1) ion with a four coordinate planar geometry.llJ

M~xrd Cu(llI), Cu(l1) valence complexes of the general formula

[Cu3(Bu~Dtc)b] [M Br+ (M = Zn, Cd, Hg) were obtained by interacting

Cu(BuzDtc)2 with MBrz and Br2 in stoichiometric amounts.lw The X-ray crystal

structure of [Cu3(Bu2Dtc)6] [CdzBr6] shows a loosely held centrosymmetric

trimer.lS

Van der Leemput et al. prepared the paramagnetic Cuz(RzDtc)&z

complexes with copper in the oxidation states(I1) and (111). These compounds

were obtained by the interaction of CuCl or CuBr and b t d s in chloroform.

'The crystal structure of Cu2(H>Dtc)3Brz consists of alternaling Cu(H2Dtc): and

Cu(H2Dtc) Br; units in chains.lX

The reaction between CuBr and b t d s in CH2Cl2 in various molar ratios

resulted in a number of polynuclear copper complexes which can be

represented by the general formula [Cu(R2Dtc); ] [CU,,.~ Br,,](-) with n varying

as a function of R group. Values of n are 7, 5, 3 for n-Bu, n-Pr and Et

Refluxing [Cu(Et~Dtc)]4 in CHCb yields a compound Cu3(EtzDtc)zCI in

which copper is in the +1 oxidation state.Iw Sholozhenkin et 01. studied the

reaction of Cu(I1) with the ambidentate ligand, carboxydecyldiethyldithio-

carbamate.I3' CuL2,HzO [HL = E~zNC(S)S(CH~)IOCOOH] was prepared from

CuSO4 and NaL in aqueous solution. The complc~x in saturated chloroform

solution exists as CuSI chromophore and with dilution isomerises to CuS30,

CuS202 and Cu01 chromophores.

Jean-Pierre Barbier et nl. have shown that irteraction of Cu(BF4)2.6H20

or Cu(ClO4)z. 6Hz0 with a solution of Cu(Et2Dtc); leads to the less common

disproportionation, copper(11) into copper(1II) and copper

4Cu1'(Et2Dtc)z +2Cu"(BFp)2 + 3CuU (EtzDtc)~ BF4 + Cu0I(EtzDtc)2 BF4

The formation of a very stable copper(1) complex, probably a cluster,

may be the driving force for this reaction.

Dani rt nl. have prepared and characterised bimetallic tetramorpholine

-4-carbodithioates of the composition MM'(med.t)+ where M = Cu(I1) or

Ni(II), M' = Zn(II), Cd(I1) or Hg(II) and medt = morpholine-4-carbo-

dithoate.1" These complexes are reported to exhibit enhanced antifungal

activity. Square planar stereochemishy around Cu(1I) and Ni(I1) has been

proposed for the polymeric complexes.

Yusuff studied the action of thionyl chloride on bis(dithiocarbamat0)

copper(1I) complex in benzene or CCl4 solution.la This provides a simple

alternative route to the synthesis of dichloro(dithiocarbamato) Cu(1II)

complexes. l 'he following reaction scheme has been proposed

Cu(Dtc)z + Cl2 -+ [Cu(Dtc)C12] + Dtc

The various types of copper dithiocarbamate complexes prepared so

far are presented rn Table 1.5.

Tuhle 1.5. Types of copper dithic~urbamute complexes found so fur r

Oxidation Number - I

I1

III

II, I

111, I

111, 11 -

Compound

[Cu(RzDtc)]4 Cu3(RzDtc)2X

[Cu(RzDtc)z]z [X Cu (R2 Dtc)]2

[Cu(RzDtc)z](+)X(-I XzCu(R2Dtc)

Cu(R2 Dtc)~ nCuBr C13Cu3 (RzDtc),

[Cu(R2Dtc)2+] [CU,.~ Br,] [Cu(R2Dtc)+2]~ [ C U ~ ~ r i - ]

[Cul(Rz ~ t c ) ? ] [MX;]z Cuz(RzDtc)+ 13 CUZ (R2Dtc)~ Br2

The types of copper dithiocarbamate complexes sun~marised above

shows the remarkable variety in the permutation of R2Dtc ligands, halide ions

and various oxidation states of copper.

1.4. I Copper complexes of dithiocarbamates derived from primary amines

During their investigations on the interi~ction of Cu(II) ion with

RHDk, Compin and Malatesta143J44 noted the generally instability of

Cu(RHDtc)z complexes. However, it was Cambi and Coriselli who first

reported that Cu(RHDtc)z complexes were unstable and rapidly decomposed

to the corresponding Cu(1) compounds."' Because of the very high

decomposition rate they could not isolate any Cu(RHDtc)z even if they are

formed. They reported that the decomposition of the complexes was

complicated. However, they claimed the formation of isothiocyanate during

the decomposilion.

Kawshik ! 111. synthesised a series of bis-N(p-ethoxyphenyl)

dithiocarbamate complexes of Cu(II), Zn(II), Cd(II), Sn(II), Ni(1I) and Pd(I1)

with composition, M[Eto-G&-NHCS2]2.'45 These complexes have been

characterised; the metal ligand ratio in all the complexes including the copper

complex was shown to be 1:2. Thermal studies of these complexes have been

carried out in static air atmosphere to dcterminc their mode of

decomposition. l n e t i c parameters have been determined.

Kaushik et nl. prepared another series of complexes,

bis-N(o-chloropheny1)dithiocarbamate and bis-N(p-chlorophenyl)

dithiocarbamate complexes of the stoichiometry M(CI-GK-NHCS2)z where

M is Cu(II), Zn(II), Cd(II) or Sn(Q.146 The complexes have been characterised

by physicochemical methods and the thermal behaviour of these complexes

In static air atmosphere has been studied.

Singh el ol. have synthesised metal complexes of 4-aminophenazone

dithiocarbamate. The formula of the complexes have been given as

(19)

where n is the oxidation state of the metal (M = C++, Fe3+, CuZt, Ni2+, Ce2+,

V02+, Zn2+, Hg2+ and TI'). Surprisingly magnetic moment of the copper

complex is given as 1.73 BM, corresponding to one unpaired electron.147

CuX (X = I, Br) or CuBrz reacted with HL (HL = RSC(S) NHPh, R = Me,

Bu) lo givv C'ui ., <'u\l3uSC(S)NFlI1h]. Will(.m: t- 1.1 ~ r l . synll~c~sisc~cl the

complexes and determined the crystal s t ru~ture ."~ For HL (R = Ph) no CuL

was obtained because of rapid elimination of PhNCS. Crystals of CuL

(R= Bu) are rhombohedra1 with one hexameric unit in the unit cell.

Matter rt r r l . prepared dimeric and polymeric Cu(1) complexes of

dithiocarbamate esters. Reactions of N-methyl Smethyl dithiocarbamate(L)

and N-phenyl S-methyl dithiocarbamate (L') with Cu(1) or Cu(1I) halides

yield [CuIL,z]z, [CuIL'&, [CuCIL'], and [CuIL'],,. Their structures have been

determined by single crystal X-ray crystallography. [CuILr]> and [CuIL1:]2 are

dimeric, Cu(1) being tetrahedrally coordinated. (CuCIL'], and [CuIL'],, are

polymeric with edge-linked tetrahedral coordination polyhedra of

Cus2cuXzCu.'~"

Iordanov rt d l . studied the interaction of some Cu(I1) chelate complexes

with NO and NO1 by EPIZ and electronic spwtroscopy.'~' 'The chelates were

Cu(acac)z, C:u(OX)>, C'u(Utc)~ and Cu(Dtp)z. (Hacac = acetyl acetone, 130X =

8-hydroxy quinoline, HDtc = H2NCSzH and HDtp = dithiophosphoric acid).

Weak reversible adducts were formed with NO, whereas NO: oxidise the

ligands in all complexes. The reaction of Cu(Dtc)z with NO2 proceeds through

Introduction 37 . .

m~xed l~gand complexes w ~ t h the participat~on of NO,-; Cu(N03)z and

d~sulphidc. front the l~gand be~ng the final products.

Salam r t r r l synthes~sed and character~sed some metal chelates of

ethylenedlam~ne-monodrth~ocarbamates. Cu2+, Ni2+, Znzt, Colt, Fez+ and Cd2+

formed complexes of the composition ML2 and Fe3+ and Colt formed ML?

(HL = HzN-CHJ-CH2-NH CSJ H) species.151

Mekil complexes of 1,3-propanediamine rnonodithiocarbamate were

prepared and characterised by Salam et nl. Divalent cations CuZ+, Ni2+, Mn2+,

Znz', Coz', Fez' and Cdz' formed ML2 and trivalent cations Feq+ and Co"'

formed ML.$(HL = H2N ( C H ~ ) Z - N H C S ~ H ) . ~ ~ ~

Enzymatic activity of copper, zinc superoxide dismutase (SOD), a

metalloprotein that catalyses superoxide radical disproportionation, involves

a cyclic Cu(lI)/Cu(l) redox process of the Cu(I1) i?n held at the active centre

in the protein. St.vcral copper (11) dithiocarbamates are known to exhibit

SOD-like activity.'jT.'3 The activity of such complexes depends on the

Cu(II)/Cu(I) rcdox process. Roberto Cao et nl. studied the SOD-like activity

of the copper(I1) complexes of the amino acids glycine, alanine, serine,

aspargine and glutatnic acid. In all these complexes copper is reported to be

existing as Cu(II) and the EPR inactivity is explained in terms of

antiferromagnetic in te ra~t i0ns . l~~

1.5 Scope and objectives

Conipin in 1920 noted the instability of Cu(I1) complexes of

dithiocarbamates derived from primary amines.143 In 1936 Cambi and

Coriselli noticed that Cu(SzCNHR)2 could not be isolated because of its rapid

decomposition to the Cu(I) species by some complicated mechanism. They

verified the presence of isothiocyanate in the decomposition products.ll'

Since 1936 there have been only very few reports, probably less than ten, on

copper complexrs with RHDtc ligands. In most ~f these reports the metal-

ligand ratio is stated to be 1 :2 the oxidation state of the metal being Cu(I1).

These reports contradict with the observation of Compin, and Cambi and

Coriselli.

The present investigation is to establish the true nature of the

interaction between Cu(II) ions and RHNCS;. The products of the reaction

between Cu(I1) ion and RHNCS; ion were carefully isolated and

characterised by elemental analysis, molecular mass determination, various

spectroscopic techniques and thermal analysis. In anticipation that the

dielectric of the reaction medium and the nature of the substituent on the N

atom of RHNCS, would dictate the nature and course of the reaction and

stereochemistry of the reaction products, interaction of Cu(1I) ions with a

range of RHNCS; ions with various alkyl, acyl and heterocyclic substituents

was studied in different solvents. With a view to evolve a strategy to achieve

redox-stabilised copper(I1) complexes of RHNCSz, the Dtc function was

anchored to a polymeric support and its interaction with Cu(l1) was

investigated mainly by electron spin resonance spectroscopy. The present

study broadly covers the following aspects:

(i) Various dithiocarbamates derived from primary amines are prepared

in solution condition by the nucleophilic addition of primary amines

on CSr in alkaline medium. Dithiocarbamates with a variety of

N-substituents-alkyl, simple aryl, heterocyclic, aralkyl, aryl with

either electron donating or electron withdrawing groups-are

prepared with a view to study the effect of N-substituents on the

course and nature of the reaction between Cu(1I) and RHDtc.

(ii) The primary amine-dithiocarbamates generated in solution are

allowed to interact with cupric ion in aqueous medium. The reaction

products, both in solid state and in solution are isolated and purified.

The compounds are systematically analysed by various analyl~cal and

spectral methods which suggest the occuirence of a redox process

during the interaction, forming polymeric copper(1) dithiocrbamates

and thiuram disulphides, RHNCS2-SzCHNK.

(iii) To establish the identity of the oxidised form of the ligands, thiuram

disulphides, formed on interaction of copper(I1) with Dtc, all the

dithiocarbamates are oxidised separately and individually with iodine

solution and their composition and spectral properties compared with

those obtained through copper(I1) interaction.

(iv) Since preparation of dithiocarbamate ligands and their complexation

with metal ions in ethanol medium have been reported, the generation

of various dithiocarbamates, RHDtc and their interaction with

copper(I1) are tried in ethanol medium also. The course of the reaction

is found to be different for N-alkyl and N-aryl substituents though the

occurrence of the expected redox process is confirmed.

(v) Since the reaction medium is found to affect the redox potential of a

process," the interactions are studied in TIIF and/or DMF expecting

that solvents with a lower polarity might stabilise the copper(1I) state.

The products are isolated in the pure state and analysed by various

physicochemical methods including molecular mass determination.

Though redox process does occur, structure and stereochemistry of the

resulting Cu(1) complexes are found to be different from those formed

in aqueous or alcoholic medium. Various oligomeric complexes,

dimers and tetramers, could be isolated.

(vi) Complexation with metal ions is expected to facilitate interesting

reactivity pattern of RHDtc. With this in view electrophilic

substitution, like benzoylation, is attempted on the various oligomeric

and polymeric Cu(1)RHDtc formed during the interaction studies. 'The

difference in the reactivity of the Cu(1) complexes towards

benzoylation is found helpful, to a great extent, to infer the structure

and stereochemistry of the complexes.

(vii) With a view to prepare cupric dithiocarbam:~te by inhibiting the redox

process, the ditluocarbamate function is immobilised by anchoring to a

polymer matrix. Three polymer bound dith ocarbamates derived from

ethylenediamine, p-aminophenol and o-antinophenol are separately

treated with Cuz+(aq). The polymer beads are analysed mainly by EPR

which indicates the redox stabilisation of copper(I1) during the

interaction.

(viii) Attempts are made to generate mixed ligand Cu(1I) complexes

involving N-monosubstituted Dtc on polymer matrix. Each of the

dithiocarbamate functionalised resins is treated separately and

individually with six different Cu(I1) complexes of Schiff bases,

P-diketones and 8-hydroxy quinoline. All the eighteen polymer metal

complexes generated are studied by EPR to characterise the complexes

formed on the polymer matrices.

(ix) To studv the effect of the N-substituent and slereochemistry of the

complex on the mode of decomposition and thermal stability, and also

to confirm the structure and compositiori proposed for the Cu(1)

complexes prepared by interaction of Cu(1.) with RNHCSk), thermal

analysis of the various dimeric, tetrameric and polymeric Cu(I)

dithiocarbamate complexes are carried out by TG and DTG. With the

help of the nine mechanism-based equations proposed by Satava

various kinetic and thermodynamic parameters are calculated from the

thermograms. Suitable mechanisms are proposed for all the major

decomposition stages.

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