Quaterly Reviews & the Chemical Socity. Q.rev.Chem.soc_1965!19!303

8/3/2019 Quaterly Reviews & the Chemical Socity. Q.rev.Chem.soc_1965!19!303

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---THE STRUCTURAL CHEMISTRY OF MERCURY

By D. GRDENIC.(LABORATORY OF GENERAL AND INORGANIC CHEMISTRY, FACULTY OF SCIENCE,

UNIVERSITY, ZAGREB, YUGOSLAVIA)

1:- Introduction- ,IN recent years knowledge of the structures of mercury compounds hasbeen greatly expanded, mainly by X-ray work. Wells! has provided a goodgeneral summary, and there have been recent reviews ofmercury-nitrogen,'halogenomercurateur)," and mercurous compounds.' This review dealswith the structural chemistry of mercury in both molecular and macromolecular compounds. The applications of ligand field theorys- 7 tod10-metal ions,8,' though promising, are not yet numerous, and it is hopedthat the present survey will stimulate further work in this fieid ;-- -

The nearest neighbours of mercury in a crystal structure arc those in"contact" with it. In a free molecule (e.g. in the vapour state) these contacts arc chemical bonds and their number is defined by valency or coordination number. With a crystal structure the position is less clear, andit is necessary to restrict the consideration ofnearest neighbours (the atomsin the co-ordination sphere) ofmercury to those within a defined distance.This is difficult for mercury, as frequently the distances between the surrounding atoms and mercury do not follow the additivity rule of the atomicradii currently accepted, yet are less than the sum of the 'van der Waalsradii. Even when only atoms of one element surround mercury, markedlydifferent distances may be found. Consequently, the adoption of a suitable set of atomic radii should precede any discussion of the structuralchemistry ofmercury.(a) The Metallic R a d i l l s . ~ T h e p a r a m e t e r s of the crystal structure ofsolid mercury'? at low temperature have recently been determined veryaccurately.'! The structure differs from the common metallic close packed

1 Wells, "Structural Inorganic .Chern ist ry," 3rd edn . Clarendon Press; Oxford,1962, pp. 890-900. .". 'Lipscomb, Annals New York Acad. Sci. 1957,427-435. Deacon, Rev. Pure and Appl. Chem., 1963, 13, 189; see also Bergerhoff, Angew,Chem. Intemat. Edn., 1964,3; 686. -. Tarayan, Uspekhi Khim., 1953,22, 1002, _ Nyholm, "Report of the JOth Solvay Conference in Chemistry," Brussels, 1956,p.225. - ' Gillespie and Nyholm, ProgriStereochem., 1958,2,261., Orgel, "An Introduction to Transition-Metal Chemistry-Ligand Field Theory,"Methuen and Co. Ltd., London, 1960. Orgel, J., 1958,4186. Dunitz arid Orgel, Ad. Inorg: Chem. Radiochem., 1960,2, 34.

10 McKeehan and Cioffi, Phys, ReI'., 1922, 19, 444.11 Barrell, Acta Cryst., 1957, 10, 58.

, 2. Radii ofMercury " .. .. ., .

303


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structure, and may be int erpreted as a packing of flattcned rotationalellipsoids instead of spheres. There are two groups of six equal interatomicdistances, 3000 and 3466 A.I: Each mercury atom is surrounded by sixcloser mercury atoms in an octahedron drawn along one of its triad axesand six equatorial, more distant mercury atoms in a hexagon (Figure I).This gives mercury twelve-co-ordination in a fiat cuboctahedron.

304 QUARTERLY REVIEWSj

,,

-,--

,,,,,

, II I, II ,, III

",,,,,

FIG. 1. The environment of mercury atoms ill the crystal of metallic mercury: eachmercury atom has six closer neighbours 01 3000 A (dashed lines) in a distorted actahedron and six more at 3466 A (dotted lines) in a hexagon. The resulting co-ordination.. polyhedron is .a flattened cuboctahedron which relation 10 the rhombohedral crystallattice (a = 3-000 A, a = 70 32') is shown. _ - ' -As a single Hg-Hg distance cannot be given for the metal, both values

.. have to be used in defining the metallic radius, p (Hg), so that 150 A.s;; p (Hg) .s;; 173 A. In amalgams, this means that Hg-Hg distances of290--324 A represent true contacts between mercury atoms, while thoselonger than 3466 A do not.(b) The van der Waals Radius.-In a discussion of the stereochemistry-.o f. mercury, the distinction between bonded and non-bonded atomssurrounding mercury is very important. The van der Waals radius ofmercury, R(Hg), is expected to be nearly equal to its metallic radius. Theexperimental results are rather unsatisfactory, because there are very fewcontacts between mercury and non-bonded atoms in crystal structures.The lowest value of R(Hg) (1'58 A, from the Hg .... C contact") is onlyslightly larger than the lower metallic radius of mercury. Thus the valueR(Hg) = 150 A will henceforth be used for the van der Waals radius.Provided R(Hg) < 173 A, some form of bonding is considered to occur.(c) The Ionic Radius.-Anhydrous mercuric fluoride is the only mercury11 Pauling, "Th e Nature of the Chemical Bond", 3rd edn., Cornell University Press,Ithaca, 1960... Kitaigorodski i, Khotsyanova, and Struchkhov, Zhur, fiz, Khim., 1953, 27, 780.

I

I

J,II!


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GRDENIC: THE STRUCTURAL CII E111STRY OF MERCURY 305compound which is purely ionic. It crystallises in a fluorite type structure"with a = 554 A, and Hg-F = 240 A. Thus R ( H g ~ = 104 A {usingPauling's value12 for R(F-).} [The value R ( H g ' ~ ) = 110A, which is usuallyquoted in tables was probably obtained by using Goldschmidt's value forR(F-).](d) The Covalent Radii.-As stated in section (b), each Hg-L (L isany atom, ion, etc., in a mercury compound) approach of R(Hg) < 173 Acan be considered as bonding. The larger Hg-L distances, indicative ofweak interactions, are not observed in the vapour or in apolar solvents, butonly in crystal structures,melts, and polar solvents. This review is concernedwith the environment of mercury in free molecules or crystal structures.

Digonal covalent radius. The most familiar mercury compoundsare those of the type L-Hg-L (e.g. mercuric chloride) and L-Hg-L' (e.g.methylmercuric chloride). In addition there are also many polymeric _mercury compounds with linear L-Hg-L bonding (e.g. arnidomercuricchloride, which contains the polymeric ion +NH ..HgNH.+Hg). Thediffering values of r(Hg) derived from Hg-L distances in these compoundsare usually explained by the postulation of double bonding or partial ionicbonding for relatively short or long bonds respectively.Because the influence of surrounding atoms on L-Hg-L bonds is significant (see section 5), themost reliable digonal covalent radius ofmercury,r[Hg(2)], should be one derived from data obtained for free L-Hg-Lmolecules in the vapour. The Hg-L bond lengths obtained by electrondiffraction or microwave spectroscopy arc given in Table 1.TABLE I. Hg-L bond lengths (in A) observed in the vapour phase byelectron diffraction (e) or microll'a1'e (mil') spectroscopy .Hg-CI. Mercuric chloride: 220 (e)"; 234 (e)'; 227 (e)'. Mcthylmercuricchloride: 2282 (mw)",Hg-Br. Mercuric bromide: 2'40 (e)"; 244 (e)'; Methylmercuric bromide:2406 (mw)",Hg-I. Mercuric iodide: 255 (e)"; 261 (e)'.Hg-C. Dimethylmcrcury: 220 (c) '; 223 {c}'. Methy!mcrcuric chloride: 2061(rnw)", Mcthylmercuric bromide: 2074 (mw)","Braune and Knocke, Z. phys, Chem., 1933, BlJ, 163. 'Gregg et 01., Trans, FaradaySoc. 1937.33,852. 'Maxwell andMosley. Phys. R. .. 1940,57. 2t. "Gordyand Sheridan.J. Chern. Phys., 1954. 22, 92. 'Brockway and Jenkins, J. Amer, Chem, Sac. 1936. 58,2036.To derive the bicovalent radius of mercury it is preferable to use data

from compounds in which the electronegativity difference x(Hg) - x(L)is as low as possible, i.e. in Hg-C and Hg-I derivatives.* Thus fromHg-I (2'61 A)*r[Hg(2)] = 128 A, which is close to the values of 129and I 30 A derived from C-Hg data for methylmercuric chloride and


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306 QUARTERLY REVIEWS

bromide. The corresponding Hg-Cl and Hg-Br data give values of 129and 127 A, respectively. H owever the value ob tained from the Hg-Cdistance of dimethylmereury (2,23 A) is significantly higher {r [Hg(2)] =146 A}. Although the Hg-C bond in rnethylmercuric chloride or bromidemust be influenced by the halogen, the extent of bond shortening fromMe.Hg to MeHgX seems surprising, and a re-investigation of the structureof Me.Hg is desirable. .Theoretically r[Hg(2)] could be defined as half the observed Hg-Hgdistance in mercurous compounds. However, this distance varies withchange of L in LHgHgL derivatives. For example, in mercurous halldest!the Hg-Hg distance increases from 243 A in the fluoride to 269 A iniodide. The crystal structure'< of mercurous dihydrate contains [H.OHgHgOH.J2+ ions with an Hg-Hg distance of 254 A, which givesr[Hg(2)] = 127 A. As this agrees well with the above r[Hg(2)] values, the

. . co .. _ : : _ ~ . _ _ l ..... f_ ...... .. . o ti, ,;tu ~ ; r r " . r " . n r p . ..-Il-To\ I I \ I hconOltlon UJ J lUI lU l lUUl \ , . J"- ' , ," ,UU.II" '6 u.aJ . . . . . . . w . . . . . . . . . w .. \ ...o , -X \ - - ,/ mus. _cfulfilled., By taking into account the influence of the eleetronegativity difference'!(Schomaker- Stevenson coefficient = 0,03), the mean value for the digonalcovalent radius is 130 A. This obeys the additivity rule satisfactorily forthe free L.Hg or LHgL' molecules. In crystal structures additivity is notpreserved because of the close approach' of neighbouring groups to mercury. Support for the value of r[Hg(2)] comes from the fact that it is lessthan the metallic radius of mercury. The bieovalent radius for sp (or ds-see section 6) hybridisation should be much shorter than the correspondingmetallic radius, especially as metallic, bonding is relatively weak withmercury.

In the literature the bicovalent and tetrahedral covalent radii ofmercuryhave often been confused.TABLE 2. Bond lengths (in A) in crystal structures having tetrahedrallyco-ordinated mercuryHg-I. Mercuric iodide (red): 278' ; 2 ' 8 0 ~ 2'77'; 280". Silvcrtr) tetraiodomercurate: 2'74"; Copperu) tetraidomercurate: 264".He--S. Mercuric, sulphide (black, metadnnabarite): 2'53''/; 254'; 2'525';CobaH(u) tetrathiocyanatomercurate: 2'50'; Bisethylenediaininecopper(n)

tetrathiocyanatornercurate: 2'561; Niekel(u) tetrathiocyanatomercurate dihydrate: 2 ' 5 0 ~ Potassium tctrathiocyanatornercuratc: 254'.Hg-Se, Mercuric selenide: 262.Hg-Tc. Mercuric telluride: 278."HavisllUrsl, Amer. J. S ci., 1925. 10, 556. ' Bijvoe l, Cla ssen , and Karsscn, Proc, k,ned. Akad. Wctenschap., t926, 29, 529. 'Huggins and Magill,J. Amer, Chem. Soc., 1927,49,2357. dKetclaar ,Z. xn; 1931,80, 190. Lehmann, Z. Krist., 1924 ,60, 379JKolkrneijcr and Bijvoct, Proc, k. ned. Akad. Wetenschap., 1924,27,390,847. 'Buckley andVernon, Mincralog: Alag., 1925, 20, 382. hGold schmidt, Geochemlsche Verteilungsgesctze der Elemente, Skrifter Norske Videnskaps Akad.. vol. 8. 1926. 'Jeffery, Nature,1947,159, 6106. IScouloudi , Acta Cryst. , 1953,6, 1953. lK'uo-Hsiang Chou and PoraiKoshits, Krlstallografiya, 1960,5,462. 'Zvonkova, Zhur.fis, Khim., 1952,26,1952."Grdenic and Djordjcvic, J., 1956, 1319.


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, ,I

G R D ~ N I C : TilE STRUCTURAL CHEMISTRY OF MERCURY 307Th e tetrahedral co va lent radius. Inleratomic Hg-L d istances, deter

mined by X-ray o r neutron-diffraction methods, for crystals in whichmercury has regular tetrahedral stereochemistry arc recorded in Table 2.The H g L ~ group is either a complex ion [as in KzHg(SCN),] or the structural element of a layer (as in red mercuric iodide) or ofa three-dimens ionalframework of the zinc blende type (as in rnetacinnabarite).

From the Hg-L distances and the covalent radii12 of suiphur (1'04 A),selenium (1-14 A). tellurium (1'32) A, and iodine (1'28 A), the tetrahedralcovalent radius of mercury is found to be 148 A. This radius is not ofgeneral use, as those structures in which mercury has distorted tetrahedralstereochemistry are actually derived from bicovalently or tricovalentlybonded mercury, with additional close approaches of other, usually moreelectronegative, atoms or ions. To summarise, the following mercury radiiare adopted and used in this review.Metallic radius, p(Hg}: 150 .,;; p(Hg} .,;; 173 A

van der Waals radius: R(Hg} = 150 AIonic radius: R(Hg'+} = 104 ADigonal covalent radius: r[Hg(2)] = 130 ATetrahedral covalent radius: r[Hg(4}] = 148 A

3. Valency Bond Angle of MercuryThe only compounds of mercury that exist as discrete molecules orgive them in vapour or solution are of the type LHgL (e.g, mercuricchloride) and LHg'HgL (e.g. mercurous trichloroacetate). They are notvery numerous, especially the latter. The presence of linear bonds has beenconfirmed in free molecules (vapour phase) by electron diffraction andmicrowave spectroscopy, and in some crystalline compounds by X-ray

diffraction. A slight departure from linearity has been found ' for theC-Hg-C bonds in mercury diethylene oxide" (bond angle 176) and inmercuric cyanide' s (bond angle 171), but in all other crystal structures oforganomercurials the C-Hg-C or C-Hg-X bond angles are close to 180.However, the dipole moments of various diarylmercurials in benzene anddecalin solution are unexpectedly not zero,l9-21 suggesting either a bentC-Hg-C skeleton or at least significant oscillation of the molecules awayfrom linearity in solution. Further investigations as to the configurationsof free molecules of the diaryl mercurials seem desirable. In the crystalstructures of these compounds, because of a combination of inter- andintra-molecular factors, linearity of C-Hg-C bonds seems a requirementfor minimum energy, but this is not necessarily true for free molecules.

In other crystalline mercury compounds, particularly those with halo-gen, oxygen, or sulphur atoms, nonlinear L-Hg-L skeletons are frequently" Grdenic, ACla Cryst., 1942,5,367.


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308 QUARITRLY REVIEWSfound, due to the clcctronegativity of the ligands or the packing conditions in the crystal structure.

4. Co-ordination Chemistry of Mercury .All atoms surrounding mercury at a distance of less than the sum of the

van der Waals radii [D(Hg-L)


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GRDENIC: TI lE STRUCTURAL CHEMISTRY OF MERCURY 309number n is an extension of the characteristic co-ordination number toinclude all l igands fulfilling the condition D(Hg-L) < R(Hg) + R (L).This a rises f rom the close approach of adjacent molecules, ctc., in a crystalstructure. Thus HgLm becomes HgL;where I l>m. The value n = 6 is thehighest known (Table 4). Only when m = 2 or 3 can additional ligands beaccommodated. With m = 4, n = 4, digonal characteristic co-ordinationleads to distorted trigonal, square planar, tetrahedral, pyramidal, oroctahedral stereochemistry, while trigonal co-ordination gives elongatedtrigonal bipyramidal, or very distorted tetrahedral effective co-ordination.Linear bonds in digonal characteristic co-ordination become bent when themolecule acquires distorted trigonal or tetrahedral effective co-ordination.The new ligands lengthen the Hg-L bonds of the characteristically coordinated ligands. To summarise, in effective co-ordination two groups ofD(Hg-L) distances are observed: (I) shorter ones, which represent characteristic co-ordination and (2) longer ones, due to close approaches incrystal structures. This criterion is also applicable to systems involvingdifferent ligands. Examples of all known effective co-ordination arrangements arc given in , Figures 3-5 , the most common being distortedoctahedral, based on digonal characteristic co-ordination.

TABLE 3. Characteristic co-ordination (m) 01 mercury ill molecules,complex iOIlS, and crystal structures in relatlon to the electronegativity

(XL) 01 the ligands, Covalent

D(Hg-L) =r[Hg(2)] + r(L)

D(Hg-L)D(Hg-L)=r[Hg(4)] + r(L)D(Hg-L)


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D(Hg-L)peciesHgOHg(NH:J2CI2Hg(CN).HgPY2C l2NH,HgCI3K2HgCI,H.oHgBr.HgT2(yel low)HgS(cinnabar)Hg2X .

II6

TABLE 4. Effective co-ordination (n) of mercury ill crystal structures ill.relation to the characteristic co-ordination (rn) and bonddistances D(Hg-L)Characteristic bonds11/ = 2 11/=3O-Hg-ON-IJg-NCI-Hg-ClBr-Hg-BrC-Hg-CI-Hg-I -S-Hg-Sandmixed

5Distortedsquarepyramid5Elongatedtrigonalbipyramid

(MeShHg

Mc3SH gT 3Hg,NHBr.

Probablebut notobservedyct

xIHg/ "XX = Br or I

11/ distances withD(Hg-L; 'r[Hg(2)] + r(L)and n - 111 distances withD(Hg-L)< R(Hg)+R(L)

4(Me,N)HgBr3(Me,N)HgCI3(Me,N)HgT3

XIHg/ "XX = ci , Br, or T

HgSO, O-Hg-O(NCHg).o G-Hg-DHgCI.)HgO 0"''' ,....,-ng-\...IHg(SCN)"KSCN S-Hg-S

3Planar or Mercuryncarly diethylene G-Hg-C -planar oxide(c) Factors Influencing Co-ordination.s-Tue following are the mainfeatures of the crystal chemistry of mercury, (I) Linear covalent bond


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GRDENIC : TilE STRUCTURAL CII EMISTRY OF MERCURY" 311,

, C '

o ',

101 0 "" .. 2,041 9 ~ ~ 0: . . ,C. 53 158' 0b

.: ~ ' 7 ~ O216 H 216. . 9 .2' 16 / _ \ 2 '66 .11-20o Cl

c .

,

o' 1 4 ~ 2 ~

IS9 (H9 I 144_ .. / ' ~ t:",'t,oe

SHg )102

/ 2 ~ / \ JI . .Sf

FIG. 3. Examples of trigonal (a) and tetrahedral effective co-ordination of mercury:(a) mercury diethylene oxide,11 (b) mercuric oxycyanlde,IS (c) trimercurlc ox)'chloride,ll(d) bisitrtphenylarsine oxlde'ynercuric c1rloride,ua (e) anhydrous mercuric sulphale,H.(f) mercuric dithizonate,"

S\3 ,24

S 2 3& 3 24 S- -- H g_S.----- S" o

Sr13 '09Sr.......... 2-60

120\ Hg SrS r \ .Sr b c . '.'

.. .

FIG. 4. Examples of tetrahedral (a), bipyramidal (b), and pyramidal, (e) effecti veco-ordination o f mercury, The distorted tetrahedron (a) ill (Me.N)HgBr,," and thetrigonal bipyramld (b) ;11 imidomercuric bromide" are based on tr igonal characteristic- co-ordinaticn, while- the -pyramid-in dimethylthto -mercury"' . originated from dlgon..i !characteristic co-ordination. " .compounds from ionic to molecular, and mercury co-ordination could becorrelated with all influencing factors, e.g, ionic radii, polarisability,electronegativity, etc. It has been found that the electronegativity difference [x(Hg) - x(L)1is most suitable for a rational classification of thecrystal structures of mercury compounds. The value x(L) = 25 is acritical one, because for x(L) > 25 the characteristic co-ordination"isgenerally digonal and for x(L) < 25 the characteristic and effective


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312

)QUARTERLY REVIEWS

Sr 0 8 r ~ 1 2 ' 8r !?>-.I ~ 5S K7S' 5H g 282 ' . H . _ ' J 6 - - H g ~B r I "--Sf -: 9 .,"'..0_ 10,2 48 o 2'031 S-- /23;:-S8r 0 Sa b c

N N CI 90' Br 2'06/ 76' a-r ___ ~ B - - . . . . . . . . . . H ~ 1

208307 Hg 1'0' 85'(" 1.> : --.:.... 33. 9 190' - 1 - - = - H g ~Sr 12'17 Br S r ~ / 3 ~ B r 1"33. I -r

N Nd - - Ce f

9

I k

aN I,,. N< , Hg ) .5 'N ,o>l:-N

CS. ~

C l i l / N70'1 Hg 2 80------ -...::..:: NCl /2 -2.Cl

o1--....... I ~ H Hg 333I I -----12-03o h

028-O 210\- : 2 -


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- .GRDCNIC : TIl E STRUCTURAL CHEMISTRY OF MERCURY 313

groups depending whether the basic L-Hg-L skeleton is linear or bent.The first group contains the point groups C/ (very frequent), D'h' C, .,D.h , C." C., C" Cl> and the second the point groups C" Cae , C., and C1(Figure 5). Tetrahedral effective co-ordination is very rare for x(L) > 25and may be the result ofsteric factors inthe crystal structure, e.g., mercuricsulphate" or cyanomercuric oxide, (CNHg),O.2' The size of the atomsand packing conditions do influence the final arrangement, but x(L) givesa satisfactory correlation for most structures.

5. Crystal and Molecular Structures of Mercury Compounds(a) Halogenomercurateiuy Compounds:-:The crystal structures of themercuric halides are an example ofa morphotropic transition dependent onthe electronegativity of the halogen (4'0, 3'0, 28, and 25 for F, Cl, Dr, andI, respectively). The structures of the chloride, bromide, and iodide arcgiven in Figure 6. While mercury has distorted octahedral stereochemistry

,B Br B BrI '-, ?Sr\."Hg ' l 1'" ,.?BrHg V A.. HgBr'l " ~ H ... Br: / '" ,// Br '// BrBr Br b

.. .

Cl Cl\ \Hg Hg\ \Cl Cl Cl Cl Cl

" / :... /H9. H9./ \ ...... / v-,CL cict Ct "CL/ IHg HgI /Cl Cl

I II \ I \ ,

I / \ Hg/ \ Hg /\ Hg ~ / Hg>4/"-Hg > 4 / ~ Hg' )4/\ .~ ,X /

I I , -- . . I " .C

FIG. 6. The environment of mercury in the crystal structures of mercuric chloride (a),bromide (b), and iodide (c).

o

. "

in the chloride, the structure is essentially molecular.t! because two pairsofHg . .... CI distances (3'34 and 363 A) are larger than the sum of thevan del' Waals radii (3,30 A). Mercuric bromide has a layer structure of adeformed brucite type," with six Hg-Drdistances less than R(Hg) + R(Br).


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314 QUARTERLY REVIEWSJ

Thus the effective co-ordination is oc tahed ra l (Figures 5 and 6). The crysta l'structure of the iodide is composed of la yers of fully corner-linked HgT.tetrahedra." The mercuric halides are a good example of allot ropy.The yellow unstable modification of the iodide crystallises with the HgBr.structure,"S,29 while the bromide probably has an unstable m odificationwith the HgCl, structure': ' In the vapour state, mercuric chloride, bromide;and iodide consist of linear molecules. The behaviour of the fluoride isnot known, but should be similar in the vapour.Halogenomcrcurates of the type (Ml+)._'a/,HgaX. (M is a simple orcomplex cation, X is Cl, Br, I) are well known, the most common belongingto the classes (M+).HgX, and (M+)HgXa- Recently some pyridinium tetra- .fluoromercuratefu) derivatives have also been made.s? The properties ofthese compounds in 'solutions and in the melt have been extensively investigated' and have been recently reviewed." Here the discussion isrestricted to their crystal structures.TIle electronegativity rule is again relevant. The characteristic co-ordination for chloro-complexes is generally digonaI , though there arc exceptions (see below), while that for iodo-complexes is trigonal or tetrahedral.Relatively little is known about the structures of crystalline bromonercurates, and the structures of the tctrafluoromercuratess" have nol yet beendetermined. .The crystal structure of yellow, tetragonal Ag.HgI, has cubic do se

packed iodine atoms, with some of the tetrahedral holes filled by silverand mercury atoms in a regular manner, It is ordered below 50'7 , butthe red 'cubic modification obtained above this temperature has a diso rdered zinc biende structure." The isomorphous Cu,HgT, shows analogous therrnoehromic properties. Tetraiodomercurate ion are ' found in[Me.SJ,HgT,," bUI the complete structure has not yet been reported. Thestructure of [Mc.S ]HgJ. (m = 3) has been mentioned (Figure 2, Table 4),while [Me,N1HgT. is isomorphOilS" with [Me,N1HgBr. , the structure ofwhich :IS has been considered (Figure 4, Table 4).There is one exception to the clectronegativity rule conclusion that in

chIoromcrcurate the characteristic co-ordination of mercury should bedigonai. The tetrachloromercuratc of the alkalcid pcrloline containstetrahedral HgCI,'- ions.'e In [Me,N1HgCI., which is isomorphous"with the corresponding tribromo-derivative.s! the characteristic co-ordination is trigonal (Table 3). A classification of the other known chloro-. . Havighurst, Amer, J. Scl., 1925, 10, 556; Bijvoet, Clausen, and Karssen,Proc.k.lled.Akad. Wetenscliap., 1926, 29, 529.to van Nest, Z. Krist., 1909,47,263 .. . Dotzer and Meuwsen, Z . anorg; Chem., 1961,301,79." Wait and Janz, Quart. Rev., 1963, 17, 225."Ketelaar, Z. Krist., 1931,80,190; 1934, (A), 87, 435; Wells, ref. I, pp. 174,533." Fenn, Oldham, and Phillips, Nature, 1963, 198, 381." Fatuzzo, Nitsche, Roetschi, and Zingg, Phys. Rev., 1962, 125, 5t4 ." White, Acta Cryst., 1963, 16, 397.3 \1 Jeffreys, Sim, Burnell, Taylor, Corbett, Murray, and Sweetman, Proc, Chem. Soc...

I,I,,


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o

7,GRDENIC: TilE STRUCTURAL C I I E ~ I l S T R Y OF MERCURY 315

mercurate structures has been given.37 As 11/ = 2 and II = 6 in all thesecompounds, then the con figuration of the anion does not follow from thestoicheiometry. Thus K2HgCI.,H20 does not contain discrete HgCI .2ions,38 ,39 but distorted HgCI . octahedra form columns sharing two oppos ite edges (Figure 7a). There is a linear CI-Hg-Cl skeleton (Hg-Cl229 A), with two pairs of longer bonds (2,29 and 313 A).'8Iftwo of these

Cl Cl CLCl I CL I Cl I Clfig ./" 'Hg/ 'Hg/

Cl / Ih/ \Cl/ ICtCl Cl C b

.. '" .

cCl Cl Cl

Cl Cl Cl Cl

cr" e Cl"- t 'c( j , 'Cl\ \ 1 / 1 \ / 1/ H 9 , cyHg , Cy Hg, ClCl Cl Cl I Cl

Cl Cl Cl

.......

FIG. 7. The chloromercurate anions ill the crytsal structures of mercury chloro-complexes (idealised): (a) ribbon o f (HgCI,).' - in K,J1gCI"H,O,"" (b) twofoldribbon 0/ (Hg,CI .). ' - in NaHgCl," and related compounds ... ... and (e) layer of(HgC I,). - ill NH.HgCI, ."


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316 Q U A R T ~ R L R E V I ~ W SI

o

. b . .

linked octahedral columns are condensed by sharing three edges of anoctahedron.the polymeric (Hg.CI,)a'a- ion is formed. This has been foundin the structures of NaHgC13 .'o NaHgCI,,2H .O,41 and Na.Hg.CI.OH(Figure 7b)." NH.HgCI3 is not isomorphous with the sodium salt, buthas a layer structure" in which HgCI, octahedra share four chlorineatoms (at 2'96 A), and have colinear Hg-Cl bonds of 234 A (Figure 7c).The corresponding cresium compound has a cubic structures" in whichmercury and cesium occupy perowskite positions, the environment ofeach mercury being two chlorines at 229 A arid four at 270 A. Threeacids, H.HgCl.,3H.O, H.Hg.CI"xH.O, and HHg.C1., have been idcntificd in the solid state}' The first is isostructural with K:HgCl.,H.O andthus may be written (H30).HgCl.,H.O. The detailed structures of theothers are not known, but are considered to involve HgCl, octahedra."(b) Mercuric Oxide, Oxyhalides, and Related COlllpoll/lds. -Bothmodifications, orthorhombic'! and hexagonal", of mercuric oxide, are

1 I _ -r.,. ,.... TT 1""\ TT ' 1... IC' 0\ t........ 1 oascc on u'ng'v"ug'v 'ng ig..zag cnams (r -igure oj, wrncn arc pranar 1:1the former and spiral in the latter. In both 11 = 6 and II I = 2, and thecharacteristic Hg-O distance is 203 A. Four oxygen atoms from t j n chains are only slightly closer than the sum of the van der Waals radii,viz. at 279 A (two) and 290 A (two) in the hexagonal form, and at 282 A(four) in the orthorhombic form. The crystal structure parameters havebeen refined by neutron-diffraction methods, and hence are very accurate.o 020Y " / -,Hg 109" H;)m" Hg-, / -, /o 0

H H H H\/ \/2 .10 N+ N..

/ " - - " , - . . . - : - ' \ ' 80 ' / - ,Hg 109" Hg) Hg-, /


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-GRDENIC: TI lE STRUCTURAL CHEMISTRY OF MERCURY 317

TABLE 5. S tructure types 0/ mercuric oxylialides Hg.C1 ,O._l ill relationto oxygen :mercury atomic ratioa Formula O/Hg Structure Ref.I HgCI. 0 CI-Hg-Cl 263/2 . 2HgCl"HgO 1/3 (CIHg).o+C1- 49, 503 HgCI,,2HgO 2/3 Hg(OHgCI), 514 HgCI,,3HgO 3/4 Related to kleinite, a naturally occur- 52,44,ing oxychloride containing nitrogen. 47,2Probably distorted tridymite-like struc-ture5 HgCI,,4HgO 4/5 ( H g ~ C I ~ ) H ( H g . O . ) I - 535 HgBr,,4HgO 4/5 ( H > B r ~ ) I + (Hg.O.)I- 53*00 HgO III Chains of Hg-O-Hg-O-Hg-O 44-47 Added ill Proof.-The structure of HgDr,,4HgO, as determined recently byAurivilius (Ark;" Kemi, 1965,23,469), is best defined as Hg(OHg),Br,.Many basic mercuric halides or oxyhalides of the general formula

Hg.CI.O._1 (a = 15,2,3,4,5) have been described." Recent investiga-,. tions have shown that the characteristic co-ordin ation is digonal [x(Cl) = '3,0, x(O) = 3,5), and the effective co-ordination is tetrahedral or octahedral. ~ - . The structures are strikingly dependent on the oxygen: mercuryratio, and each species has its own distinct structure (Table 5).With increasing oxygen content, the structures become more differenti

ated into mercury-ehlorine and mercury-oxygen structural elements,until complete separation into alternating layers of complex halide cationsand complex oxide anions occurs" with an atomic ratio O:Hg = 4:5.The structure of 2HgCI.,HgO contains the tris(chloromercuri)oxoniumion. This should be pyramidal if the Hg-O links are covalent single bonds.However X-ray diffraction has shown the ion to be approximatelyplanar>"o (Figure 9c), which might indicate partial double-bond characterfor the Hg-G links. Neutron-diffraction analysis" shows a slight deviation from planarity. The O-Hg-CI bonds arc linear (Hg-G = 203 A,Hg-Cl = 228 A) and mercury has a very distorted octahedral environment.The structures of the analogous tris(methylmercuri)oxonium compounds"' a re now being investigated. ..,.. Aurivilius and Heidenstarnm, Acta Chem, Scand., 1961, IS, 1993." Aurivilius, Acta Cryst., 1956, 9, 685 .. . Mellor, "A Comprehensive Treatise on Inorganic and Theoretical Chemistry,"vol, 4. Longrnans, Green and Co., London, 1957.. . Weiss, Nagorsen, and Weiss, Z. Noturforscli. , 1953, 8b. 162; Weiss, Nagorsen, andWei ss. Z. anorg, Chem., 1953,274,151." Grdenic and S C a v n i c . ~ r , Nature, 1953, 172, 584; SCavniear and Grdenie, ActaCryst., 1955,8,275." Seavniear, Acta Cryst., 1955, 8, 379.


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31 8 QUARTERLY REVIEWS !Mercuric oxide forms compounds with salts other than those ofmercury.

The crysta l structure of (HgO)"NaI consists of layers formed from HgOchains intermingled with iodide ions.56The effective co-ordination of eachmercury is octahedral, if th e fairly close approach of a mercury atom inan adjacent layer is considered to be significant (H g . - . . . Hg = 3334 A).(c) SalIS ofOxy-acids.-Digonal characteristic co-ordination is expected[x(O) = 3'5]. The following crystal structures have been investigated:(I) anhydrous salts: HgSO?,,57(2) hydrated salts: HgSO.,H.058(3) basic salts: HgSO.,2Hg059-6., 2HgSO..HgO,2H,O,61

Hg(X03)"HgO,H,06' (X is ci, Br)(4) complex or double salts: K3[Hg(NO,).]NOa- 23Mercuric sulphate and selenate are isomorphous, as are dioxotrimercuricsulphate, selenate, and chromate.s?In anhydrous mercuric sulphate the stereochemistry of mercury isdistorted tetrahedral. The O-Hg-O bond angles in two mutually normalplanes are 1590 and 144, while the Hg-O OUJlU lengths are 214 (two) forthe former, and 208 and 228 A for the latter OHgO groups. The hydratehas a completely different structure based5sa on H,OHg05 octahedraarranged in parallel chain..which arc crosslinked through sulphate ions.

The characteristic H,O-Hg-O bonds arc bent to 169, the H,O-Hg andHg-O distances being 224 and 217 A, respectively.P" The Hg-O distancesof the more remote neighbours are 2-50 A (two) and 251 A (two) [N.B.R(Hg'+) + R(O'-) = 244 A]. The structure of the basic mercuric sulphate,Hg30,SO., (turpcth mineral)!" has been solved quite recently.60,61 Thestructure is formed from layers of polymeric mercurioxonium cations(Figure 9d) with sulphate anions between them. Thus the compound is(Hg30,),,+(SOI-). The characteristic Hg-O distances are 203 A, andmercury has octahedral effective co-ordination. A polymeric oxonium ion(Figure 9b) has also been Iound'" in crystals of basic mercuric chlorate andbromate monohydrate, Hg(X03)"H,O, and the correct formulation ofthese salts is therefore (HgOH).+(X03-). ' The occurrence of rnercurioxonium ions in ba sic mercuric salts is thus fairlv general. Similar behaviour is found for ammonia derivatives of mercury compounds, wheremercuriammonium ions occur. The compound 2HgSO.,HgO,2H,O has acomplex structurc'" with two crystallographically different mercury atoms.. . Aurivilius, Acta Chem, Scan d., 1960,14,2196; 1964, 18, 1305.I l Aurivilius and Malmros, Acta Chem, Scand., 1961,15, 1932.. . (a) Bonefacic, Acta Cryst., 1961, 14, 116; (b) Templeton, Templeton, and Zalkin,Acta Cryst., 1964, 17,933 (refinement by the use of three-dimensional X-ray diffractiondata).. . Paie, Bull. S oc. chim. France, 193 0,47,1254 ; Paic, AIII/ . Chlm. (Frallce), t933, 19,4 2 .. . Nagorsen, Angew, CMIII., 1962,74, 119."B onefacie, Thesis, University of Zagreb, 1963; Bonefacic, Acta Cryst., 1963, 16,A30. .

,I,I,

I,I,,;,


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I- GRDENIC: TilE STRUCTURAL CHEMISTRY OF MERCURY 319

Hg 2'S ' Hg 21$ -H2O OH2- '- ,- '

H H Cl , , . , . .'I I , I +0 +0 , - Hg " ./ ')( ?3 / " " - -. b I Hg , Hg . Hg Hg . '. 0, , /. ' / " 0 / . " , . O ' 0 . 0 Hg Hg' .'/ ' I r ' /, -zs -,H H H ~ Cl Cl

,

- .

d

c

FIG. 9. The oxonium ions in the crystal structures 0/ different mercury compounds:(:1) the hydrated mercurous Ion In Hg,(NO,),,2H,O," (b) the hydroxomerruric Ion IIIbasic mercuric chlorate and b r o m a l e ~ n (c) the tr isichloromercuriioxonium ion ill2HgCI"HgO,"" and (d) a (Hg,O,). '" layer In the basic mercuric slIlphate.""Only one of them ,has Hg-O bond lengths corresponding to covalentbond formation (Hg-OH2 = 212 A). All other Hg-O distances ale largerthan 241 A. There is no characteristic mercuri-oxide chain in the structure,which must be considered to be essentially ionic.The structures! of potassium tetranitritomercurate(n) nitrate also seemsto be essentially ionic. The nitrogen atoms of the nitrite ions surroundmercury tetrahedrally, but eight oxygen atoms at 24 A do not follownecessarily the tetrahedral symmetry.(d) Compounds with Mercury-Sulphur Bonding. -As the electro- . negativity of" sulphur is 2-5, both tetrahedral arid digonal characteristic

co-ordination is observed. The stable modification of mercuric sulphide(cinnabar) has octahedral effective co-ordinntion.s! The compound isisostructural with the hexagonal form of mercuric oxide.v The characteristic S-Hg-S bonds are bent to an angle of 172.4, and the Hg-S length(2,36 A) is equal to the sum of the digonal covalent radii. Two other pairsof Hg-S distances (3-10 and 330 A) are less than the sum of the van derWaals radii. Doth black mercuric sulphide, obtained by precipitation from


18/26

320 QUARTERLY REVIEWSI

Tetrathiocyanatorncrcu rates contain the tetrahedral Hg(SCN)l- ion,in which the Hg- S bond lengths nrc equal to the sum of the tetrahedralcovalent radii. The double salt, Hg(SCN)"Ni(SCN)2,2H20 , has been-showns! to be nickel tetrathiocyanatome rcurate(n) dihydrate. It containstetrahedral Hg\SCN)/- ions (Hg-S = 250 A), and nickel cations ill anoctahedral environment of two oxygen and four thiocyanate nitrogenatoms (Ni-O and Ni-N = 208 A). In the compound KHg(SCN).,mercuryhas digonal characteristic and distorted tetrahedral effective co-ordination.The characteristic S-Hg-S linkage is bent (155), and two sulphurs (at .278 A) from adjacent thiocyanate ions complete a distorted tetrahedralenvironment 'S of the type in Figure 3f. The analogous ammonium salt isisostructural, andmercuric dithizonate has a similar structure." ,The crystalchemistry of mercuric thiocyanate has been discussed.7, . ' Mixed derivatives of the type XHgSCN (X is Cl or Br) have digonally bonded mercuryin distorted octahedral cnvironrnent'" (Figure Sf).In dirnethylthiomercury, Hg(SMe2)2 , mercury has two short linearbonds and a unique square pyramidal effective co-ordination.t? Thestructures of RSHgCFl and (EtS)2Hg72 seem to need further refinement.The structure of the sulphochloride Hg,S2C12 consists of layers of polymeric mercurisulphonium cations and halide ions" and is similar to that

of the basic mercuric sulphate shown in Figure 9d. The S-Hg-S .bondsare linear (Hg- S = 244 A) with the sulphur atom at the apex of a trigonalpyramid and an Hg-S-Hg angle of ca. 95. The analogous mercuric selenoand tellurohalides have similar st ructures. Mercurisulphonium ionsprobably occur as commonly as the oxonium analogues. The suIphoniumcompounds are more stable, tris(methylmercuri)sulphonium ions havingexceptional stabi lity."!(e) Compounds with Mercury-Nitrogen Bonding. -The large number of

mercury compounds with ammonia and its derivatives were originallyelassified into three groups :7S (I) mercuric amrnines, e.g, "fusible whiteprecipitate" or the various addition products of amines with mercuricsalts ; (2)mercuric amides, e.g, "infusible white precipitate" and analogous.... ' uo ' f a- - Chou .... - Tl ,.. : Kcshits . " __... .L: _ l 0 l:f\ .::. 4.i '::1.t\. v .c :) 110 '- .I u u 1 0..>, .. u l l . . ~ , _ . , _ _

IS Zhdanov and Sanadze, Zhur, fiz, Khim . 1952,26,469.. . Harding, J., 1958,4136.17 Zhdanov, Communications au Xllfe Congres International de Chimie Pure etAppliquee, Stockholm, 1953, Editions de l'Academie des Sciences de I'U.R.S.S.,

Moscou , 1953, p. 175 .. . Zhdanov and Zvonkova, Trudy Inst, Krist, Akad. Nauk S .S.S.R., 1954, 10, 30.. . Zvonkova and Zbdanov, Zhur, fiz. Klzim., 1952, 26, 586."B radley and Kunehur, Chem, and Ind., 1962, 1240; J. Chem, Phys., 1964, 40,2258.11 Johansson, Arkiv KemiMin., Geol., Ser. A, 1939, 13, I."Wells, Z. xn. 1937, A. 96, 3 ~ "Pu ff and Kohlsehmidt, Naturwiss., 1962, 49, 299; Puff and KUster, Naturwlss.,1962, 49, 299; 49,464." Grdenic and Markuf ic, J. , 1958,2434.

iI


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-i,'.i

i,GRDENIC: TIlE STRUCfURAL CHEM ISTRY OF MERCURY 321

.. . .'..--(Hg.N)I,IIH 20(Hg.N)OH,2H 20tetrahedra

(4) three-dimensional framework:r ~ 1.+I-Hg-N-Hg-IHgI

arnidornercuric derivatives; (3) Millon's base and its sa lts. This classification still remains good in principle. X-Ray analysis has shown that thestructure of these compounds arc generally based on substituted ammonium ions. Brodersen and Riidorffhave given the following classification :'6(I) isolated linear groups: e.g. "fusible white precipitate

+H3N-Hg-NH3+ Hg(NHa).CI 2(2) polymeric zig-zag chains: e.g, infusible white precipitate-+NH.-Hg-NH.+-Hg- HgNH.CI

(3) hexagonal puckered layers: e.g, dimercury imidobrornide[Hg.(NH).l.2.+ Hg2NHBr2, viz,

[Hg.(NH)21;+(nr).(HgBra-).e.g: Millon's base and its salts

I

II

There are also some Hg-N compounds in which nitrogen apparentlyretains an unshared electron pair, e.g, mercuric derivatives of acid amidesand hydrazides.?? .In many of the known structures the mercury-nitrogen bonds are colincar, mercury having octahedral effective co-ordination. The Hg-N bondlength is equal to or less than the sum of the covalent radii. Hence the

Hg-N bond is covalent and the positive charge is located on nitrogen.Four adjacent atoms or ions approach closer than the sum of the van derWaals radii to complete the co-ordination octahedron (e.g. Figure .5d, e).Diamrninemercuric halides (NHahHgX. (X is Cl or Br) have a statisticalcubic structure, built ofunit cubes of halide ions." In each cube, one of thesix possible positions at the centre of a face is occupied by a mercuryatom, so that the linear +HaN-Hg-NHa+ ions arc randomly orientatedalong one of the three axes with nitrogen atoms approximately at the centreof each cube. The structures of the analogous alkylamine derivatives" arestill unknown. The complex PY2HgCI2 is less stable, losing pyridine atroom temperature. The Hg-N distance (2,60 A) is considerably larger thanthe sum of the covalent radii. There are two short (2'34 A) and two longer(3,25 A) Hg-CI bonds.s"" Brodersen and RUdorff, Z. Naturforsch., 1954, 9b, 164; Brodersen, ACla Cryst.,1955, 8, 723.n Brodersen, Chem. Ber., t957, 90, 2703; Brodersen und Kunkel, Z. anorg, Chem.,1959,298,34."Bijvoet and MacGillvary, Cliem, Weekblad, 1935,32,346; MacGillvary and Bijvoet,


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322 QUARTERLY REVIEWSAmidomercuric chloride and bromide are actually salts of a polymeric

mercuriarnrnonium ion (Figure 8b) . Thc bromide is cubic when purc.!'otherwise it is orthorhombic, as is the chloride.a, IDirnercurihydrazonium chloride, Hg.(N.H2)CJ2, has a layer structure .with a trails-conformation, and its formation may be interpreted as rcsulting from dehydrogenation of aminornercuric chloride :a3

An analogous chain structure has been found in ethylenediaminemercury(n) chloride." In "irnidornercury bromide", Hg2NHBr2, layers ofHgBr3- . ions alternate with [Hg3 (NH ) 2 ] . ' a + layers which also containbromide ions."- The compound is therefore written: [Hg3(NH)J .'a+(Dr). (HgDr3- ) u- The cation layer has the same structure as (Hg30J.'a+(Figure 9), oxygen atoms being replaced by NH groups. There are twokinds of effective mercury co-ordination in the structure, octahedral andtrigonal bipyrarnidal, Steric conditions do not allow the accommodationoran anion other than bromide. .Millon's base and its salts are known in cubica'a and hexagonala'b,.omodifications. In the cubic form, only the nitrate is stable, whereas thehydroxide and other salts are stable in the hexagonal modification.ssThe cubic modification has a cristobalitc structure, and the hexagonal forma tridimite type of structure in which silicon is replaced by nitrogen andoxygen by mercury. The NHg. framework so formed is a polymericcation, (NHg 2).a+, having interstitial holes sufficiently large to accommodate different anions. A regular tetrahedron of mercury atoms surrounds each nitrogen (Hg-N = 204-209 A, depending on the anion).Additional close approaches give mercury octahedral effective co-ordination. The structure has been confirmed by infrared spectroscopy'? andanion-exchange experiments. The fluoride, HgNH2F, is a doublefluoride of the ammonium ion and Millon's base'" [(NHgJF,NH.F].The interstitial space is sufficiently large to accommodate ammoniumas well as fluoride ions. The hydrated acid fluorides" should probably beIormuluted (NHgJF)(H30)F) w _ i l ~ ~ h p . nitrate, sulphate, phosphate , and" Riidorff and Brodersen, Z. anorg: Chem., 1952,270, 145; Brodersen and RfidorfT,Z. anorg. Chem., 1954, 275, 141... Lipscomb, Acta Cryst., 195J, 4, 266; Nijsen and Lipscomb, Acta Cryst., 1952,5,604... Brodersen, Z. anorg, Chem., 1956,285,5; Brodersen, Z. anorg: CMm., 1957,290,24.. . Brodersen, Z. anorg: Chem., 1959,298, 142-. . (a) Lipscomb, Acta Cryst., 1951,4, 156; (b) N ijsen and Lipscomb, Acta Cryst.,1954,7, 103. .II Brodersen and RfidorfT, Augew. Chem., 1952, 64, 617; RUdorfT and Brodersen,

Z. anorg: Chem., 1953,274,323.et Brodersen and Becher, Chern, Ber., 1956,89,1487.. . Arora, Lipscomb, and Sneed, J. Amer, Chern. Soc., 1951,73, 1015; Seeger andPualuan, BDl. Soc. Chilena Q/lim., 1962, 12,25.


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, T STRUCTURAL H R OF MERCURY 323arsenate should have a similar structural framework.90 The structure of thedegradation product of Millon's base has been studied ,"!The structures of a number of phosphine and arsine derivatives of

mercuric salts have also been investigated;" In PbHgPH mercury issurrounded by four phosphorus atoms in a slightly deformed tetrahedron 93with Hg-P = 252 A.(f) Compounds with Mercury-Carbon Bondillg.-Potassium tetracyanomercurate(lI), which is isomorphous to the analogous zinc compound.v'was the first mercury compound to be examined by X-ray methods, butthe exact structure is sti1l not known. Mercuric cyanide is molecular rather

than ionic. 9s The full structure was determined by Zhdanov and Shugam, 9Sand neutron diffraction has subsequently been used to obtain the lightatom positions accurately.t" The Hg(CN) 2molecules, which are linear insolution or vapour, are bent in the crystal lattice because of Hg-N inter-actions. The effective co-ordination is distorted octahedral (Figure 5),the C-Hg-C bond angle being 171, and the Hg-C and Hg-N bondlengths 1986 and 270 A, respectively, The former is significantly shorterthan the sum of the digonal covalent radii, hence the Hg-C l ink may havesome double-bond character. Mercuric oxycyanide, HgO,Hg(CN)2, isactually cyanomercuric oxide, (NCHg}20.M.99 Mercury has digonalcharacteristic co-ordination (Hg-C=I'97 A), but the two mercury atoms ineach (NCHg}20 unit have different effective co-ordination numbers,viz, tetrahedral (C, 30) and octahedral (C, 0 , 4N). The closest mercury-tooxygen approach between two molecules, 253 A, is exceptionally shortfor an intermolecular contact (cf. Hg-O bond length of 204 A), butthis does not influence the Hg-C length. However, in the structure ofKI,Hg(CN)2 the close approach of four iodine atoms (3,383 A) causes aslight lengthening of the Hg-C bonds10o (2,079 A).Comparatively little is known about the crystal structures of R 2Hgorganomercurials. Both phenyl radicals of diphenylmercury are copianar lOl , l02 in the crystal structure, but this seems less likely in solution.19- 21All cyclic organomercurials so far investigated contain essentially' linear C-Hg-C bonds. Thus "o-phenylcnernercury" which \V aS considered "" Airoldi, AIII I , chlm, (Rome), 1958, 48,491... Weber, Naturwlss., 1957,44,465.. . Evans, Mann, Peiser, and Purdie, J., 1940, 1209.13 Krebs and Ludwig, Z. anorg, Chem., 1958,294,257."Dickinson, J. Amer, Chem, Soc., 1922,44,774.13 Hassel, Z. Krist., 1926, 64, 218.If Zhdanov and Shugarn, Zhur, fix. KI,IIII., 1945. 19, 433." Hvoslef, ActaChem. Scand., 1958, 12, 1568... S C a v n i ~ r , Acta Cryst., 1960, 13, A, 57; SCavniear, Z. Krlst., 1963, Jl8, 248.II Weiss and Hofmann, Z, Naturforsch., 1960, ISh, 679.' 00 Kruse, Acta Cryst., 1963, 16, 105.


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324 QUARTERLY REVIEWSto be 9,10-dimercura-anthracene is actually Irimeric,l o3 the moleculecontaining six linear C-Hg-C bonds.I M "Pentamethylenemercury" forwhich a structure having a ring of five carbons and onc mercury atomhas been suggcs ted.l'" is actually dimeric,lo but is not isostructural withmercury diethylene oxide.':" The latter has a twelve-membered ring, inwhich the C-Hg-C bonds arc slightly bent (176), due to mercury-oxygeninteraction (Hg . . . . . 0 = 221 A). "2,2-Biphenylenemercury" is not 9mercurafluorcne, but is a cyclic tetrarner.t'"The crystal structures of mixed organomercurials RHgX (X is CI, Dr,or I) are characterised by linear C-Hg-X bonds and octahedral effectiveco-ordination. In alkyl derivatives the structure is dictated by a tendencyfor parallel alignment of CHgX groups .l'" There is disagreement betweenthe Hg-Cl distance for McHgCl as obtained by X-ray methods (solidstate)IO' and microwave spectroscopy (vapour state)1I0 (2'50 and 2282 A,respectively). Such a difference between the bonding in the two statesseems improbable. The X-ray value is doubtful, since it was obtained fromtwo-dimensional Fourier synthesis from a small number of reflexions.Thus a three-dimensional Fourier synthesis of the electron density isdesirable. For rnethylmercuric bromide the difference is much smaller[2'50 A (X-ray), 2406 A (microwave)]. Chlorovinylmercuric bromide hasa sim ilar structure!'! with an Hg-Br length of 243 A.The available data suggests that the Hg-C bond is longer in arylthan in alkyl-mercuric derivatives, and thus may be more ionic in theformer. For crystalline aryl derivatives, the dependence of the Hg-C bondlength on the nature and position of substituents in the aromatic ringhas yet to be determined. A common property of most known organomercurials is a low ability to accept further ligands. Thus, in crystals ofmixed mercurials the additional Hg . . . . X contacts are only slightly Jessthan the sum of the van der Waals radii. However, pcrfluoroalkylmercurials1l2 and pcrfluoroarylmercurials'P do give co-ordination derivatives.Thus Hg(CF3). forms the complex anions,1I2 Hg(CF3).X- andHg(CF3).X.'- (X is halogen), while Hg(C.Fs). forms bipyHg(CoF.).ne(g) Mercurous Compouuds.-Derivatives containing mercury .bonded iohalogens, oxygen, and nitrogen arc known. The compounds are of particular interest because of the presence of metal-metal bonding. Althoughmercury has a formal oxidation state of one, the structures arc based on' " Wittig and Bickclhaupt, Chem, Bcr., 1958,91,883.m Grden ic, Chem, Ber., 1959, 92, 231.' " Hilpert and Gruttncr, Ber., 1914,47,186. 00 Grdenic and Gorican, unpublished molecular weight determination.' " Grdcnic, Acta Cryst., 1952,5,367., . . Willig and Lehmann, Chem, BeT., 1951,90,876.,Ot Grdenic and Kitaigorodskii, Zhur. fir. Khim., 1949, 23, 1162.l lD Gordy and Sheridan, J. CMIII. Phys., 1954,22,92.11 l Kitaigorodskii, [nest. Akad. Nauk, S.S.S.R., Otdel, khim. Nauk, 1947,259.m Lagowski, Quart. Rev., 1959, 13,233. and references therein.l IS Chambers, Coates, Livingstone and Musgrave, J., 1962,4367.

,I1IIi

tsc


23/26

._ GRDENI(';:3HE STRUCTURAL CHEMISTRY OF MERCURY - - 325digonally co-ordinated mercury as in mercuric compounds. The presenceof linear XHg'HgX bonds in mercurous halides has been long established!8 but less is known about the structures of other derivatives.The most important interatomic distances in the structures of themercurous halides are given in Table 6. The Hg-Hg bond distance increasesfrom fluoride to iodide, although the opposite is expected because of the

probable higher ionic character of the Hg-F bond. This behaviour can beattributed to the repulsion between the electron clouds of the ligand andthe secondary mercury atoms, i.e. by a trans-influence, which thereforeweakens the Hg-Hg bond more in the iodide than in the flu oride. However,the Hg-Hg bond distance of 290 A found in mercurous diacetylhydrazide,lU Hg.N z(CO'CH3b cannot be explained in this way, as the nitrogenelectron cloud is unlikely to exert such a strong repulsive influence, andthis behaviour is not yet understood.

..

Interatomic distances in crystal structure oj mercurous halidesHc,F, HC3Cl. Hg3B r3 Hg,!,231' 2'52"; 2'41" 2'53"; 2'45" 268"243 ' 2'53; 245" 2'58"; 2500 269"Hg-XHg-Hg

X" 'Xalong the (c-axlsdirection) no contact 3'33"; 3'700 3'40"; 3'55" 3'55"aHavighurst, J. Amer. Chem, Soc., 1926,48,2113. 'Belovand Mokeeva, Trudy lnst,Krist; Akad. Hank S.S,S.R . 1949,5,57.


24/26


the Hg-Hg-to-oxygen relationship are found in the structures ofmercurousoxychlorides. The X-ray diffraction pattern of the mineral eglestonite isbest interpreted by the formula H g 6 C 1 ~ O . 1 1 S The effective co-ordination issimilar to that ofmercurous chloride (Figure 10). Two chlorine atoms arereplaced by one oxygen atom and the Hg-Hg groups are reorientated. Theoxygen is quite loosely accommodated in an octahedral hole at the centreof the cube, the Hg-O distance being 280 A, which is more than the greatest Hg-O distance observed at all (2'66 A in the addition compound ofmercuric chloride and 1,4-dioxan"6). The Hg-Hg distance is 241 A.Oxymercuric mercurous chloride,"? 2HgO,Hg.CI., consists of slightly deformed mercurous chloride molecules and mercuric oxide chains withrather close intermolecular approaches. The dipole moment ofmercuroustrichloroacetate, 265 D, excludes a rigid chelate structure.v"The nature of "alkylrnercurous" compounds (RHg) is not yet fullyunderstood. To explain their high electrical conductivity, it has been suggested that they arc organic metals.P? Recent results exclude their formula

tion as organomercurous compounds, RHg'HgR, or as amalgams betweenmercury and organic free radicals.P"

(h) Amalgams.-In a comprehensive discussion of the structuralchemistry of mercury, the amalgams should not be entirely omitted. Asalloys they haveto be treated from tliestandpoint ofihe theory ofmetals;and their structural inter-relations in terms of the crystal chemistry ofinterrnetallic phases. Most of them have a peculiar structure in which somemercury atoms are grouped more closely than others. Characteristicarrangements arc triangles and pentagons in Mn,Hg,,121 isolated squaresin Na3Hg, ,122 or squares shared in puckered ribbons in NaHg.U2

I,,

i,

,II,I,,. 1 .,,I,

6. General Concepts of the Stereochemistry of MercuryThere arc two main features in the stereochemistry of mercury which

have to be explained: (I) reduction of the characteristic co-ordinationnumber from six or four (as with zinc and cadmium) to two, and (2)additional ligand approaches leading to distorted effective co-ordination,generally distorted octahedral.

An explanation of both phenomena has been given by Orgel.t- In thethird transition series d-s mixing is more energetically favourable for d'O. ' ions than in the first or second series. Hybridisation of the dz' and sorbitals gives two new orbitals, and, depending 'on which of these is filled,

11 $ HedIik, Tschermaks mlneralog: II . petrotog: Mitt., 1949, I, 378.,I t Hassel and Hvoslef, Acta Chern, Scand., 1954, 8, 1953 . .lU SCavniear, Acta Cryst., 1956,9,956. . . . lU Davidson and Sutton, J., 1942,565.lU Coates, Quart. Rev., 1950,4,217. '' " Gowenlock and Trotman, J., 1957, 2114; Gowenlock, Pritchard, Jones, andOvenall, J., 1958, 535.

, 121 de Wet, Acta Cryst., 1961, 14, 733.m Nielsen and Baenziger, Acta Cryst., 1954, 7, 277; Duwel and Baenziger, ActaCryst, 1955,8,705; 1960, 13,476. /


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mercury can either form two short strong bonds in the z direction andfour weak ones in the xy plane, or four strong ones in the xy plane andtwo weak in the z direction. The fonner arrangement is preferred, thoughno explanation is yet available. The tendency for digonal co-ordination canalso be explalned'P by the fact that mercury has a larger s-p separation thancadmium or zinc, which makes sp' or Sp2 hybridisation more difficult formercury, hence favouring sp hybridisation. Nyholm'P has shown that,while the dl o -7 d's separation favours the third series when the charge onthe metal is high, the order is not maintained when the charge is low.However, in the latter case the dlOs -7 d10p separation in the third seriesis high, favouring sp hybridisation.

For distorted octahedralmercury complexes based on digonal characteristic co-ordination, the residual charge on mercury in the linear L 2H gskeleton must be reasonably high or further co-ordination would not occur.Thus it is valid to use ds hybridisation to explain the stereochemistry.However the low acceptor properties of organomercurials show that inthese compounds the charge on mercury is low, and sp hydridisationprobably predominates.

Added ill Proof.c-Sosx structural data have been obtained recentlyconfirming the main structural principles outlined above.Seb). It has been established by X-ray single-crystal and powder methodsthat the alkali metal oxomercuratesftt) are isostructural.r" The HgO t2ion is linear with an Hg-O bond length of 195 A. There arc no othermercury-to-oxygen contacts in the structure and the effective co-ordination

of mercury is octahedral if four equidistant mercury atoms at 342 A areincluded. An interesting oxygen bridge structure has been found for thedimeric 1:1 tri(phenylarsine oxide) adduct with mercuric chloridc.P!The HgCI 2 molecule is as strongly deformed as in the 2:1 ndduct.P' In theI :I addition product of mercuric chloride with cyclohexane-Lc-dionev"the CI-Hg-Cl bond angle is 173 and the Hg-CI and Hg : . . - 0 distancesarc 230 and 279 A respectively. The effective co-ordination of the mercuryatom is a deformed octahedron.

Remarkable refinements of the structure of trischloromercurioxoniumchloride have been made by neutron diffraction12S as well as by threedimensional X-ray diffraction mcthods.P" The oxygen atom is displacedout of the plane of mercury atoms by 043 004 A so that the oxoniumion has thc shape ofa very fiat pyramid with a vertex angie of 175.5.S(c). In anhydrous mercuric sulphate's" the mercury atom is coordinated by four oxygen atoms at distances 212 (two atoms), 2-29, andm Nyholm, Proc. Chem. Soc., t961, 273.'u Branden, Acla Chem. Scand., 1963. 17, 1363.'" Hoppe and Rohrborn, Z. anorg: Chem., 1964,329, 110.m Branden, Arkiv kemi., 1964, 37, 485.U1 Groth and Hassel, Acta Chern, Scand., t964, 18, 1327.


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. ' . , ,328 QUARTERLY REVIEWS

I . .; ,-."

243A in a deformed tetrahedron, the b-ond angle corresponding to the two. shortest Hg-O approaches being 157.4.Sed). Interesting structural differences have been found among themercury mercaptidcs, In t-butylmercury mercaptide!" mercury hastetrahedral co-ordination in a polymeric structure with Hg-S bond lengthsto 259 and 266 A. The ethyl derivatives according to the authors' unpublished results has a simplemolecular structure. The adducts ofmercuricchloride with diethyl sulphidev" and with tetrahydrothiophcri'P arc sulphonium salts, i.e., (Et.SHgCI)+C1- and (C.H.SHgCl)+Cl-. The characteristic mercury co-ordination in both structures is digonal with a departurefrom linearity shown by angles of 158 in the former and 143 in the lattercompound. The effective co-ordinations are octahedral and trigonalbipyrarnidal, respectively. The Hg-S bonds (2,41 and 240 A) are shortenedin comparison with those given in Table 2.

In the crystal structure of the clathrate CoHg2 (SCN) . ,CGH. mercuryagain has tetrahedral co-ordination-s but it is deformed in such a way thata side of the benzene molecule approaches the mercury atom withC ..... Hg distances of 352 and 366 A. Co(NCS).-octahedra are joinedby mercury-sulphur bridges, the benzene molecules being accommodatedin holes. In the 1,6-dithiacyclodeca-cis-3,cis8-diene adduct (I :2) onemolecule .of mercuric chloride is .linear and the. second i s ~ e . r 0 ! I D _ c ~ ~ u s .. .In the bis(thiourea) complex a cation (thiourea),HgCI with mercury inthe planar trigonal characteristic co-ordination has been found, theeffective co-ordination having not been reported.r" The adduct withphenoxathiin'< has the mercury atom in an effective octahedral co-ordinationwith the structural pattern similar to that found in bispyridine adduct." The preliminary structure investigation of two mercury thiourea com-plexes has also been reported.':" "5(f) The crystal of di-p-tolylmercury contains planar centrosymrnetricalmolecules so that the high dipole moment in solution must be due either

to atom polarisation or to solvation.':"The author thanks Professor R. S.Nyholm, F.R.S., for suggesting this reviewand for stimulating discussion.U I Kunchur, Nature, 1964,204,468.'" Branden, Arkiv keml, 1964,22,83.m Branden, Arklv kemi, 1964,22,495 and p. 561.UI Grenbaek and Dunitz, Helv: Chim, Acta, 1964, 47, 1889. " ..m Cheung, McEwen, and Sim, Nature, 1965,205,383.m Korczynski, Roczniki Chem., 1963,37, 1645; 1963,37, 1647.' " Kunchur and Mathew, Proc. Chem. Soc., 1964,414.

Quaterly Reviews & the Chemical Socity. Q.rev.Chem.soc_1965!19!303

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