42

Zintl Phases

Zintl Phases Electropositive cationic compound (alkali metal, alkaline earth metal,

lanthanoid) and an anionic compound of moderate electronegativity

Zintl Phases Resembles ionic solids in composition but are quite different

1. Metallic property especially metallic luster

2. Not full-value metals, not malleable and ductile but brittle

3. Electrical properties: semiconductivity or moderate metallic conductors

4. Obeys the 8-N rule

“Due to the ionic nature the structures of these compounds are driven by cationic or anionic

nature of the main group elements in the zintl phase”

To find out the nature of the main group elements in the Zintl phase the understanding of the

“Generalized 8-N rule” is important

Generalized 8-N rule: Expression of an octet principle

For binary compounds: MmXx

X – Main group element (Group 4 to 7): e(X)

M – Electropositive metal: e(M)

m. e(M) + x. E(X) = 8x

43

If covalent bond exists between M atoms [b(MM)] and M retains the non-bonded atoms E

Similarly if the X atoms require lesser electrons if it involves covalent bonds with each other

M [e(M) – b(MM) – E] + x [e(X) + b(XX)] = 8x

Rearranging the equation

m. e(M) + x. e(X)/x = 8 + {m [b(MM) + E] – x. B(XX)}/x

VEC (X) = m. e(M) + x. e(X)/x

B(XX) = 8 – VEC (X) + m/x {b(MM) + E} Generalized 8 – N rule

Special cases:

1. Elements

m = 0, VEC (X) = e(X) = N; b(XX) = 8 – VEC(X)

ex: S atom b(SS) = 8-6 = 2

2. Polyanionic compounds

b(MM) = 0 and E = 0; b(XX) = 8 – VEC(X)

Ex: Na2O2: VEC(O) = 7; b(OO) = 8 -7 = 1

Comparing the above two equations

“The geometric arrangement of the atoms in a polycationic compound corresponds to the

arrangement in the structures of the elements of the fourth to seventh main groups when the

number of covalent bonds per atom b(XX) is equal” – E. Zintl

44

3. Polycationic Compounds

b(XX) = 0

b(MM) + E = x/m {VEC (X) – 8} all valance electrons to be considered while applying

this equation including that for forming M-M bonds

Example: Hg2Cl2; e(Hg) = 2 ; VEC(Cl) = 9; b(HgHg) = 1

If 10d electrons of the Hg are considered then VEC(Cl) = 19; E = 10; b(HgHg) = 1

4. Simple ionic compounds

b(MM) = b(XX) = 0

VEC(X) = 8 (Octet rule)

The b(MM), b(XX) and E values can not adopt negative values

VEC(X) < 8: Polyanionic

VEC(X) = 8: Simple ionic

VEC(X) > 8: Poly cationic

InBi = Bi – Bi contacts and have metallic properties

Similarly K8Ge46 = Meshed framework of Ge atoms enclosing K+ ion

The electrons donated by K+ ion are not taken over by Ge; they form a band

Solid solution Ge as solvent for K+ ion and solvated electrons

K8Ge46 metallic properties

K8Ga8Ge46 Same structure, all K-electrons are required for framework, semiconductor

45

POLYANIONIC COMPOUNDS

Homopolyatomic anions: Zintl ions

According to octet rule monoatomic anions with charges (such as As3-) equal to 8-N is possible.

Elements of Groups 4 to 6 often fail: because of the great negative charge densities on none-too-

electronegative atoms; needs loss of substantial atomization energies on going from element to

monoatomic anions.

But can form homopolyatomic anions (such as As73-): lower charge densities per atom, not all

element-element covalent bond energies need to be lost.

Homopolyatomic anions both ions and oligomers or polymers

Degree of polymerization of homopolyatomic anions are often less than parent elements

If the parent element is a cluster then the homopolyatomic anions form more open cluster with

lesser element-element links

Reason the LUMOs of the parent elements are generally anti-bonding and hence addition

of extra electrons reduces the bond order – rearrangement of geometry from parent element

Examples of homopolyatomic anions: (I – I – I) – ; Pb52– ; Sn9

4– ; As73–

Homopolyatomic anions of the main group element is

not isolated in pure form but are isolated as metal

complexes

46

Zintl anions and Zintl phases

Examples of polyanionic

compounds which have integral

VECs per anion atom.

In agreement with the 8 – N rule,

structures like those of pure

elements with the corresponding

numbers of valence electrons occur

for the anionic components.

For a particular anion having a specific number of connectivity can exhibit different connection geometry.

For example, CaSi2 has layers of (Si¯)∞ with the three coordinate silicon atoms in the pyramidal geometry

as in As. Under pressure CaSi2 is transformed to the α - ThSi2 type. Contrary to the expectations based

on 8 – N rule the Si atoms in α - ThSi2 do not have pyramidal coordination, but planar coordination. The Si

atoms in α - ThSi2 are located in the centre of the trigonal prisms formed by the cations.

47

Compounds with non-integral VEC(X) values

Fractional value of VEC(X) occurs for fractional number of covalent b(XX) bonds…..

Anions having structurally different atoms results in fractional number of covalent bonds

S32– chain structure

Terminal S atoms b(XX) = 1; Central S atom b(XX) = 2

“An atom of the N-th main group that participates in 8 – N covalent bonds obtains a formal

charge of Zero. The sum of all formal charges is equal to the ionic charge”

Diversified and complicated structures are known for the polyanions

For alkali and alkaline earth metals 50 different binary polyphosphide structures are known

For other metals more than 120 binary polyphosphide structures

Structures like chains, rings and cages etc. can be obtained for the anionic compounds…

Cages example; As4S4 and P4S3; Every P atom that replaces S atom as P–

Atoms assigned formal negative charges can actually have negative charges and can easily be

recognized from the structure. Ex. NaP5: 4 neutral P atoms and one P–. Neutral P atoms form

ribbons of connected rings in chair conformation. P– atoms are in close contact with Na+ ions.

48

Examples of the anionic structures in polyphosphides, polyarsenides and polyantimonides

Layer structures can be regarded as sections of the structures of black phosphorus or arsenic. Other

structures correspond to fragments of the structure of fibrous red phosphorus

Binary Polyatomic compounds Direct synthesis from elements

The first evidence for Zintl ions was reported by Joannis in 1891 from studies of the reaction of

sodium in liquid ammonia with a variety of metals.

He observed a blue solution of sodium which colored the ammonia solution to green (presence of

dissolved electrons in NH3) with excess lead giving new solids.

49

More attention was given to the solid precipitates which were found to have the compositions

NaPb4 or NaPb2 depending on conditions.

The isolation of any solid derivative of the Zintl anions has only been accomplished in the last 40 years. Intact cage like anions can be extracted from the solids by

offering a complexing ligand to the cations. The Na+ ions in

Na2Sn5 can be captured by cryptand molecules

[N(C2H4OC2H4OC2H4)N] as [NaCrypt]2Sn5

NaTl Classic example of a zintl phase Na+ Tl – : The term zintl phase more often than zintl

ions to represent these compounds because of its polar salt like phase.

Tl

TlTl

Tl

Tl

Tl TlTl

Tl

Thallium partial structure has the

diamond (diamondoid) like structure

Ti – Tl bond distances 324pm in the

diamondoid unit is shorter than the

contact distance 343pm in metallic Tl

SrGa2 has similar VEC(X) values like NaTl, but do

not form a diamond like structure for Ga rather

forms a layer as in Graphite (similar to MgB2).

The numbers of such known zintl phases are very

large….

50

Ternary Zintl Phases:

Similar structure principles for the elements as in binary zintl phases: (KSnSb-)∞ is similar to As

Additionally the anionic part these ternary zintl phases resembles halo or oxo anions or

molecular halides.

Examples of the anion partial structures in ternary ZINTL phases

Ba4SiAs4 SiAs48– tetrahedra isostructural to SiBr4

Ba3AlSb3 Al2Sb612– isostructural to Al2Cl6

Ca3AlAs3 polymeric chains linked tetrahedra of (AlAs36–)∞ similar to chain silicates

(SiO32–)∞

The compound Ca14AlSb11 = [Ca2+]14[Sb3-]4[Sb37-][AlSb4

9-] contains three kinds of anions,

namely single ions Sb3-, ions Sb37- that are isostructural with I3

- and tetrahedral AlSb49- ions.

Although a lot of structures of zintl phases resembling silicate tetrhedra, the possible varieties are

far greater than the simple linking found in silicate tetrahedral geometry.

51

Polycationic compounds

The number of known polycationic compounds is very less than that of polyanionic compounds.

Many of them contain S, Se, and Te. Since these elements are oxidizing agents and Lewis acids,

their preparations involve different methods from that used to obtain polyanionic compounds.

For example S82+ is oxidized by AsF5 in liquid SO2 to yield S8

2+

S8 + 3AsF5 (SO2) S8 [AsF6]2 + AsF3

A strongly oxidizing peroxide compound FO2SOOSO2F in a strong acid solvent like

fluorosulfuric acid oxidizes Se to Se42+.

4Se + FO2SOOSO2F (HSO3F) [Se4][SO3F]4

S42+ ; Se4

2+ ; Te42+ all have a square structure having a 6π electron

system

The structures of S82+ and Se8

2+ can be interpreted with the 8 – N

rule: a bond is generated across an S8 ring, resulting in two atoms

having three bonds and one positive formal charge each. The new

bond is remarkably long (289 pm as compared to 203 pm for the

other bonds), but the occurrence of abnormally long S–S bonds is

also known for some other sulfur compounds.

Several varieties are known of Te4

2+ ions, in which triply bonded Te+ and uncharged Te atoms

are quite visible.

52

The structure of the Te3S32+ and Te6

4+ ions can also be

understood in terms of the 8 – N rule. Te64+ can be

described as a trigonal-pyramidal structure in which one

prism edge has been strongly elongated; according to the 8

–N rule, this edge would not be considered to be a bond.

However, some weak bonding interaction must still be present, otherwise the structure would not

be as it is.

Polyatomic cations of the other group elements: Bi forms interesting polyhedral cations such as

Bi53+ and Bi9

5+ which are isoelectronic with Sn52- and Sn9

4-.

The most familiar homopolyatomic metal cations are the Hg(I) cation Hg22+ which occurs in

aqueous solutions.

Hg32+ , Hg4

2+ and an infinite [Hg2.86+]∞ are also known.

Boning in these Hg cations involves Hg – 6s orbitals in contrast to Hg – 6p based orbitals in the

mercuride anions.

Apart from that several late transition metal ions have the tendency to form interesting

polyatomic cations.

Example: Au42+, Ag6

4+, Rb75+ etc.

Note on Homopolyatomic Zintl phases (anionic):

The tendency of the group 4 elements to form the nine-atom clusters of the type M4E9 leads to a

new zintl phase for the homopolyatomic compounds. Ex: M = Na, K, Rb, Cs and E = Si, Ge, Sn,

Pb

Similarly there are a large number of other types of zintl phases such as M12E17 known for group

4 elements. Ex: Rb12Si17, Rb12Sn17 and Na12Ge17

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Some homopolyatomic Zintl anions: (a) Sn94-, (b) Sn9

3-, (c) Ge92-, (d) Pb5

2-, (e) Bi42-, (f) Sb7

3-

structures obtained from X-Ray crystallography

Clusters and Wade rules:

This rule was initially framed for explaining bonding in borane clusters

In main group elements the bonding electrons are located at the s and p orbitals. The s orbital due

to its spherical geometry can overlap with pz orbital to form two sp hybrid orbitals.

54

Combinations of atomic orbitals leading to molecular orbitals in an octahedral cluster such as B6H62-

One of the two sp orbitals points radially to the centre of the cluster where as the other orbital

point radially outwards. The orbital projecting outwards can be used for bonding with other

external atoms.

The sp orbital pointing towards the cluster gives one bonding orbital and n-1 non-bonding or

anti-bonding orbitals.

The px and py orbitals are oriented tangentially to the cluster to give n bonding and n anti-bonding

orbitals for the cluster skeleton.

From this follows the WADE rule: a stable closo cluster requires 2n+2 skeleton electrons or

n+1 electron pairs.

WADE stated some further rules for open clusters that are interpreted as deltahedra with missing

vertices. They are of special importance for boranes and other main group naked clusters

nido cluster: one missing polyhedron vertex, n+2 bonding skeleton orbitals

arachno cluster: two missing vertices, n+3 bonding skeleton orbitals

hypho cluster: three missing vertices, n+4 bonding skeleton orbitals

The WADE rules can be applied to ligand-free cluster compounds of main-group elements. If we

postulate one lone electron pair pointing outwards on each of the n atoms, then g – 2n electrons

remain for the polyhedron skeleton (g = total number of valence electrons)

55

The calculation also works if some of the atoms bear ligands (instead of lone pairs) and others

have no ligands but lone electron pairs.

The examples should not give the misleading impression that bonding in clusters is a clear and

simple matter. Next to many examples for which the WADE rules work well, they do not do so

in many other cases, or additional assumptions have to be made.

56

Electron Counts for closed deltahedral clusters n Polyhedron p-block VEC d-block VEC NFE

5 Tigonal bipyramid 22 72 12

6 Octahedron 26 86 14

7 Pentagonal bipyramid 30 100 16

8 Dodecahedron (cuboctahedron) 34 114 18

9 Tiicapped trigonal prism 38 128 20

10 Bicapped square antiprism 42 142 22

11 Octadecahedron 46 156 24

12 Icosahedron 50 170 26

n 4

(a) (b) (c)

Deltahedra and deltahedral fragments for

boranes and borane anions.

(a) The Closo boranes containing n boron

atoms (b) Corresponding nido boranes (c)

Corresponding arachno boranes

Diagonal lines show the fragmentation

patterns leading to the structures of the

nido and arachno boranes

5

6

7

8

9

10

11

12

57

Cluster Compounds and Bonding Types:

Links between atoms Compensates lack of electrons for attaining next noble gas

configuration

Common pair between two atoms to gain one electron in the valance shells of each atom

Two centre two electron Bond or 2c2e bond

Most polyanionic compounds share electrons between each other

Polyanionic compounds sufficient number of electrons per atoms to form 2c2e bonds and

hence generalized 8 – N rule is fulfilled for polyanionic compounds

Additionally the excess of electrons due to anionic charges is explained by Wade’s rule of n+1

electron pairs for bonding

Electropositive Elements Inferior number of valance electrons and have to supply

electrons to the more electronegative partner.

They can gain electrons in two ways

1. By complexation by ligand acquisition

2. By combining their own atoms with each other. This results in the formation of the cluster.

“A cluster is an accumulation of three or more atoms of the same element or similar elements

that are directly linked with each other”

If the cluster formation yields a sufficient number of electrons to allow one electron pair for a

connecting line between two adjacent atoms, then each line (bond) can be taken to be a 2c2e

bond

electron precise clusters

58

For low values of valance electrons (< 3 for main group elements) covalent 2c2e bonds are not

sufficient to overcome the electron deficiency

Electron deficient compounds multi-center bonds

Simple ones are the 3c2e bonds where 3 atoms share an electron pair

Other multi-center electron pairs are also possible but the degree of tight

bonding proportionally decreases

The electron pair in a 3c2e bond essentially is located in the center of the triangle defined by the three atoms

The location of electrons linking more than three atoms cannot be illustrated as easily. The

simple, descriptive models must give way to the theoretical treatment by molecular orbital

theory.

Completely closed, convex, single-shell clusters are called closo clusters; their atoms form a

polyhedron.

If the polyhedron has only triangular faces, it is also called a deltahedron.

Depending on the number of available electrons, we can distinguish four general bonding types

for closo clusters:

1. Electron precise clusters with exactly one electron pair per polyhedron edge

2. Clusters with one 3c2e bond for every triangular face

3. Clusters that satisfy the WADE rules (discussed on pages 54 and 55)

4. Clusters not matching any of these patterns

59

Electron Precise Clusters

Molecules such as P4 and the polyanionic clusters such as Si44- or As7

3- are representatives of

electron precise closo clusters.

Numerous clusters with electron numbers that account for exactly one electron pair per

polyhedron edge are also known for the more electron-rich transition group elements (beginning

with group six). In addition, every cluster atom obtains electrons from coordinated ligands, with

a tendency to attain a total of 18 valence electrons per atom. The easiest way to count the number

of electrons is to start from uncharged metal atoms and uncharged ligands.

The electrons supplied by the ligands and the valence electrons of the n metal atoms of an Mn

cluster are added to a total electron number g. The number of M–M bonds (polyhedron edges)

then is:

main group element clusters: b = ½ (8n – g); transition element clusters: b = ½ (18n – g)

n1, n2, n3, and n4 are the numbers of polyhedron vertices at which 1, 2, 3 or 4 polyhedron edges (M–M bonds) meet, respectively

Number of electrons supplied by ligands to metal atoms in complexes when the metal atoms are

considered to be uncharged. µ1 = terminal ligand, µ2 = ligand bridging two atoms, µ3 = ligand bridging

three atoms; int = interstitial atom inside a cluster. This mode of calculation is called the EAN rule

60

The cluster mentioned last, [Mo6Cl14]2- consists of an Mo6 octahedron inscribed in a Cl8 cube; each of the eight Cl atoms of the cube is situated on top of one of the octahedron faces and is coordinated to three molybdenum atoms. The formula [Mo6Cl8]4+ applies to this unit; in it, every Mo atom is still short of two electrons it needs to attain 18 valence electrons. They are supplied by the six Cl- ions bonded at each octahedron vertex. Picture on the right side is an array of Mo6Cl8 clusters linked via chlorine atoms to a layer in

Mo6Cl12

61

If electrons are added to an electron precise cluster, cleavage of bonds is to be expected for every

additional electron pair g increases by 2 and b decreases by 1. An example is Os3(CO)12(SiCl3)2

with a linear Os–Os–Os group; by attaching two SiCl3 groups to triangular Os3(CO)12, two more

electrons are supplied, and one Os–Os bond has to be cleaved.

Clusters with 3c2e bond:

If there are not enough electrons for all of the polyhedron edges, 3c2e bonds on the triangular

polyhedron faces can be the next best solution to compensate for the lack of electrons. This

solution is only possible for deltahedra that have no more than four edges (and faces) meeting at

any vertex. These include especially the tetrahedron, trigonal bipyramid and octahedron.

B4Cl4: every boron atom takes part in four bonds, one 2c2e B–Cl

bond and three 3c2e bonds on the faces of the B4 tetrahedron. In

this way every boron atom attains an electron octet. Eight of the

valence electrons take part in the multicenter bonds; the other

eight are needed for the B–Cl bonds.

In the Nb6Cl184- ion the octahedral Nb6 cluster can be assumed to have eight 3c2e bonds on its

eight octahedron faces. A chlorine atom bonded with two Nb atoms is situated next to each octahedron edge. This makes twelve Cl atoms in an Nb6Cl12

2+ unit. The remaining six Cl- ions are terminally bonded to the octahedron vertices.

62

Clusters with interstitial atoms

Clusters derived from metals which have only a few valence electrons can relieve their electron

deficit by incorporating atoms inside. Common octahedral clusters

The interstitial atom usually contributes all of its valence electrons to the electron balance.

Nonmetal atoms such as H, B, C, N, and Si as well as metal atoms such as Be, Al, Mn, Fe, Co,

and Ir have been found as interstitial atoms.

Transition metals of groups 3 and 4 form many octahedral clusters that are isostructural with

those of the less electron-deficient elements of the following groups, but they contain additional

atoms in their centers

Ex: Nb6Cl14, we can substitute the niobium atoms by

zirconium atoms; Nb6 Zr6 Loss of six electrons;

partial compensation by introducing a carbon atom in

the Zr6 as Zr6CCl14 octahedron.

Slightly inferior number of electrons: the cluster in

Zr6CCl14 is stable due to some changes in the bonding.

The more electronegative carbon at the center electron

density inwards: weakening the Zr–Zr bonds, but

stronger bonding interactions of Zr with the C atom.

On the other hand, the metal–metal bonds are strengthened when the interstitial atom is a metal

atom.

Ex: Nb6F15 to Th6FeF15: Same structure but the later has an interstitial Fe atom at Oh centre.

Nb6F15 has one electron less than required for the eight 3c2e bonds; in Th6FeF15 a further six

electrons are missing. The intercalated Fe atom (d8) supplies these seven electrons; the eighth

electron remains with the Fe atom.

63

Endohedral Clusters:

Clusters with similar sizes consisting of metal atoms are not stable if they are hollow; the bonds

at their surfaces are too weak. However, they can be stabilized by interstitial atoms, even if the

interstitial atoms do not contribute with their electrons endohedral clusters

Examples: Ih clusters [Pt@Pb12]2¯and [Cd@Tl12]12¯ with a Tl1412¯ cage: These clusters obey

Wades rule for Neutral Pt and cationic Cd2+

Condensed Clusters: Another possibility for relieving the electron deficiency consists of joining clusters to

form larger building blocks. Among the known condensed clusters the majority

consist of M6 octahedra linked with each other. Figure shows a possibility for the

condensation of M6X8 clusters. Merging trans vertices of octahedra to a linear chain

requires that opposite faces of the X8 cubes also merge; every X atom is thus shared

by two cubes. The resulting composition is M5X4. Compounds with this structure are

known with M = Ti, V, Nb, Ta, Mo and X = S, Se, Te, As, Sb, e.g. Ti5Te4. They have

12 (Ti5Te4) to 18 (Mo5As4) skeleton electrons per octahedron. Eight of the electrons

form four 2c2e bonds at the four equatorial edges of the octahedron; the remaining

electrons are oriented along the other octahedron edges, and their interaction in the

chain direction results in metallic energy bands.

Condensed M6X8 in

Ti5Te4