Solid-State Hypervalent Compounds

We have seen that hypervalent compounds frequently have unusual geometries and this is also true in the solid-state area. The usual Zintl-Klemm counting rules that were presented in Section 13.5 have been extended to electron-rich phases [23]. The basic premise is that these compounds have an occupied valence s AO, which lies at a low energy with respect to the p AOs. Consequently, the 

Building up the molecular orbitals of a chain consisting of p AOs interacting in a a fashion for three (a), five (b), and infinite (c) chains. S and A refer to the MOs being symmetric or antisymmetric, respectively, to the central mirror plane of the molecule. 

The optimal electron count for a square net can be determined in a straight forward manner [23], as follows. We have shown that the hypervalent linear chain is one which has seven electrons per A atom. Bringing an infinite number of parallel chains together generates 14.27. Recall that there is one electron in a p AO that runs along each chain. Therefore, removing one electron from the in-plane lone pair on 

With six electrons in a square net there also exist several ways to generate Peierls distortions [23]. Several were diagramed in 13.67-13.69. Each case generates classical structures where all of the atoms are two-coordinate and, therefore, follow the Zintl-Klemm formalism. Ladder structures, 14.3 I, can also form where the A atoms still are at a six electron count [23]. A diamond chain of vertex sharing 

2Ba + gives Bi3 = 19 electrons There is one electron per unit cell too many. Electronic structure calculations show [31] that the extra electron partially stays in Ba valence orbitals and Bi—Bi antibonding states. Another structure, which contains hypervalent Sn, is given by the simple binary compound, LiSn, 14.37 [33]. Recall that KSn exists as electron precise tetrahedra with Sn—Sn distances of 2.98 and 2.96 A. 

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Neutral compounds of heavy group 14 elements having donor atoms in monodentate ligands can be tetrahedral in the solution. However, in this case the environment for the element in the solid state can be five-coordinate due to hypervalent bridges. Selfassociation can be also observed for pentacoordinate species when the donor atom forms a unidentate bridge across the six-coordinate centers. The self-association is also characteristic of anionic species (Sections IV.A and IX.A). For a tin atom the intermolecular coordination is observed very frequently . As a rule it results in a polymeric structure mainly via the oxygen atom . However, dimers and other simple cyclic hypervalent structures are formed only seldom. 

In the condensed phase Me3SiF molecules show no intermolecular interactions, while the same germanium derivatives are associated as a dimer due to intermolecular F Ge coordination. According to the tendency for tetrahedral main-group 14 elements to expand the coordination sphere, organotin fluorides show a strong tendency to associate in the solid state and even in triorganotin fluorides the tin atom is five-coordinate A common feature of this class of compounds in the solid state is coordination expansion of the tin atom due to hypervalent interaction, which in turn often results in formation of polymeric materials. 

Alloys Chalcogenides Solid-state Chemistry Hypervalent Compounds Magnetism of Extended Arrays in Inorganic Solids Magnetism of Transition Metal Ions Semiconductors. 

The tendency to form covalent rather than ionic bonds is crucial in the formation of hypervalent compounds. Incorporation of fluoride as a ligand usually stabilizes covalent species. For example, PF5 is covalent. However, PCI5 is characterized by the equilibria shown in equations (18) and (19). In the gas and liquid phases, the molecular form exists, while in the solid state it is ionic, consisting of PCI4+ and PClg" ions. In solution in nonpolar solvents, the molecular form is favored, while the ionic form described in equation (18) predominates in polar solvents. At low concentrations, some PCI4+ and Cl may be present owing to the equilibrium in equation (19). For equation (20) (R = alkyl and aryl), if L is a reasonably basic anion such as Cl, Br, I, or OPh, the ionic phosphonium salt (20) is favored. With L = F or C>-alkyl, the... 

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