Lone pair orbitals germanium compounds

Two factors combine to lend a greater diversity in the stereochemistries exhibited by bivalent germanium, tin and lead compounds, the increased radius of Mn compared with that of Mw and the presence of a non-bonding pair of electrons. When the non-bonding pair of electrons occupies the isotropic valence level s orbital, as in, for example, the complex cations Pb[SC(NH2)2]6+ and Pb[antipyrine]6+, or when they are donated to conductance band levels, as in the binary tin and lead selenides or tellurides or the perovskite ternary phases CsMX3 (M = Sn, Pb X = Cl, Br, I), then the metal coordination is regular. However, in the majority of compounds an apparent vacancy in the coordination sphere of the metal is observed, which is usually ascribed to the presence of the non-bonding pair of electrons in a hybrid orbital and cited as evidence for a stereochemically active lone pair . 

The yellow germanium (m.p. 18() C) and brick-red tin complexes (m.p. 1 36 C) are thermally stable but air-sensitive solids which can be sublimed in vacuo, but the purple lead analogue is rather heat and photosensitive. They are monomeric in the vapour and in their solutions in benzene. X-ray diffraction of the tin compound shows that it dimerizes in the solid state. The monomer has both a lone pair and vacant p orbital. It acts as a Lewis base towards transition metal complexes and as a Lewis acid towards pyridine. Dimerization may therefore occur through mutual acid-base interaction to form a bent tin—tin double bond. 

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