The Structures of s2Metal Complexes in the Ground and sp Excited States

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Abstract
Generally, the ground state structures of simple molecular complexes with s2 metal ions such as Ge2+, Sn2+, Pb2+, Sb3+, and Bi3+ are in agreement with the VSEPR model. The geometries are determined by a stereochemically active lone pair although there are exceptions to this rule. In terms of qualitative MO considerations these VSEPR structures are the result of a second order Jahn-Teller effect. Highly symmetrical geometries are less stable since they are characterised by a strongly antibonding s2 electron pair with a spherical distribution (inert pair). A stabilisation is achieved by a distortion which lowers the energy of this electron pair by sp orbital mixing. Simultaneously, this pair becomes stereochemically active (lone pair). Generally, the lowest electronically excited states of s2 complexes are of the metal centered sp type. In the sp state the distortion is no longer favored. Accordingly, this distortion is eliminated. As a result the emission from sp states is associated with very large Stokes shifts.

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