The Structures of s² Metal Complexes in the Ground and sp Excited States

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.

Key Words:

• s² metal complexes
• structures
• distortions
• absorption
• luminescence
• excited states