Bicapped tetrahedral, trigonal prismatic, and octahedral alternatives in main and transition group six-coordination

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We examine theoretically the bicapped tetrahedron and trigonal prism as alternatives to the common octahedron for six-coordinate main and transition group compounds. The pronounced preference of six-coordinate sulfur for an octahe- dral geometry is traced by a molecular orbital analysis to a pair of nonbonding levels, whose higher energy in the nonoctahe- dral geometries is due to the molecular orbital equivalent of a ligand-ligand repulsion. For transition element complexes we draw a correlation diagram for the trigonal twist interrelating octahedral and trigonal prismatic extremes. A possible prefer- ence for the trigonal prism in systems with few d electrons is discussed, as well as a strategy for lowering the energy of the trigonal prism by symmetry-conditioned 7r bonding. The bicapped tetrahedron geometry for transition metal complexes can be stabilized by a substitution pattern ML4D2 where D is a good u donor and M is a d6 metal center.