Chemical applications of topology and group theory: Part 35. Non-octahedral six-coordinate tris(dithiolene) and related complexes of the early transition metals

Abstract

Group theory forbids either O h octahedral or D 3h trigonal prismatic geometry for a six-coordinate early transition metal complex using a six-orbital sd 5 manifold thereby indicating that the observation of trigonal prismatic rather than octahedral metal coordination geometry is not a simple indication of the lack of p orbital participation in the chemical bonding. However, an experimentally observed C 3 geometry intermediate between octahedral and trigonal prismatic geometry is allowed by group theory for such an sd 5 manifold. Bicapped tetrahedral geometry, which is related to octahedral or trigonal prismatic geometry through combinations of various diamond-square-diamond processes, is also found in a few metal tris(dithiolenes) having saturated or benzenoid bridges between the donor sulfur atoms. The distortion of an octahedron to a trigonal prism in six-coordinate complexes of d <4 early transition metals can result from a second-order Jahn–Teller effect involving splitting of the t 1u HOMO and the t 2g LUMO in order to allow mixing of the resulting e′ orbitals in the trigonal prismatic geometry. This effect is favored when the ligands are strong σ-donors but weak π-donors and the metal is not too electropositive such as is the case with many metal tris(dithiolenes). The MS 2C 2 chelate rings in metal tris(dithiolene) complexes may be regarded as resonance hybrids of ethylenedithiolate and dithiodiketone canonical forms having different metal oxidation states. The stereochemical non-rigidity of trigonal prismatic metal tris(dithiolenes) observed experimentally by NMR requires interchange of the ligands on the top and bottom rings of the trigonal prism so that a simple trigonal twist through an octahedral intermediate is not adequate to account for this observation. A ‘rotary electric switch’ mechanism has been proposed for this process but rearrangement mechanisms through bicapped tetrahedral intermediates also appear reasonable.