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

Group theory forbids either Oh octahedral or D3h trigonal prismatic geometry for a six-coordinate early transition metal complex using a six-orbital sd5 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 C3 geometry intermediate between octahedral and trigonal prismatic geometry is allowed by group theory for such an sd5 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 t1u HOMO and the t2g 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 MS2C2 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.