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 O-h octahedral or D-3h trigonal prismatic geome try for a six-coordinate early transition metal complex using a six-orbital sd(5) manifold thereby indicating that the observation of trigonal prismat ic rather than octahedral metal coordination geometry is not a simple indic ation of the lack of p orbital participation in the chemical bonding. Howev er, an experimentally observed C-3 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 s aturated or benzenoid bridges between the donor sulfur atoms. The distortio n 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. Thi s effect is favored when the ligands are strong <sigma>-donors but weak pi -donors and the metal is not too electropositive such as is the case with m any metal tris(dithiolenes). The MS2C2 chelate rings in metal tris(dithiole ne) complexes may be regarded as resonance hybrids of ethylenedithiolate an d dithiodiketone canonical forms having different metal oxidation states. T he 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 obs ervation. A 'rotary electric switch' mechanism has been proposed for this p rocess but rearrangement mechanisms through bicapped tetrahedral intermedia tes also appear reasonable. (C) 2001 Elsevier Science B.V. All rights reser ved.