MOLECULAR-STRUCTURES OF THE ETA-4-TETRAPHENYLCYCLOBUTADIENE COMPLEXES MCP(ETA-4-C4PH4)CL2 (M=NB,MO) AND THEIR RELATIONSHIP TO BENT-SANDWICH MCP2CL2 COMPLEXES

The structures of the eta4-tetraphenylcyclobutadiene complexes MCp(eta4-C4Ph4)Cl2 (M = Nb (1), Mo (2)), when compared to the bent-sandwich complexes MCP2Cl2, are consistent with the LUMO of d0 complexes and the HOMO of d1 and d2 complexes of both types being in the MCl2 plane and parallel to the Cl-Cl vector. Extended Huckel calculations are consistent with this interpretation and also show that the eta4-cyclobutadiene ligand is best represented as a dinegative ligand when formal oxidation states are assigned. The EHMO calculations were also used to account for the various factors that influence the Cl-M-Cl bond angle. In particular, the Cl-M-Cl angles are found to decrease as the d electron count increases (97.3(l)-degrees for 1 (d0) and 90.0(1)-degrees for 2 (d1)). Crystal data for 1: monoclinic, space group P2)(1)/n; Z = 4; a = 11.138(3) angstrom, b = 14.584(5) angstrom, c = 16.305(e) angstrom, beta = 94.36(3)-degrees; V = 2641(1) angstrom3; T = 295 K; R = 0.0295; R(w) = 0.0292 based on 2763 reflections for F(o) greater-than-equal-to nsigma(F(o)) (n = 3). Crystal data for 2: orthorhombic, space group Pbca; Z = 8; a = 27.118(39) angstrom, b = 11.420(12) angstrom, c = 19.322(15) angstrom; V = 5984(11) angstrom3; T = 295 K; R = 0.057; R(w) = 0.057 based on 1824 reflections for F(o) greater-than-or-equal-to nsigma(F(o)) (n = 3).