STRUCTURE AND BONDING OF HETEROBIMETALLIC FISCHER CARBENE COMPLEXES

Heterobimetallic Fischer carbene complexes (CO)5CrC(OM')(R) [M' = Ti(O-iPr)3, R = n-Bu (3); R = 2-furyl (4); R = C(Me)CH2 (5); M' = TiCp2Cl, R = C(Me)CH2 (6)] are readily synthesized by metalation of pentacarbonylchromium acylate intermediates. X-ray crystallographic determinations have been performed at -120-degrees-C for these compounds, as well as for the acylate precursor [NMe4][(CO)5CrC(O)-(CMeCH2)] (7). Structures of 3-5 are the first to be reported of bimetallic carbene systems in which the Lewis acidic metal contains only oxygen ligands. The heterobimetallic complexes display unusually short Cr-C(carbene) and O-C(carbene) bond distances, with respect to standard alkoxycarbene analogues. 3 is monoclinic, P2(1)/n, with a = 12.694 (2) angstrom, b = 14.409 (3) angstrom, c = 13.552(3) angstrom, beta = 96.12 (2)-degrees, and R = 0.044. 4 is monoclinic, P2(1)/n, with a = 12.431 (4) angstrom, b = 10.775 (2) angstrom, c = 17.476 (6) angstrom, beta = 95.17 (3)-degrees, and R = 0.046. 5 is triclinic, P1BAR, with a = 9.555 (2) angstrom, b = 11.135 (3) angstrom, c = 12.613 (4) angstrom, alpha = 108.33 (2)-degrees, beta = 110.73 (2)-degrees, gamma = 97.51 (2)-degrees, and R = 0.038. 6 is monoclinic, P2(1)/n, with a = 12.369 (3) angstrom, b = 12.964 (3) angstrom, c = 12.993 (4) angstrom, beta = 107.47 (2)-degrees, and R = 0.031. 7 is orthorhombic, Pnma, with a = 11.603 (3) angstrom, b = 9.327 (3) angstrom, c = 14.993 (7) angstrom, and R = 0.039. The relevance of structural and spectroscopic data to bonding in these systems is discussed: the substitution of an O-bound electron-deficient metal for an O-alkyl group of a Fischer alkoxycarbene complex appears to increase pi-electron delocalization across the carbenoxy unit in such a way as to strengthen both M-C(carbene) and O-C(carbene) interactions.