EARLY-TRANSITION-METAL KETENE COMPLEXES - SYNTHESIS, REACTIVITY AND STRUCTURE OF KETENE COMPLEXES OF BIS(TRIMETHYLSILYL)NIOBOCENE, X-RAY STRUCTURE OF [NB(ETA(5)-C5H4SIME3)2BR(PH2C=C=O-C,O)

The "carbenoid-like" complex [Nb(eta-5-C5H4SiMe3)2Br] 1a reacts with 1 equivalent of several ketenes, R1R2C=C=O, to give the niobium(V) complexes [Nb(eta-5-C5H4SiMe3)2Br(R1R2C=C=O-C,O)] (2a, R1 = R2 = Ph; 3a, R1 = R2 = Me; 4a, R1 = Ph, R2 = Me; 5a, R1 = Ph, R2 = Et) with the expected C=O bonding mode found in several early-transition-metal moieties. The protonation of these complexes with 1 equivalent of H+ (an ethereal solution of HBF4) affords the acyl cationic niobocene [Nb(eta-5-C5H4SiMe3)2Br(R1R2HCC=O)]+ (6a, R1 = R2 = Ph; 7a, R1 = R2 = Me; 8a, R1 = Ph, R2 = Me; 9a, R1 = Ph, R2 = Et). The ketene complexes 2a and [Nb(eta-5-C5H4SiMe3)2Cl(R1R2C=C=O-C,O)] 2b (hereafter b refers to the chloro-complexes) undergo a two-electron reduction without transformation of the ketene moiety to give the same anionic niobium(III) species [Nb(eta-5-C5H4SiMe3)2(eta-2-(C,O)R1R2C=C=O)]- 10 by an ECE process. The structure of 2a was determined by X-ray diffraction methods. The crystals are triclinic, space group P1BAR with Z = 4 in a unit cell of dimensions a = 16.063(7), b = 19.108(8), c = 10.696(6) angstrom, alpha = 99.89(2), beta = 94.64(2), gamma = 111.95(2)-degrees. The structure was solved from diffractometer data by Patterson and Fourier methods and refined by blocked full-matrix least-squares on the basis of 9706 observed reflections, to R and R(w) values of 0.0391 and 0.0567 respectively. The diphenylketene and the two Cp' rings (Cp'= C5H4SiMe3) are eta-2(C-O) and eta-5 respectively. The niobium atom is also bonded to a Br atom. If the centroids of the Cp' rings and the midpoint of the C-O ketene bond are considered, the Nb atom displays a distorted tetrahedral coordination.