Seven-Coordinate Titanium and Vanadium Carbonyls: Synthesis and X-ray Crystal Structures of [f-BUSi(CH₂PMe₂)₃]TI(CO)₄and [f-BuSi(CH₂PMe₂)₃]V(CO)₃H

Reductive carbonylation of the tripod phosphine complexes [£-BuSi(CH₂PMe₂)₃]MCl₃(thf), where M is Ti or V and thf is tetrahydrofuran, with sodium naphthalenide in thf under carbon monoxide gives the titanium(O) carbonyl [£-BuSi(CH₂PMe₂)₃]Ti(CO)_4>1, and the vanadium(I) hydride [t-BuSi-(CH₂PMe₂)₃]V(CO)₃H, 2. X-ray crystallographic studies of these two compounds reveal seven-coordinate 4:3 piano stool geometries, with slightly different orientations of the fourfold and threefold faces; the titanium and vanadium structures may also be described in terms of a face-capped trigonal prism and a face-capped octahedron, respectively. Important structural parameters are as follows: Ti-CO = 1.98 (1), 2.00 (1) A; Ti-P = 2.612 (3), 2.632 (5) A; C-Ti-C(cis) = 69.8 (5), 71.7 (5)°; C-Ti-C(trans) = 109.9 (5)°; V-CO = 1.852 (9), 1.862 (7) A; V-P = 2.440 (2), 2.470 (2) A; C-V-C(cis) = 82.0 (3)°; C-V-C(trans) = 96.9 (3)°. The chelating nature of the phosphine ligand lends these complexes unusual kinetic and oxidative stability. Spectroscopic data for 1 and 2 are discussed: in particular, the large downfield^{1 2 3 4 5 6 7 8 9 10 11 12 13}C NMR chemical shift and low CO stretching force constant in 1 indicate that there is unusually strong Ti —> CO ir back-donation.