Comparison of H-H versus Si-H sigma-bond coordination and activation on 16e metal fragments. Organosilane, N-2, and ethylene addition to the agostic complex W(CO)(3)(PR3)(2) and dynamic NMR behavior of the latter

Variable-temperature P-31{H-1} NMR spectroscopy of the agostic complexes M(CO)(3)(PCy3)(2) (M = Mo, W) indicates dynamic behavior as evidenced by collapse below -20 degrees C of a singlet to an AB signal plus a shifted singlet. The inequivalency of the phosphines is possibly due to the presence of conformational isomers resulting from hindered rotation of the M-P bond or, less likely, a geometric isomer with pseudo-cis PCy3 ligands. Further studies on the coordination chemistry of W(CO)(3)(PR3)(2) (R = iPr, Cy) were performed. The bridging dinitrogen complex [W(CO)(3)(PiPr(3))(2)](2)(mu-N-2) (1) was cleanly formed in the reaction of W(CO)(3)(PiPr(3))(2) with N-2. Complex 1 was structurally characterized and compared with other bridging dinitrogen compounds of tungsten. The ethylene complex W(CO)(3)(PCy3)(2)(eta(2)-C2H4) (2) was synthesized and characterized by X-ray crystallography in order to compare the binding mode of ethylene with that of H-2. Phenylsilane reacted with W(CO)(3)(PR3)(2) (R = iPr, Cy) to form the thermally unstable oxidative addition (OA) products WH(SiH2Ph)(CO)(3)(PR3)(2) (3, R = Cy; 4, R = iPr). Diphenylsilane reacted with W(CO)(3)(PiPr(3))(2) at 60 degrees C to form the bridging silyl species [W(CO)(3)(PiPr(3))-(mu-SiHPh2)](2) (5), which was confirmed by spectroscopic techniques and X-ray crystallography to have two 3-center 2-electron W ... H ... Si interactions. Detailed comparisons of the binding and activation of silanes versus H-2 on various 16e metal centers suggest a high degree of similarity, but relative ease of OA depends on the electrophilicity of the metal-ligand fragment and other factors such as bond energetics. Increasing the electrophilicity of the metal center (e.g., adding positive charge) may aid in stabilizing alkane coordination.