Comparison of H−H versus Si−H σ-Bond Coordination and Activation on 16e Metal Fragments. Organosilane, N₂, and Ethylene Addition to the Agostic Complex W(CO)₃(PR₃)₂ and Dynamic NMR Behavior of the Latter

Variable-temperature 31P{1H} NMR spectroscopy of the agostic complexes M(CO)3(PCy3)2 (M = Mo, W) indicates dynamic behavior as evidenced by collapse below −20 °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(PiPr3)2]2(μ-N2) (1) was cleanly formed in the reaction of W(CO)3(PiPr3)2 with N2. Complex 1 was structurally characterized and compared with other bridging dinitrogen compounds of tungsten. The ethylene complex W(CO)3(PCy3)2(η2-C2H4) (2) was synthesized and characterized by X-ray crystallography in order to compare the binding mode of ethylene with that of H2. 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(PiPr3)2 at 60 °C to form the bridging silyl species [W(CO)3(PiPr3)(μ-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 H2 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.