Basic Metal Complexes: Crystal Structures of Cyclopentadienylcobalt(I)-bis(trimethylphosphine) and Cyclopentadienylcobalt(I)-(trimethylphosphine)(ethylene)

The Lewis and Bronstedt basicity of transition metal in organometallic complexes is a concept widely used to rationalize stoichiometric and catalytic reactivity. [(C5H5)Co(PMe3)2] 1 is one of the prototypal representatives of basic metal complexes, and its chemistry has been extensively studied. The metal and/ or carbon-centered nucleophilic reactivity in the series [(C5H5)M(PMe3)2–n(C2H4)n] (M = Co, Rh, Ir; n = 1, 2) is of particular importance. For example, as a consequence of the variation of the metal basicity, the protonation of [(C5H5)Rh(PMe3)(C2H4)] 2a occurs at the metal to give the hydrido complex cation [(C5H5)Rh(H)(PMe3)(C2H4)], whereas in [(C5H5)Co(PMe3)(C2H4)] 2b a metal carbon bond is protonated to form an agostic ethyl complex, viz. [(C5H5)Co(PMe3)(h-C2H5)] 3.1 Here we report on the crystal and molecular structures of 1 and 2b, the latter being the precursor to 3, one of the best mechanistically characterized late-metal polymerization catalysts. Apart from catalytic applications, the series [(C5H5)Co(PMe3)2–n(C2H4)n] (n = 0 – 2) constitutes important reference molecules for MO-theoretical calculations, due to their relative simplicity and symmetrical structures. The title complexes were prepared in quantitative yield by the slow addition of two or one, respectively, molar equivalents of PMe3 to a hexane solution of [(C5H5)Co(C2H4)2] at –20 ̊C, followed by warming up to ambient temperature and removal of the solvent in vacuo. Single crystals suitable for intensity data collection were obtained by cooling pentane solutions to –20 ̊C. The structures were solved by the heavy atom method combined with structure expansion by direct methods applied to difference structure factors2 and refined by full-matrix leastsquares methods based on F2.3 All non-hydrogen atoms were given anisotropic displacement parameters. All hydrogen atoms were located in difference Fourier syntheses. During refinement, the hydrogens of the methyl groups were treated as variable metric rigid groups, with ideal bond angles. All other hydrogen atoms were freely refined. Both 1 and 2b adopt the two-legged piano-stool geometry (Fig. 2). The coordination geometry around the cobalt atom is remarkably similar in both complexes. The plane of the C5H5 X-ray Structure Analysis Online