Structural Characterization of (C5H5)Co(PPh3)(η2-alkyne) and (C5H5)Co(η2-alkyne) Complexes of Highly Polarized Alkynes

The solid state structures of the cobalt–alkyne complexes (η5-C5H5)(PPh3)Co{η2-(R3Si)C≡C(SO2Ar)} (3-TMS, R = Me, Ar = C6H5; 3-TIPS, R = CH(CH3)2, Ar = p-C6H4CH3) and the noncoordinated alkyne (Me3Si)C≡C(SO2Ph) (6-TMS) have been characterized by X-ray crystallography and, in the case of 3-TMS, 6-TMS, and (η5-C5H5)Co{η2-(Me3Si)C≡C(SO2Ph)} (5-TMS-calc), by B97D/Def2-TZVPP computational analysis. The phosphine-dissociated complex 5-TMS-calc is determined to be a ground state singlet. Analysis of bond angle and distance metrics, calculated NMR chemical shift data, and molecular orbital analysis provide strong evidence for a four-electron-donor alkyne ligand in 5-TMS-calc. The degree of asymmetry in metal–alkyne bonding, as defined by the Gladysz alkyne-slippage parameter, is dramatically reduced in 5-TMS-calc relative to that in the precursor complex 3-TMS-calc.