Atomic carbon as a terminal ligand: Studies of a carbidomolybdenum anion featuring solid-state C-13 NMR data and proton-transfer self-exchange kinetics

Anion [CMo(N[R]Ar)(3)](-) (R = C(CD3)(2)CH3 or ' Bu, Ar = 3,5-C6H3Me2) cont aining one-coordinate carbon as a terminal substituent and related molecule s have been studied by single-crystal X-ray crystallography, solution and s olid-state C-13 NMR spectroscopy, and density functional theory (DFT) calcu lations. Chemical reactivity patterns for [CMo(N[R]Ar)(3)](-) have been inv estigated, including the kinetics of proton-transfer self-exchange involvin g HCMo(N[R]Ar)(3), the carbidomolybdenum anion's conjugate acid. While the Mo equivalent toC bond lengths in [K(benzo- 15-crown-5)(2)] [CMo(N[R]Ar)(3) ] and the parent methylidyne, HCMo(N[R]Ar)(3), are statistically identical, the carbide chemical shift of delta 501 ppm is much larger than the delta 282 ppm shift for the methylidyne. Solid-state C-13 NMR studies show the ca rbide to have a much larger chemical shift anisotropy (CSA, 806 ppm) and sm aller Mo-95-C-13 coupling constant (60 Hz) than the methylidyne (CSA = 447 ppm, 1J(MoC) = 130 HZ), DFT calculations on model compounds indicate also t hat there is an increasing MoC overlap population on going from the methyli dyne to the terminal carbide. The pK(a) of methylidyne HCMo(N[R]Ar)(3) is a pproximately 30 in THF solution. Methylidyne HCMo(N[R]Ar)3 and carbide [CMo (N[R]Ar)(3)](-) undergo extremely rapid proton-transfer self-exchange react ions in THF, with k = 7 x 10(6) M-1 s(-1). Besides being a strong reducing agent, carbide [CMo(N[R]Ar)(3)](-) reacts as a nucleophile with elemental c halcogens to form carbon-chalcogen bonds and likewise reacts with PCl3 to f urnish a carbon-phosphorus bond.