A comprehensive view of M-H addition across the RC CH bond: frustration culminating in ultimate union

The reaction of MHCl(CO)L-2 (L = (PPr3)-Pr-i; M = Ru or Os) with more than a dozen terminal alkynes RC drop CH has been studied at variable temperatur es and for a variety of R groups representing a wide range of steric and el ectronic effects. This sometimes reveals (for the slower osmium examples) f ormation of an eta (2)-alkyne adduct, then the vinylidene OsHCl(C=CHR)(CO)L -2 and finally the eta (1)-vinyl complex OsCl(CH=CHR)(CO)L-2. The rate of f ormation of the vinyl complex decreases with R according to the series prim ary > tertiary > secondary and electron-withdrawing > electron-donating. De uterium labeling of OsHCl(CO)L-2 at either Os or the alkyne sp carbons show s that isotope exchange between these two sites can be competitive with vin ylidene and vinyl product formation, and thus can confuse some attempts to trace the fate of the hydride. When this complication is absent, convention al syn addition of Os-D to HC drop CR is established, to give Os(E-CH=CDR). The rate of conversion to the vinyl product is not suppressed by added fre e (PPr3)-Pr-i. Taken together, these results are consistent with a mechanis m of vinyl complex formation involving neither the adduct with H trans to R C drop CH, nor the vinylidene, but rather with direct alkyne attack cis to the hydride, which is also consistent with the considerable steric influenc e on the rate of vinyl formation. DFT (B3PW91) calculations show that the v inyl complex is the thermodynamically most stable product and thus is alway s the final observed product. The calculations also show that the "direct" addition of the alkyne occurs via approach of the alkyne cis to M-H inside the H-M-Cl quadrant. This direct route is in fact calculated to be a multis tep process with an alkyne intermediate that is not in a deep well and thus cannot be observed experimentally. Calculations also agree with the fact t hat the vinylidene and the vinyl complexes are obtained through two indepen dent routes.