Kinetics and mechanism of nucleophilic addition at the bridging vinyl ligand of the cluster [HOs 3 (η 2 ‐CHCH 2 ) (CO) 10 ]

Kinetic and spectroscopic studies are reported for the addition of a range of neutral phosphorus and nitrogen nucleophiles to the bridging η 2 ‐vinyl ligand of the cluster [HOs 3 (η 2 ‐CHCH 2 )(CO) 10 ]. With PPh 3 , the reaction is reversible, and the rate law, k obs = k 1 [Nuc] + k −1 , is observed. This rate law is consistent with direct bimolecular addition ( k 1 ) of PPh 3 to the η 2 ‐vinyl ligand. With more basic triarylphosphine nucleophiles, the simple expression, k obs = k 1 [PR 3 ], is obeyed. The second‐order rate constant, k 1 , varies markedly with the phosphine basicity. For the series P(4‐XC 6 H 4 ) 3 , a linear Brönsted plot of log k 1 vs p K a is obtained with a slope α of 0.46. The reversible addition of cyclohexylamine to this cluster also obeys a two‐term rate law. Overall, nucleophilicity towards [HOs 3 (η 2 ‐CHCH 2 )(CO) 10 ] decreases in the order C 6 H 11 NH 2 > P(4‐MeOC 6 H 4 ) 3 > P(4‐MeC 6 H 4 ) 3 > PPh 3 , which quantitatively parallels that previously found for the related mononuclear cation [CpFe(CO) 2 (η 2 ‐CH 2 CH 2 )] + .