Hydride transfer reactions of transition metal hydrides: Kinetic hydricityof metal carbonyl hydrides

Hydride transfer from neutral transition metal hydrides (MH) to Ph3C+BF4- g ives M-FBF3 and Ph3CH. The rate law -d[Ph3C+BF4-]/dt = k[Ph3C+BF4-][MH] was established from kinetic measurements using stopped-flow methods. Second-o rder rate constants determined in CH2Cl2 solution at 25 degrees C range fro m k(H)- = 7.2 x 10(-1) M-1 s(-1) to k(H)- = 4.6 x 10(6) M-1 s(-1). The orde r of increasing kinetic hydricity is (C5H4-CO2Me)(CO)(3)WH < (CO)(5)MnH < C p*(CO)(3)CrH < Cp(CO)(3)WH < HSiEt3 < cis-(CO)(4)(PCy3)MnH < cis-(CO)(4)(PP h3)MnH < (C5H4Me)(CO)(3)WH < Cp(CO)(3)MoH < Cp*(CO)(3)WH < (indenyl)(CO)(3) WH < (CO)(5)ReH < Cp*(CO)(3)MoH < cis-(CO)(4)(PPh3)ReH < Cp(NO)(2)WH < tran s-Cp(CO)(2)(PCy3)MoH < trans-Cp(CO)(2)-(PPh3)MoH < trans-Cp(CO)(2)(PMe3)MoH (Cp = eta(5)-C5H5, Cp* = eta(5)-C5Me5, Cy = cyclohexyl). Ranges of activat ion parameters for hydride transfer from trans-Cp(CO)(2)(PMe3)MoH, trans-Cp (CO)(2)(PCy3)MoH, cis-(CO)(4)(PPh3)ReH, and Cp*(CO)(3)MoH are Delta H doubl e dagger = 3.0-5.9 kcal mol(-1) and Delta S double dagger = -18 to -24 cal K-1 mol(-1). The rate constant for hydride transfer (k(H)-) from cis-Cp(CO) (2)(PCy3)MoH at -55 degrees C is 3 orders of magnitude lower than that for trans-Cp(C0)2(PCy3)MoH. Phosphine substitution for CO generally enhances th e kinetic hydricity, with trans-Cp(CO)(2)(PMe3)MoH being 10(4) times as rea ctive as Cp(CO)(3)MoH. The electronic effect of phosphine substitution is a ttenuated by steric factors when the phosphine is cis to the metal hydride. The hydride transfer kinetics reported here are interpreted to be single-s tep hydride transfers, rather than a multiple-step mechanism involving an i nitial electron transfer followed by hydrogen atom transfer. A distinction is made between hydricity and nucleophilicity of metal hydrides.