THERMODYNAMIC ACIDITIES AND HOMOLYTIC METAL-HYDROGEN BOND-ENERGIES OFGROUP-8 PROTONATED DECAMETHYLMETALLOCENES CP-ASTERISK(2)MH(+) (M=RU, OS)

The first thermodynamic acidities and homolytic metal-hydrogen bond di ssociation energies (BDE) of metal-protonated metallocenes in solution are reported. The pK(a) of Cp2OsH+ in acetonitrile has been determin ed as 9.9 +/- 0.1 from proton-transfer equilibrium measurements. For C p2RuH+, the pK(a) has been estimated as 4.3 on the assumption that pK (a) differences in acetonitrile are straight-forwardly related to diff erences in the reported heats of protonation of CpOs-2 and Cp*Ru-2 in 1,2-dichloroethane. The pK(a) data are used in conjunction with rever sible oxidation potential data for the metallocenes to obtain metal-hy drogen BDE values of 284 kJ mol(-1) for Cp2RuH+ and 298 kJ mol(-1) fo r Cp2OsH+ respectively. The BDE value for Cp*2RuH+ is essentially equ al to that of Cp(2)RuH(+) in the gas phase.