Unusual kinetics of gas-phase proton transfer to Mn(CO)₅⁻ and Re(CO)₅⁻

Abstract

Rate constants for gas-phase proton transfer to Mn(CO)₅ ⁻, Re(CO)₆ ⁻ and Cl⁻ from reference acids and metal hydrides are determined. Proton-transfer efficiencies to Mn(CO)₅ ⁻ are small, and less than the efficiencies for proton transfer to Cl⁻; proton-transfer efficiencies to Re(CO)₆ ⁻ are larger than those for Mn(CO)₅ ⁻, but still generally less than for transfer to Cl⁻. Proton-transfer efficiencies to Mn(CO)₅ ⁻ and Re(CO)₅ ⁻ are substantially more dependent on the nature of the proton donor (e.g. metal hydride versus oxy-acid). The thermoneutral proton transfer between Mn(CO)₅ ⁻ and ¹³CO-labelled HMn(CO)₆ is particularly inefficient, occurring only in about 1 in 100 collisions. The results are discussed in view of evidence for a short lifetime of the collision complex, and the effects of electronic and structural reorganization of Mn(CO)₅ ⁻ and Re(CO)₅ ⁻, including loss of fluxionality of the metal species, which occur on protonation.