The insertion of germylene into the H-H bond;: rate constant limits and thermochemistry.: Ab initio and DFT calculations on the reactions of GeH2 and SiH2 with H2

The technique of laser flash photolysis in the gas-phase has been used to set limits on the rate constants for the bimolecular reaction of germylene (GeH2) with deuterium (D-2) at both ambient and elevated temperatures (585 K). These limits show that the activation energy for the insertion of GeH2 into the H-H bond is at least 19 (+/-6) kJ mol(-1). Thermochemical arguments place the activation energy approximately in the range 63-84 kJ mol(-1). DFT B3LYP/6-311++G(3df,2pd) and ab initio QCISD(T)/6-311G++(3df,2pd)//QCISD/6-311G(d,p) calculations have been carried out on the potential energy surfaces of reactions ZH(2) + H-2 --> ZH(4) (Z= Ge, Si). Both methods predict the same mechanisms for germylene and silylene insertion which include formation of loose prereaction complexes and transition states of similar structure. The prereaction complex is only about half as strong in the case of germylene (DeltaH (298 K) = -9 (-11) kJ mol(-1)) as in the case of silylene (DeltaH (298 K) = -16 (-21) kJ mol(-1)) (QCISD values cited with B3LYP values in parentheses). The differences in activation energies are even more significant. Germylene insertion has a very high barrier of 58 (56) kJ mol(-1) compared to that of silylene 13 (6) kJ mol(-1). Calculated activation parameters for both reactions are in reasonable consistency with experimental results. Reasons for the enhanced H-H insertion barrier for germylene compared with silylene are discussed.