The prototype Ge-H insertion reaction of germylene with germane. Absolute rate constants, temperature dependence, RRKM modeling and the potential energy surface

Time-resolved studies of germylene, GeH2, generated by laser flash photolys is of 3,4-dimethylgermacyclopentene-3, have been carried out to obtain rate constants for its bimolecular reaction with monogermane, GeH4. The reactio n was studied in the gas phase over the pressure range 1-100 Torr, with SF6 as bath gas, at five temperatures in the range 292-520 K. The reaction of Gel-H-2 with GeH4 to form digermane, Ge2H6, is pressure dependent, consiste nt with a third-body assisted association reaction. The high-pressure rate constants, obtained by extrapolation, gave the following Arrhenius equation : log(k(infinity)/cm(3) molecule(-1) s(-1)) (-11.17 +/- 0.10) + (5.2 +/- 0. 7 kJ mol(-1))/RT ln 10. These Arrhenius parameters are consistent with a mo derately fast reaction occurring at approximately one-fifth of the collisio n rate. RRKM modeling,based on a variational transition state, used in comb ination with a weak collisional deactivation model, gave good fits to the p ressure dependent curves, for a suitable choice of the critical energy, E-o , for reverse: decomposition of Ge2H6. The step size (energy removed in a d own collision) was chosen by analogy with the corresponding system for Si2H 6 (collisional efficiency (beta(c)) of ca. 0.7 for SF6). The value obtained for E-o was 155 kJ mol(-1). Corrected for thermal energy and combined with the insertion activation energy this gives Delta H degrees = 166 kJ mol(-1 ) for the decomposition of Ge2H6. There is no previous experimental determi nation of this quantity. From it we derive Delta H(f)degrees(GeH2) = 237 +/ - 12 kJ mol(-1), in reasonable agreement with earlier estimates. From bond dissociation energy values the Divalent State Stabilization Energy (DSSE) o f germylene (119 kJ mol(-1)) is larger than that of silylene (94 kJ mol(-1) ). Ab initio calculations at the correlated level reveal the presence of tw o weak complexes (local energy minima) on the potential energy surface corr esponding to either direct or inverted geometry of the inserting germylene fragment. Surprisingly, the latter is the lower in energy, lying 25 kJ mol( -1) below the unassociated reactants. These complexes rearrange to digerman e with very low barriers. The implications of these findings and the nature of the insertion process are discussed.