The Prototype Ge−H Insertion Reaction of Germylene with Germane. Absolute Rate Constants, Temperature Dependence, RRKM Modeling and the Potential Energy Surface

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Abstract

Time-resolved studies of germylene, GeH2, generated by laser flash photolysis of 3,4-dimethylgermacyclopentene-3, have been carried out to obtain rate constants for its bimolecular reaction with monogermane, GeH4. The reaction 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 GeH2 with GeH4 to form digermane, Ge2H6, is pressure dependent, consistent with a third-body assisted association reaction. The high-pressure rate constants, obtained by extrapolation, gave the following Arrhenius equation: log(k∞/cm3 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 moderately fast reaction occurring at approximately one-fifth of the collision rate. RRKM modeling, based on a variational transition state, used in combination with a weak collisional deactivation model, gave good fits to the pressure dependent curves, for a suitable choice of the critical energy, Eo, for reverse decomposition of Ge2H6. The step size (energy removed in a down collision) was chosen by analogy with the corresponding system for Si2H6 (collisional efficiency (βc) of ca. 0.7 for SF6). The value obtained for Eo was 155 kJ mol-1. Corrected for thermal energy and combined with the insertion activation energy this gives ΔH° = 166 kJ mol-1 for the decomposition of Ge2H6. There is no previous experimental determination of this quantity. From it we derive = 237 ± 12 kJ mol-1, in reasonable agreement with earlier estimates. From bond dissociation energy values the Divalent State Stabilization Energy (DSSE) of germylene (119 kJ mol-1) is larger than that of silylene (94 k J mol-1). Ab initio calculations at the correlated level reveal the presence of two weak complexes (local energy minima) on the potential energy surface corresponding 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 digermane with very low barriers. The implications of these findings and the nature of the insertion process are discussed.

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