Time resolved studies of silylene, SiH2, generated by the 193 nm laser flash photolysis of phenylsilane, have been carried out to obtain rate coefficients for its bimolecular reactions with methyl-, dimethyl- and trimethyl-silanes in the gas phase. The reactions were studied over the pressure range 3–100 Torr with SF6 as bath gas and at five temperatures in the range 300–625 K. Only slight pressure dependences were found for SiH2
+ MeSiH3
(485 and 602 K) and for SiH2
+ Me2SiH2
(600 K). The high pressure rate constants gave the following Arrhenius parameters:
ReactionLog(A/cm3 molecule−1 s−1)Ea/kJ mol−1
SiH2
+ MeSiH3−10.05 ± 0.11−3.91 ± 0.81
SiH2
+ Me2SiH2−10.07 ± 0.05−3.48 ± 0.35
SiH2
+ Me3SiH−10.37 ± 0.10−4.65 ± 0.76
These are consistent with fast, near to collision-controlled, association processes. RRKM modelling calculations are consistent with the observed pressure dependences (and also the lack of them for SiH
2
+ Me
3SiH).
Ab initio calculations at both second order perturbation theory (MP2) and coupled cluster (CCSD(T)) levels, showed the presence of weakly-bound complexes along the reaction pathways. In the case of SiH
2
+ MeSiH
3 two complexes, with different geometries, were obtained consistent with earlier studies of SiH
2
+ SiH
4. These complexes were stabilised by
methyl substitution in the substrate
silane, but all had exceedingly low barriers to rearrangement to product disilanes. Although methyl groups in the substrate
silane enhance the intrinsic SiH
2 insertion rates, it is doubtful whether the intermediate complexes have a significant effect on the kinetics. A further calculation on the reaction MeSiH + SiH
4 shows that the
methyl substitution in the silylene should have a much more significant kinetic effect (as observed in other studies).