Time-resolved studies of silylene, SiH₂, generated by laser flash photolysis of phenylsilane, have been carried out to obtain rate constants for its bimolecular reaction with monosilane, SiH₄. The reaction was studied in the gas phase over the pressure range 1–100 Torr, with both Ar and SF₆ as bath gases, at six temperatures in the range 298–665 K. The reaction of SiH₂ with SiH₄ to form disilane, Si₂H₆, is pressure dependent, consistent with a third-body assisted association reaction. The high-pressure rate constants, obtained by extrapolation, gave the Arrhenius equation: log(k^∞/cm³ molecule^–1 s^–1)=(–9.91 ± 0.04)+(3.3 ± 0.3 kJ mol^–1)/RT In 10. These Arrhenius parameters are consistent with a fast, nearly collision-controlled, process. RRKM modelling, based on a variational transition state, used in combination with a weak collisional deactivation model, gave good fits to the pressure-dependent curves. The step sizes (energies removed in a down collision) corresponded to collisional efficiencies (β_c) of ca. 0.5 for SF₆ and ca. 0.2 for Ar.

The rate constants for the insertion and reverse decomposition (of Si₂H₆) have been combined to obtain a precise value of the equilibrium constant K_p at 552 K. Using the third-law method, a value for Δ_fH°(SiH₂)= 273 ± 2 kJ mol^–1 is derived which represents the most precise experimental value for this quantity yet obtained.

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 silylene fragment. Surprisingly, the latter is the lower in energy, lying 51.5 kJ mol^–1 below the unassociated reactants. These complexes rearrange to disilane with very low barriers. The implications of these findings and the nature of the insertion process are discussed.