Absolute rate constants for the head-to-tail [2+2]-dimerization of 1,1-diphenylsilene and 1,1-diphenylgermene have been determined in hexane and isooctane solution at 23 °C by laser flash photolysis, using the corresponding 1,1-diphenylmetallacyclobutanes as precursors. This requires knowledge of the molar extinction coefficients at the UV absorption maxima of the two transient species, which have been determined by a transient actinometric procedure. The rate constants for dimerization of the two compounds are similar and within a factor of about 2 of the diffusional rate constant in both cases. The 1,3-dimetallacyclobutane dimerization products have been prepared by direct photolysis of the corresponding 1,1-diphenylmetallacyclobutanes in dry isooctane. In addition, 1,1,3,3-tetraphenyl-1-germa-3-silacyclobutane has been prepared by photolysis of a 1:1 mixture of the two metallacyclobutanes. The solid-state structures of the three 1,3-dimetallacyclobutanes have been determined by single-crystal X-ray crystallography, and their photochemistry has been studied. Laser flash and/or steady-state photolysis experiments indicate that all three compounds cyclorevert to the corresponding 1,1-diphenylmetallaenes upon photolysis in hydrocarbon solvents, most likely via the same (•M−C−M‘−C•) 1,4-biradical intermediates which link the metallaenes with their corresponding dimers via stepwise dimerization mechanism. The quantum yield for photocycloreversion of the digermacyclobutane is roughly one-fourth that of 1,1-diphenylgermacyclobutane. However, it is about 3 times higher than that for the silagermacyclobutane and roughly 15 times higher than that for the disilacyclobutane. Singlet lifetimes have been determined for the metallacyclobutanes and the disila- and digermacyclobutanes using single photon counting techniques. The implications of these results for the mechanisms of the head-to-tail [2+2]-dimerization of silenes and germenes are discussed.

Absolute rate constants have been measured for the reaction of methoxytrimethylsilane with a series of transient, para-substituted 1,1-diphenylsilenes (H 2 CdSi(C 6 H 4 X) 2 , where X ) H, Me, F, Cl, and CF 3 ) in hexane solution at room temperature. The data correlate with Hammett substituent constants, affording the reaction constant F ) +0.9 ( 0.2. For the parent compound and the 4,4′-bis(trifluoromethyl) derivative, rate constants have been determined over the 0-60 °C temperature range in hexane, 1,2-dichloroethane, and acetonitrile solution. The rate constants for reaction of the parent compound increase in the order hexane ≈ 1,2-dichloroethane < MeCN, but are relatively insensitive to solvent in the other case. However, differences are revealed in the Arrhenius activation energies for reaction, which are negative for both compounds in all three solvents. Those for the 4,4′bis(trifluoromethyl) compound in particular change to more positive values with increasing solvent polarity. The data are consistent with a two-step mechanism involving reversible preassociation of the silene and the alkoxysilane to form a Lewis acid-base complex, which collapses to product by intramolecular transfer of trimethylsilyl from oxygen to the silenic carbon.