Arrhenius parameters for the head-to-tail dimerizations of 1,1-diphenylsilene and 1,1-diphenylgermene in solution

Arrhenius parameters for the head-to-tail dimerization of 1,1-diphenylsilen e and 1,1diphenylgermene have been determined in hexane solution by nanosec ond laser flash photolysis. Both compounds exhibit near-zero activation bar riers over the range 0-60 degreesC and preexponential factors of ca. 10(9) and 10(10) M-1 s(-1), respectively. While the results do not allow one to d istinguish conclusively between concerted and stepwise mechanisms for the r eaction, they provide an indication of what the kinetic behavior of the put ative reaction intermediates must be if dimerization proceeds nonconcertedl y.