A Marcus-type model for steric and electronic effects of ligands in atom transfer reactions of metal carbonyl radicals

The rate constants for reaction of a series of Re(CO)₄L (LCO, PR₃, P(OR)₃) radicals with various atom or functional group donors in hexane at room temperature have been measured using laser flash photolysis. The data in each series have been analyzed in terms of the linear free energy relationship, In k=aE_i+bS _i+c, where E_irepresents a measure of the donor character of ligand L (the ¹³C chemical shift of the CO groups in LNi(CO)₃), and S_i represents a measure of its steric requirement (the ligand repulsive energy parameter, E_R). Expressing the atom or group transfer process in terms of the Marcus model for atom transfer, the variation in electron donor-acceptor character of L is related to variation in the overall free energy change in the atom transfer step, ΔG°′. The variation in ligand steric requirement is related to variation in the intrinsic barrier for the process, ΔG^*(O). The model leads to a new prediction: for a group of related reaction sets, such as the atom transfer reactions of the Re(CO)₄L radicals, the ratio a/b decreases monotonically with increasing exergonic character. That is, the relative sensitivity of the rate to variation in the electronic character of the ligand as compared with variation in its steric character decreases as the reaction becomes more exergonic. The predicted behavior is in good agreement with the experimental results for the large body of atom and group transfer data presented.