Re‒C Bond Homolysis in Alkyl- and Arylrhenium Trioxides: A Qualitative MO Interpretation

In many cases, the decomposition of methyltrioxorhenium (CH₃ReO₃, MTO) and other alkyl and aryl derivatives appears occur through the homolytic cleavage of the Re‒C bond. Under these circumstances, the already deficient metal [Re(VII)] gives up half of the electron pair donated by the alkyl anion, and the ensuing Re(VI) species has a finite lifetime as a free radical. The problem is tackled from the viewpoint of qualitative MO theory. From a detailed examination of all the orbital interactions between metal and the coordinated atoms in pseudotetrahedral MTO, it is established that the metal receives up to nine electron pairs from the oxygen atoms and CH₃. Accordingly, the complex is better formulated as an 18- rather than the widely accepted 14-electron species. The study of the dynamics of the most critical frontier MOs on elongating the Re‒C bond provides significant chemical information. Particularly relevant for the one-electron transfer from carbon to the metal atom is the avoided crossing between the HOMO and the LUMO. With the flattening of both separating fragments taken into account, the two levels converge to the accidentally quasi-degenerate σ-CH₃ and ReO₃ Z² orbitals. In the model, the effects of substituting groups such as phenyl or tert-butyl for methyl are also pointed out.