SUBSTITUENT EFFECTS ON THE STRENGTHS OF SOME MANGANESE-MANGANESE BONDS

Kinetic studies have been made of the thermal decomposition (in decalin and under an atomosphere of oxygen) of the axially substituted complexes Mn2(CO)8L2 [L = PPhEt2, PPh2Et, PPh(OMe)2, PPh2(OMe), or P(p-MeOC6H4)3], The activation enthalpies for these [O2]-independent reactions are assigned to homolytic fission of the Mn-Mn bonds and are found to fall on or near a smooth plot of ΔH‡ hf against cone angle of L. A more detailed analysis of the steric contribution to the values of ΔH‡ hf shows that they do not interpolate linearly between the values for the “unmixed” ligands P(OMe)3 or PEt3 and PPh3, and suggestions are made to account for this. The analysis also allows a value of 127 ± 8° to be assigned to the cone angle of triphenylarsine. © 1979, American Chemical Society. All rights reserved.