Spontaneous and Induced Homolysis of Bis(triphenylphosphine)octacarbonyldimanganese (Mn-Mn)

Article abstract of DOI:10.1021/ja00303a011

Kinetic studies show that the complex Mn2(CO)8(PPh3)2 reacts thermally by two paths with C2H2Cl4, C16H33I, O2, NO, P(OEt)3, and P-n-Bu3 in decalin or cylcohexane.Reactions with CO or P(OPh)3 proceed only by one of these paths.The two paths are approximately equal importance and both show a very close fit to the same, rather complex, form of rate equation.The equation is quite inconsistent with any form of rate-determining dissociation, but it is consistent with two forms of reversible homolysis, one spontaneous and one induced.The latter involves initial, reversible formation of a reactive isomer of Mn2(CO)8(PPh3)2 which undergoes homolysis when attacked by a sufficiently reactive reagent.Both paths are therefore operative in reactions with the more reactive reagents, but reactions with CO and P(OPh)3 proceed only via spontaneous homolysis because these reagents are evidently unable to induce homolysis of the reactive isomer of the complex.A possible structure for the reactive isomer is one formed by metal migration, i.e., it can be formulated as (Ph3P)(OC)4Mn(μ-CO)Mn(CO)3(PPh3) which contains a bridging CO ligand, no Mn-Mn bond, and a vacant coordination site on one Mn atom.Attack at this site by suitably active reagents is postulated to lead to fragmentation.These results show that behavior previously thought to be uniquely indicative of spontaneous homolysis could also be explained by reversible homolysis induced by the reactant after isomerization of the complex.Rate constants are derived for halogen transfer from C2H2Cl4 or C16H33I to .Mn(CO)4(PPh3) and for nucleophilic displacement of PPh3 from .Mn(CO)4(PPh3) by P(OPh)3, P(OEt)3, and CO.