Kinetic and Thermodynamic Study of Reactions of Some Substituted Manganese(I) and Manganese(II) Tricarbonyl Complexes Using Spectrophotometric and Electrochemical Techniques

Electrochemical oxidation and reduction processes enable the species fac-[Mn(CO)3(L-L')X]0,+ and mer,-[Mn(CO)3(L-L']0,+ to be synthesized and their thermodynamic and kinetic properties to be studied via electrochemical and spectrophotometric measurements ((L-L')=bidentate phosphorus or arsenic ligand: X=Cl or Br). In oxidation state II, the 17-electron manganese complexes are extremely labile and several reactions occur. The rate of isomerism, fac-Mn(II)→mer-Mn(II). studied by a fully computerized version of double potential step chronoamperometry, is not markedly dependent on solvent, bidentate ligand, or X. A twist mechanism is proposed to explain this isomerism. By contrast the activation energy in the thermodynamically favored 18-electron oxidation state I isomerism must be extremely high because no isomerism was observed to occur in 24 h. mer-Mn(ll) complexes generated via fac-Mn(II) isomerism undergo slow solvolysis and redox reactions in the dark. They are also extremely light sensitive and are rapidly reduced in sunlight. Reactions of manganese(II) carbonyls are therefore considerably more complex than previously thought.