Excited states of metal-metal bonded diimine complexes vary from extremely long lived to very reactive with formation of radicals or zwitterions

Abstract

The metal-metal bonded complexes Re(ML n )(CO) 3( α-diimine) [ML n = Mn(CO) 5, Re(CO) 5, SnPh 3, etc.] have a lowest 3σπ ∗ excited state, in which σ represents the ReM bonding orbital and π ∗ the lowest empty orbital of the α-diimine. This excited state, which can be occupied via the Re→α-diimine MLCT states, is normally reactive, giving rise to homolysis of the ReM σ bond. Only the complex Re(SnPh 3)(CO) 3(bpy) was found to be photostable and much longer lived in its 3σπ ∗ state than related complexes having a lowest 3MLCT state of comparable energy. This was ascribed to the strength of the ReSn bond and to the weak distortion of this complex in its 3σπ ∗ state with respect to the ground state. In the complexes Ru(E)(E′)(CO) 2(α-diimine) the axial ligands E and E′ can vary strongly and so do their excited state properties. Of special interest are the compounds in which both E and E′ are SnP 3 ligands, strongly bonded to Ru by a high-lying σ orbital. Because of the strength of these bonds and the strong mixing of the σ(SnRuSn) orbital with the π ∗(α- diimine ) orbital, the structural effects of the σ → π ∗ transition are small. This is refl in very small shifts of the CO-stretching vibrations with respect to the ground state and in a very long lifetime of the 3σπ ∗ state ( τ = 264 μs for Ru(SnPh 3) 2(CO) 2(iPr-DAB) in a 2-MeTHF glass at 77 K). Similarly, irradiation of a cluster Os 3(CO) 10(α-diimine) in an apolar solvent gives rise to homolysis of an OsOs bond with formation of a biradical, which was detected with EPR and time-resolved absorption spectroscopy ( τ = 5–110 ns). In a coordinating solvent such as CH 3CN, the homolysis reaction is accompanied by the coordination of a solvent molecule, which induces an intramolecular electron transfer. A zwitterion is then formed, which has a lifetime of seconds in CH 3CN. The biradical can also be transformed into such a zwitterion if a Lewis base is added to the apolar solvent. In this article the mechanistic details of these reactions are presented and discussed.