Lowest unoccupied molecular orbital transition-metal complexes

It is important to note for the following discussion that in electron-transfer processes the reductant s highest occupied molecular orbital (HOMO) should combine with the oxidant s lowest unoccupied molecular orbital (LUMO) of the same symmetry to ensure proper overlap of reductant and oxidant orbitals to initiate electron transfer. That is, electron transfer will occur readily from n to n orbitals on different species or from a to a but not n to a in a linear arrangement of atoms [e.g., A-B-C in Appendix I (following references at the end of this chapter)]. In the case of outer-sphere electron-transfer processes, n- to 7r-electron transfers are favored over a to a because (1) such transfers do not require major changes in bond lengths in the precursor complex (lower activation energy) and (2) the n orbitals are more diffuse or better exposed than a orbitals. This process is well documented for transition metals. For inner-sphere electron-transfer processes, both n- to n- and a- to n-electron transfers are most favored (Purcell and Kotz, 1980). 

To illustrate the tuning aspects of the MLCT transitions in ruthenium polypyridyl complexes, the well known [RuLs] (L = 4,4 dicarboxylic acid-2,2 -bi-pyridine) type of complex can be considered. This complex shows strong visible band at 466 nm, because of CT transition from metal t2g highest occupied molecular orbitals (HOMO) to jr -lowest unoccupied molecular orbitals (LUMO) of the ligand (Fig. 3). The Ru(II)/(III) oxidation potential is at 1.3 V, and the ligand based reduction potential is at —1.5V... 

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