Electronic characteristics of an extensive series of ruthenium complexes with the non-innocent o-benzoquinonediimine ligand: A pedagogical approach

Density functional theory (DFT) calculations are carried out on an extensive series of ruthenium complexes with the non-innocent (redox active) o-benzoquinonediimine (bqdi) ligand, namely [Ru(WXYZ)(bqdi)](n+) where WXYZ are a range of spectator ligands including ammonia, phosphines, 2,2'-bipyridine, 2,2',2 ''-terpyridine, carbon monoxide, water, halide, acetonitrile, triazacyclononane, nitrosyl, cyclam, etc. In addition, a smaller series, Ru(acac)(2)(R-bqdi) is explored, where acac = 2,4-pentanedionate, and R = H, Cl, Me, NO2 and N-SO2Me. A range of properties including Mulliken and Natural population analysis (NPA) charges, Mayer bond orders (Ru-N, C=N, C=C, etc.), net sigma-donation and net pi-back donation, and percentage Ru 4d pi in the LUMO, are derived and correlated with experimental properties including oxidation and reduction potentials and ligand electrochemical parameters, E-L(L). The various properties are understood in terms of the primary involvement of pi-back donation to the pi*-LUMO of bqdi. Net pi-back donation is derived from the contribution of the pi*-LUMO (and higher virtual orbitals) of bqdi, to filled molecular orbitals of the complex. The question of whether these species should be considered exclusively as being represented as [(RuL4)-L-II(bqdi)] or [(RuL4)-L-III(sqdi)] (sqdi = o-benzosemiquinonediimine) is briefly considered and evidence presented for the former electronic structure. This is written as a pedagogical treatise rather than a detailed research discussion of the electronic properties of these molecules. (C) 2010 Elsevier B.V. All rights reserved.