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, CN, CC, etc.), net σ-donation and net π-back donation, and percentage Ru 4dπ in the LUMO, are derived and correlated with experimental properties including oxidation and reduction potentials and ligand electrochemical parameters, EL(L). The various properties are understood in terms of the primary involvement of π-back donation to the π*-LUMO of bqdi. Net π-back donation is derived from the contribution of the π*-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 [RuIIL4(bqdi)] or [RuIIIL4(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.