The very covalent diammino(o-benzoquinonediimine) dichlororuthenium(II).: An example of very strong π-back-donation

The syntheses and X-ray structures of the complexes Ru( S-dmso) Cl-2( opda) ( 1) and Ru( NH3)(2)Cl-2( bqdi) ( 2) are described ( opda = o-phenylenediamine, bqdi = o-benzoquinonediimine). Optical absorption and emission, vibrational ( resonance Raman), and electrochemical data are discussed. We explore the nature of the ruthenium benzoquinone electronic interaction in species 2 primarily within the framework of density functional theory ( DFT) but also using INDO/S to extract Coulombic and exchange integrals. The resonance Raman and emission data were understood in terms of a common set of coupled vibrations localized primarily within the ruthenium metallacycle ring. Experimental and computational data were also compared among a select group of ruthenium bqdi species with other spectator ligands, specifically ammonia, 2,2'-bipyridine, and 2,4-pentanedione. The changes in the electrochemistry, optical spectroscopy, and vibrational spectra with changing spectator ligand donicity were explained within a common theoretical ( DFT) model which further provided a detailed analysis of the variation in the molecular orbital descriptions. With the application of an extended charge decomposition analysis ( ECDA), a detailed picture emerged of the bonding between the bqdi ligand and the metal atom, illustrating the coupling between the orbitals of each fragment as a function of orbital symmetry and charge transfer between the fragments of the complex. Metal-to-bqdi pi-back-donation is seen to be very important.