From Electron Transfer Reactions to the Effects of Backbonding

Abstract

The initial motivation for extending the chemistry of ruthenium ammines was to provide new reagents for research on electron transfer reactions. Quite early in pursuing the preparative chemistry, an unexpected capac -ity of Ru(II) for backbonding was uncovered, and the systematic investi -gation of its influence on physical and chemical properties became a goal in its own right. The new directions led to the discovery of a num -ber of novel reactions, which in some cases led to novel products. The research has provided documentation of the effect of backbonding on ligand-to-metal charge transfer, ligand-metal distances, the intensities of infrared absorption by π-acceptor ligands, the acid-base properties of such ligands, the properties of co-ligands exerted by the electronic-with -drawing power of π-acceptor ligands, and on Ru(III)/Ru(II) redox potentials and enthalpies of complex formation. Some early results, which demonstrate the superior capacity of the osmium ammines to engage in backbonding, are also described. ONE OF THE THEMES OF MY EARLY RESEARCH at Stanford, begun late in1961, was the investigation of the mechanisms of substitution reactions of metal complexes by the application of isotope effects, mainly of oxygen. For example, we carried out kinetic and tracer studies in the alkaline hydrolysis of [Co(NH3)5(02CCF3)] 2+ (I) and attempted to generate intermediates such as might be implicated in SN1 mechanisms (e.g., we hoped to generate [Co(NH3)5] 3+ by the nitrosation of [CoiNH^N^P) (2). Mechanisms of Electron TransferThe major emphasis of the research in my group, however, was the investiga tion of the mechanisms of electron transfer reactions. In key experiments done

Back-bonding Manifestation on the Stability of a Pyridinethiolate Surface Modifier at a Gold Electrode

“…The data collected on the system described here strongly demonstrated that the π back-bonding effect, so well established by Taube's research group, , from the [Ru(CN) 5 ] 3- metal center , plays an important role in the stability of the RupySAu improving the assessment of the ET of cyt c . To extend such study to other π-donor systems, experiments with [Fe(CN) 5 ] 3- , [Ru(NH 3 ) 5 ] 2+ , and [Os(NH 3 ) 5 ] 3+ metal centers are currently under investigation at our laboratory.…”

“…We undertook the [Ru(CN) 5 ] 3- metal center as a π-donor system coordinated to the pyS ligand to design the first inorganic complex applicable to the assessment of the inherent rate of the cyt c ET reaction. A key point is that the coordination of a π-donor transition metal center to a nitrogen on a pyridine ring of the pyS would affect its electronic density, enhancing the C−S bond strength with consequent stability gain of the adlayer.…”