Elecronic Structure of Bis(o-iminobenzo-semiquinonato)metal Complexes (Cu, Ni, Pd): The Art of Establishing Physical Oxidation States in Transition-Metal Complexes Containing Radical Ligands

摘要

The researchers have clearly demonstrated that o-iminobenzosemiquinonato complexes of Co, Ni, Pd, and Cu have well-defined metal and ligand oxidation states which are closely related to the molecular properties such as the bond-length pattern, electrochemical behavior, electronic absorption spectra, magnetochemistry, and EPR spectroscopy. As in the case of dioxolene complexes, it was demonstrated that the bond lengths within the o-iminobenzosemiquinonato ligand are highly indicative of the ligand oxidation state. Electrochemical behavior also closely reflects the oxidation states. In general, each ligand present in the coordination sphere can be oxidized and reduced to the corresponding quinone and catecholate, respectively. This is characteristic of the intermediate (i.e., semiquinone) ligand oxidation state. It is worth noting that each o-iminobenzosemi-quinonato ligand in ML_2 or CoL_3 complexes is reduced and oxidized separately, by one electron. This is due to a relatively strong interaction between the o-iminobenzosemiquinonato ligands, which is also manifested by magnetochemical data and by the electronic adsorption spectra. It is very interesting to see how all the various properties fit together, consistently with the description of these complexes as containing the metal atom and the noninnocent o-iminobenzosemiquinonato ligand in well-deined oxidation states. Importantly, these oxidation states have been determined experimentally and they have nothing to do with formal oxidation states and electron-counting rules. It follows that metal and ligand oxidation states in complexes of noninnocent ligands should be regarded as experimental observables and related to molecular properties. This conclusion is in full accordance with the concepts of "preponderant configuration," spectroscopic, magnetochemical, and electrochemical oxidation states, that were introduced in the 1960.~5-7