Iridium dioxygen complexes in the oxidation of substrates: kinetics, mechanism, and steric and electronic effects in the oxidation of carbon monoxide, carbon dioxide, triphenylphosphine, and sulfur dioxide by RIr(O2)(CO)L2 (R = methyl, phenyl, neopentyl; L = tri-p-tolylphosphine, triphenylphosphine, methyldiphenylphosphine, tri-p-anisylphosphine)