Ligand field analysis of the electronic spectra of [M(H2O)(6)](3+), [M(NH3)(6)](3+), [M(en)(3)](3+), [M(chxn)(3)](3+) and [M(tacn)(2)](3+) complexes (M=Co-III, Rh-III, Ir-III). Spectrochemical strength and nephelauxetic effect of aqua, ammine and amine ligands

Gaussian analyses of the electronic spectra of homoleptic aqua, ammine, ethane-1,2-diamine (en), trans-cyclohexane-1,2-diamine (chxn) and 1,4,7-triazacyclononane (tacn) complexes of cobalt(III), rhodium(III) and iridium(III) have provided transition energies that are not distorted by the overlap of absorption band envelopes. This has uncovered the true (1)A(1g)-->T-1(2g) ligand field transition of [Ir(NH3)(6)](3+) and [Ir(en)(3)](3+). Apart from an interchange observed for cobalt(III), the spectrochemical series was found to be H2O much less than NH3 < chxn < en < tacn for all three metal ions. The spectrochemical order thus follows the order of am(m)ine ligand gas-phase basicities. The anomaly of [Co(chxn)(3)](3+) has been explained in terms of steric crowding around the metal centre. For all three metal ions the nephelauxetic series was found to be H2O much less than am(m)ine, with the am(m)ine ligands all giving rise to quite similar degrees of nephelauxetism.