A detailed infrared and far-infrared spectral investigation of the alkyl- and (non-alkyl)cobalt(III) complexes of 3,8-dimethyl-5,6-benzo-4,7-diazadeca-3,7-diene-2,9-dione dioxime and a study of ground state trans-effect in tetragonally distorted octahedral cobalt(III) complexes by far-infrared and electronic absorption spectroscopy

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The tetraaza quadridentate chelate, 3,8-dimethyl-5,6-benzo-4,7-diazadeca-3,7-diene-2,9-dione dioxime [(DOH) 2bzo], forms stable tetragonally distorted octahedral cobalt(III) complexes with monodentate ligands such as halogens, pseudohalogens and Lewis bases in the axial sites besides forming pseudooctahedral alkylcobalt(III) complexes. A detailed IR and far-IR spectral investigation has been made for nine alkyl- and nineteen (non-alkyl)cobalt(III) complexes, reported recently. These complexes exhibit IR absorptions such as ν(OH), ν(CH), ν(NO), ν(CN), ν(CC), ν(CN), ν(CC) and CCH 3 symmetric and asymmetric deformation vibrations characteristic of the (DO)(DOH)bzo ligand framework and absorptions characteristic of the axial ligands. In the case of the complexes of aromatic Lewis bases vibrations such as ring deformation, ring breathing mode, ring stretchings and ring torsion characteristic of the aromatic rings are observed. Spectral evidence is obtained for the formation of an intramolecular hydrogen bond. The effect of hyperconjugative electron release and π-backbonding are manifested in the spectra of the complexes of 4-methylpyridine and triphenylphosphine, respectively. The far-IR spectra show vibrations such as ν(CoN), ν(CoBr), ν(CoC) and ν(Co-base) (Lewis base in the axial site). The ground state trans-effect in a series of tetragonally distorted octahedral (non-alkyl)cobalt(III) complexes of the type [LCo((DO)(DOH)bzo)Br] + (where L=a Lewis base such as pyridine, imidazole, methylimidazole, 4-methylpyridine, benzimidazole, 5,6-dimethylbenzimidazole, ethylamine, diethylamine, triethylamine, 2-aminopyridine, triphenylphosphine) is studied by following the variation of the CoBr stretching frequency on the field strength, Dq z , of the Lewis base in the axial site, trans to Br −. A plot of ν(CoBr) frequency versus Dq z is linear and as the Dq z increases ν(CoBr) frequency decreases. As the electron donating power of the axial Lewis base increases it transmits more electron density to Co 3+ with a concomitant transmission of electron density to Br −, trans to the Lewis base, thereby weakening the CoBr bond. This is the first report of the ground state trans-effect being studied using a quantitative measure of the electron donating ability of a series of test ligands on the ground state property of a probe ligand involving a heavy atom in six-coordinate cobalt(III) complexes.