Allowed and Forbidden d-d Transitions in Poly(3,5-dimethylpyrazolyl)methane Complexes of Nickel(II)

Absorption spectra of four nickel(II) complexes with poly(pyrazolyl)methane ligands are presented in the NIR-VIS-UV region and the band system corresponding to the lowest-energy spin-allowed and spin-forbidden transitions is analyzed. A quantitative theoretical model involving coupled electronic states provides precise energies for the lowest-energy triplet and singlet excited states and allows comparisons between complexes with a variable number of nitrogen and oxygen ligator atoms. Singlet energies between 12 840 and 13 000 cm−1 are determined for heteroleptic complexes. These energies are in an intermediate range between those for homoleptic complexes with either nitrogen or oxygen ligator atoms with singlet states at approximately 12 000 and 14 000 cm−1, respectively. The new theoretical approach is compared to the traditional ligand-field parameters obtained from the maxima of the broad, spin-allowed absorption bands.