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

Abstract

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 lowestenergy spin-allowed and spin-forbidden transitions is analyzed. A quantitative theoretical model involving coupled electronic states provides precise energies for the lowestenergy 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 21 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 21, respectively. The new theoretical approach is compared to the traditional ligand-field parameters obtained from the maxima of the broad, spin-allowed absorption bands.

Crystal-field analysis for d8 ions at D4h symmetry sites: Electronic states in trans-NiCl2(H2O)4 complex

“…The absorption spectra of nickel(II) complexes are presented in the near-infrared and visible spectral regions [1][2][3][4][5][6][7]. For these complexes, the absorption bands are related to the transition metal ions Ni 2+ with 3d 8 configuration and originate from electronic transitions in the unfilled 3d electronic shell.…”

“…For these complexes, the absorption bands are related to the transition metal ions Ni 2+ with 3d 8 configuration and originate from electronic transitions in the unfilled 3d electronic shell. The optical associated spectra are characterized by the presence of broad absorption bands due to the coupling between electronic transitions and host crystal phonons [1][2][3][4][5][6][7].…”

“…Crystals of trans-NiCl 2 (H 2 O) 4 Á 2H 2 O crystallize as very regular monoclinic green prisms [6,7]. The Ni 2+ ions occupy a site with D 4h symmetry in trans-NiCl 2 (H 2 O) 4 .…”

“…The Ni 2+ ions occupy a site with D 4h symmetry in trans-NiCl 2 (H 2 O) 4 . The shortest separation between nickel centres is 6.5 Å without shared ligands or bridging water molecules.…”

“…Larger metal-metal distances in this structure involve hydrogen bonds through at least one non-coordinated water molecule [6,7]. A detailed spectroscopic study of the optical characteristics of trans-NiCl 2 (H 2 O) 4 complex was presented by Bussie`re et al [6,7]. In their publications, a crystal field calculations based on the angular overlap model (AOM) were performed for this complex.…”