Spectroscopic and ligand-field analysis of the spin–orbit interaction between the 1Eg and 3T2g states in bis(1,4,7-triazacyclononane)nickel(II)

Abstract

The low-temperature single-crystal absorption spectrum, far-infrared spectrum and single-crystal structure determination of [Ni(tacn)₂][ClO₄]₂·H₂O (tacn = 1,4,7-triazacyclononane) are reported. The compound crystallizes in the monoclinic space group P2₁/a(Z= 2) with a= 9.614(3), b= 13.451(6), c= 9.648(4)Å and β= 120.11(1)°. The [Ni(tacn)₂]²⁺ complex ion is centrosymmetric, possessing approximate D_3d molecular symmetry. Trigonal splitting of the NiN₆ octahedral modes of vibration as well as several macrocyclic ring-deformation modes were observed in the far-infrared spectrum. From a detailed ligand-field analysis using the angular overlap model, the relative contributions of low-symmetry splitting and spin–orbit coupling within the ³T_2g state versus spin–orbit mixing between the ¹E_g and ³T_2g states were determined in relation to the anomalous double-humped ³A_2g→³T_2g band envelope. Although the trigonal-field splitting of both ³T_1g states is large, it is quite small for the ³T_2g state, consequently, the ³A_2g→³T_2g bandshape is shown to arise predominantly from spin–orbit mixing between the ¹E_g and ³T_2g states even though the ¹E_g state contributes less than 10% to the overall band envelope. The double-humped bandshape feature is shown to be the result of two relatively narrow bands corresponding to transitions to spin–orbit levels which contain significant spin-singlet character. The best fit ligand-field parameters are e_σ(N)= 4340, B= 840, C= 2830 and ζ= 500 cm^–1. The Racah parameters B and C are in good agreement with those found for other NiN₆ complexes.