Spectroscopic and ligand-field analysis of the spin–orbit interaction between the 1 E g and 3 T 2g states in bis(1,4,7-triazacyclononane)nickel( II )

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 D3d 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 3T2g state versus spin–orbit mixing between the 1Eg and 3T2g states were determined in relation to the anomalous double-humped 3A2g→3T2g band envelope. Although the trigonal-field splitting of both 3T1g states is large, it is quite small for the 3T2g state, consequently, the 3A2g→3T2g bandshape is shown to arise predominantly from spin–orbit mixing between the 1Eg and 3T2g states even though the 1Eg 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 eσ(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.