Single-crystal electronic and electron spin resonance spectra of tetra-(6-aminohexanoic acid)copper(II) diperchlorate

Abstract

The polarised single-crystal electronic spectra of Cu(HA)₄(ClO₄)₂ are assigned in S₄ and D_2d symmetries with a d_xy ground state and yield the one-electron orbital sequence: d_xy > d_x²–y² > d_z²d_xzd_yz. The e.s.r. and electronic spectra yield the orbital reduction factors r_‖= 0·79 and r⊥= 0·77 indicative of negligible π-bonding. These results are compared with the corresponding properties of CaCu(AcO)₄,6H₂O (with the d_x²–y²→d_xy transition assigned to the weak band at 10·8 kK) and a correlation is suggested between the copper–oxygen bond lengths, in-the-plane and out-of-the-plane, the values of T or T*, and the energy of the d_z²→d_xy transition. This implies weak bonding by the off-z-axis oxygen atoms in both complexes, but stronger in CaCu(AcO)₄,6H₂O, and favours a distorted dodecahedral description of the stereochemistry of the CuO₄O′₄ chromophore.