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 D2d symmetries with a dxy ground state and yield the one-electron orbital sequence: dxy > dx2–y2 > dz2dxzdyz. 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 dx2–y2→dxy 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 dz2→dxy 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.