Observation of a magnetic dipole origin in the spectrum of the square planar CuCl42− ion

The low temperature d-d absorption spectrum of bis(2-aminobenzothiazolium) copper tetrachloride is reported. The lowest energy B2g(xy) ← B1g(x2 − y2) transition has a relatively sharp electronic origin which has not previously been observed in a CuCl42− ion. The polarisation properties of this electronic origin show it to be magnetic dipole allowed. The transition also has extensive vibrational structure and several false origins due to coupling with odd-parity vibrations. The direct observation of the electronic origin allows an accurate determination of the excited state geometry.