Metal-ligand interactions of the Cu-NO complex at the ground and low-lying excited states: an ab initio MO study

The geometrical structure and vibrational modes of the complex formed between the copper atom and NO molecule for the ground state have been studied by means of an ab initio MO method. The ground and low-lying electronic states were determined by using the singly excited configuration interaction (CI) calculations. The bent structure with a Cu-N-O angle of 117 degrees is obtained as the most stable form of the complex at the MP2/6-311G* level. Harmonic vibrational frequencies calculated for the bent form (1494 and 592 cm(-1)) are in reasonable agreement with the experimental values observed for the CuNO complex in argon matrix at 17 K by a Fourier transform infrared (FTIR) spectroscopy (1611 and 609 cm(-1)). The charge on each atom is calculated to be +0.28 for Cu and -0.28 for the NO molecule at the ground state, suggesting that the bonding nature of the complex has an almost covalent character. The absorption bands of the complex are assigned to be a pi*(in-plane) --> pi* (out-of-plane) local excitation within the NO molecule ((1)A' --> (1)A '') for the first electronic excitation, and a pi*(in-plane) --> 4s back-donation like transition from NO to Cu for the second excitation band appearing in the visible region. A third absorption band is assigned to be a pi*(in-plane) --> 4p back-donation like transition from NO to the copper atom. (C) 1997 by John Wiley & Sons, Ltd.