An Intermediate Neglect of Differential Overlap Technique for Spectroscopy of Transition-Metal Complexes. Ferrocene

Abstract

A new intermediate neglect of differential overlap (INDO-SCF-CI) method capable of calculating configuration interaction for transition-metal complexes is described. The technique is characterized by the use of atomic spectroscopic information in the formation of one-center, one-electron matrix elements and in the evaluation of two-electron integrals. All onecentered integrals that mix upon geometric rotations are found to be essential for calculation of configuration interaction and are retained. The method is applied to the calculation of the photoelectric and electronic spectra of ferrocene. Triplet and singlet state d → d* and charge-transfer transitions are considered. Three nearly degenerate triplet states are calculated for ferrocene at ~20 500 cm-1 as compared to an observed triplet state at 18 900 cm-1 Three singlet transitions of the d → d* type are calculated at 21 700, 23 900, and 31 900 cm-1, in very good accord with the experimental observations and assignments of “ at 21 800 cm-1, 1E2” at 24 000 cm-1, and 1E1” at 30 800 cm-1. The lowest charge-transfer excitation is calculated at 36 900 cm-1 compared to the experimental observation at 37 700 cm-1. A considerable number of states above 36 900 cm-1 are calculated and a possible assignment of the observed states in the higher energy region is given. The calculated energies allow spectral assignment in good agreement with experiment and help resolve prior ambiguities in the assignment. The relative energies calculated for the ionic states of the ferroceniun ion (3E2' < 2A1’ < 2E1”) are in good accord with previous ab initio calculations. The first two of these states are formed from the formal loss of an electron from a metal orbital; the third, from the loss of an electron from a ligand orbital. For all of these states relaxation upon ionization is so significant that the net charge on iron increases only negligibly from +1.9 in ferrocene to +2.0 in the ferrocene ion. This observation is also in agreement with ab initio findings and has some support from Móssbauer spectroscopy. © 1980, American Chemical Society. All rights reserved.