DFT study of the geometry and energy order of the low singlet and triplet states of [d4-η5-CpMo(CO)2X] 16-electron complexes (X = halogen, CN, H, and CH3)

Abstract

DFT methods have been used to investigate the dependence of the geometry and energy order of the low energy states of [d4-η 5-CpMo(CO)2X] 16-electron complexes on X (X = halogen, CN, H and CH3). The calculations use a double-ζ plus polarization valence basis set on all atoms and utilize relativistic ECPs on Mo and the heavier halogens. In every case two singlet and two triplet electronic states have been considered and minimized at the B3LYP level. For X = Cl, additional calculations were carried out at the BPW91, CCSD(T), and CASSCF levels. In the Cs point group, the singlet states are from the (1a′) 2(1a″)2 and (1a′)2(2a′) 2 configurations of the valence d4 electrons of the metal, and are denoted 1A′-a and 1A′-b, respectively. The triplet species are for the lowest 3A″ and 3A′ states from the (1a′)2(2a′) 1(1a″)1 and (1a″)2(1a′) 1(2a′)1 d4 configurations. For all substituents, the geometry of both the singlet and triplet states is found to distort substantially from the uniform 3-leg piano-stool structural motif, a behavior that can be related to Jahn-Teller effects. When X is a halogen or a methyl, 1A′-b is predicted to be lower than 1A′-a, while the reverse order of these two singlet states is calculated for X = H and CN. For all substituents 3A′ is substantially higher than 3A″. In turn, the energy of 3A″ is calculated to be comparable to the lower singlet state of each complex. Attempts are made to rationalize some of these results using qualitative MO theory.