Electronic Structure of N2P2 Four-Membered Rings and the Effect of Their Ligand Coordination to M(CO)5 (Cr, Mo, and W)

Abstract

In this article the biradicaloid character of the ground-state structures of N2P2R2 (R = CH3) rings is studied using the DFT and CASSCF methods, and a satisfactory agreement of the B3LYP functional and CASSCF(6,6) ab initio method has been found. In order to obtain an adequate description of the biradicaloid character, we have combined two criteria: (i) singlet–triplet energy gaps and (ii) relative values of the occupation numbers for bonding and antibonding orbitals associated with the radical sites. We have analyzed how the biradicaloid character of the N2P2R2 ring changes upon coordination to M(CO)5 (M = Cr, Mo, and W) at the B3LYP/6-311+G* level of theory. Interestingly, in some cases the biradicaloid character increases dramatically upon complexation of the N2P2R2 ligands.