Electronic structure of N₂P₂ four-membered rings

The biradicaloid character of the ground-state structures of N2P2 rings is studied by using the high-level ab initio multiconfigurational CASPT2/CASSCF method. In order to obtain accurate descriptors, we combine two criteria: 1) singlet-triplet energy gaps and 2) relative values of the occupation numbers for bonding and antibonding orbitals associated with the radical sites. The singlet-triplet energy gaps, the occupation numbers of antibonding-like orbitals, and the weights of the main configuration state functions (CSFs) of the ground-state wavefunctions, that is, Ψ((1)A1), are used to derive the biradicaloid character that ranges from 10-15%.