Electronic Structure of N<sub>2</sub>P<sub>2</sub> Four-Membered Rings and the Effect of Their Ligand Coordination
to M(CO)<sub>5</sub> (Cr, Mo, and W)
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Abstract
In
this article the biradicaloid character of the ground-state
structures of N<sub>2</sub>P<sub>2</sub>R<sub>2</sub> (R = CH<sub>3</sub>) 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 N<sub>2</sub>P<sub>2</sub>R<sub>2</sub> ring changes upon coordination
to M(CO)<sub>5</sub> (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 N<sub>2</sub>P<sub>2</sub>R<sub>2</sub> ligands
