Coupled Cluster Study of the Interactions of AnO, AnO, and AnO (An = U, Np) with N and CO.

Thermochemical and spectroscopic properties for actinyl complexes involving UO and NpO with N and CO, together with the UO-O, UO-O, and UO-NO complexes, have been studied for the first time using an accurate composite coupled cluster approach. Two general bonding motifs were investigated, end-on (η) and side-on (η) relative to the metal center of the actinyls. For end-on CO complexes, both C-coordinated (An-C) and O-coordinated (An-O) structures were examined, with the former always being lower in energy. All of the η complexes were calculated to be stable, with dissociation energies ranging from 2 to 36 kcal/mol, except for that of UO-O (the η orientation for UO-NO was not amenable to single reference coupled cluster). In agreement with a previous study, the η structure for UO-O was calculated to be relatively strongly bound, by 22.3 kcal/mol in this work. The closely related NO complex, however, had a calculated dissociation energy of just 4.0 kcal/mol. The binding energy of O to neutral UO in a η orientation was calculated to be very strong, 75.4 kcal/mol, and strongly resembled a UO(O) complex at equilibrium. The N-N and C-O bonds were found to be somewhat activated for all the side-on (η) neutral An(IV) complexes, with stretching frequencies of N or CO being red-shifted by as much as 480 cm with a 0.06 Å bond length elongation. Dissociation energies for the η complexes are strongly correlated with the extent of electron transfer from ligand to actinyl. The nature of bonding in the actinyl complexes is examined using natural resonance theory (NRT). The correlation between bonding motif and small molecule activation is in agreement with experiments in condensed phases.