Binding of nitric oxide to first-transition-row metal cations: An ab initio study
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Equilibrium geometries and binding energies have been determined for several states of the transition metal nitrosyl cations, M(NO)+, for the first-transition-row metals, scandium through copper, (M = Sc-Cu). The geometries were optimized using density functional theory (DFT) with a hybrid functional (B3LYP). Our calculations predict that the ground states for Sc(NO)+, Ti(NO)+, and V(NO)+ have side-on geometries with the N and O approximately equidistant from the metal center. In these structures, N and O both form covalent bonds with the metal center. The ground states of M(NO)+ for chromium through nickel are linearly bound at the nitrogen and Cr+-Co+ form bonds that are primarily electrostatic and dative in nature. Ground-state Ni(NO)+ is more strongly bound than the other linear M(NO)+ complexes, due to a larger contribution from NO to metal charge transfer in the bonding. Ground-state Cu(NO)+ has a bent structure with a one-electron bond between the Cu and N. All the ground-state electronic configurations are dominated by dn+1 occupations of the metals. Binding energies were calculated with both DFT and the coupled cluster approximation with single and double excitations and perturbational estimate of the triple excitations (CCSD(T)) and corrected for zero-point energy. The binding energies for the ground-state complexes calculated with respect to the ground states of the metal ions at the CCSD(T) level increase from Sc to Ti, decrease to Mn, then increase again to nickel, decreasing again to copper. We found that the DFT binding energies for the ground-state complexes in this system were larger than the CCSD(T) values by as little as 3 kcal/mol for Sc(NO)+ and Co(NO)+ and as much as 17 kcal/mol for Mn(NO)+, except for Ti(NO)+ and Ni(NO)+, where the DFT binding energies are 6.3 and 7.4 kcal/mol smaller than the CCSD(T) value, respectively. The weaker bond strengths in the middle of the transition row can be attributed to the dominance of electrostatic contributions in the bonding of these M(NO)+ complexes. Excluding Cu, the M-NO bonds are stronger at either end of the row where the contribution from covalent bonding is larger.
