BINDING OF NITRIC-OXIDE TO FIRST-TRANSITION-ROW METAL-CATIONS - AN AB-INITIO STUDY

Equilibrium geometries and binding energies have been determined for s everal 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) w ith a hybrid functional (B3LYP). Our calculations predict that the gro und states for Sc(NO)(+), Ti(NO)(+), and V(NO)(+) have side-on geometr ies with the N and O approximately equidistant from the metal center. In these structures, N and O both form covalent bonds with the metal c enter. The ground states of M(NO)(+) for chromium through nickel are l inearly bound at the nitrogen and Cr+-Co+ form bonds that are primaril y electrostatic and dative in nature. Ground-state Ni(NO)(+) is more s trongly bound than the other linear M(NO)(+) complexes, due to a large r contribution from NO to metal charge transfer in the bonding. Ground -state Cu(NO)+ has a bent structure with a one-electron bond between t he Cu and N. All the ground-state electronic configurations are domina ted by d(n+1) occupations of the metals. Binding energies were calcula ted with both DFT and the coupled cluster approximation with single an d double excitations and perturbational estimate of the triple excitat ions (CCSD(T)) and corrected for zero-point energy. The binding energi es for the ground-state complexes calculated with respect to the groun d 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 c opper. We found that the DFT binding energies for the ground-state com plexes 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 bindi ng energies are 6.3 and 7.4 kcal/mol smaller than the CCSD(T) value, r espectively. 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 bo nds are stronger at either end of the row where the contribution from covalent bonding is larger.