A Spinning Umbrella: Carbon Monoxide and Dinitrogen Bound MB₁₂^- Clusters (M = Co, Rh, Ir)

Strong binding of carbon monoxide (CO) and dinitrogen (N₂) by MB₁₂^- (M = Co, Rh, Ir) clusters results in a spinning umbrella-like structure. For OCMB₁₂^- and NNMB₁₂^- complexes, the bond dissociation energy values range within 50.3-67.7 kcal/mol and 25.9-35.7 kcal/mol, respectively, with the maximum value obtained in Ir followed by that in Co and Rh analogues. COMB₁₂^- complex is significantly less stable than the corresponding C-side bonded isomer. The associated dissociation processes for OCMB₁₂^- and NNMB₁₂^- into CO or N₂ and MB₁₂^- are highly endergonic in nature at 298 K, implying their high thermochemical stability with respect to dissociation. In OCMB₁₂^- and NNMB₁₂^- complexes, the C-O and N-N bonds are found to be elongated by 0.022-0.035 Å along with a large red-shift in the corresponding stretching frequencies, highlighting the occurrence of bond activation therein toward further reactivity due to complexation. The obtained red-shift is explained by the dominance of L←M π-back-donation (L = CO, OC, NN) over L→M σ-donation. The binding of L enhances the energy barrier for the rotation of the inner B₃ unit within the outer B₉ ring by 0.4-1.8 kcal/mol, which can be explained by a reduction in the distance of the longest bond between inner B₃ and outer B₉ rings upon complexation. A good correlation is found between the change in rotational barrier relative to that in MB₁₂^- and the energy associated with the L→M σ-donation. Born-Oppenheimer molecular dynamics simulations further support that the M-L bonds in the studied systems are kinetically stable enough to retain the original forms during the internal rotation of inner B₃ unit.