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.