[60]Fullerene-Metal Cluster Complexes: Understanding Novel η1 and η2[6:5] Bonding Modes of Metallofullerenes

We performed extensive density functional calculations on various metallofullerene complexes and their polyanions to gain insight into novel eta(1) and eta(2[6:5]) metal (M)-C-60 bonding modes. For LnMC60 (L = ligand), the eta(1) mode is calculated to be the most stable, followed by eta(2[6:5]) and eta(2[6:6]) for -3 anions, in contrast to eta(2[6:6]) >> eta(2[6:5]) approximate to eta(1) for neutral cases. This observation is responsible for the transformation from 1121661 to eta(1) for LnM3C60, such as [Os-3(CO)(9)C-60], upon successive electron reductions. Our energy partitioning analysis (EPA) indicates that the pi-type character of eta(2[6:6]) is much larger than that of eta(2[6:5]). An electron addition decreases the pi-type interaction of both the eta(2[6:6]) and eta(2[6:5]) modes by about 35%, whereas it has little effect on sigma-type interactions. Because of the large proportion of pi-character in eta(2[6:6]) coordination, the stability of eta(2[6:6]) coordination decreases steeply as electron reductions continue. On the basis of the EPA results, we could explain why the reaction of [Os-3(CO)(8)(CNR)(mu(3)-eta(2[6:6]), eta(2[6:6]), eta(2[6:6])-C-60)] (R = CH2Ph) with CNR (4e donor) produces [Os-3(CO)(8)(CNR)(mu(3)-CNR)(mu(3)-eta(1),eta(2[6:5]),eta(1)-C-60)]. The eta(1) and eta(2[6:5]) bonding modes of M-C-60 are crucial to fully understand the bonding nature of M-C-60 bonds in exohedral metallofullerene complexes.