[2 + 2]-type Reaction of Metal-Metal σ-Bond with Fullerene Forming an η 1 -C 60 Metal Complex: Mechanistic Details of Formation Reaction and Prediction of a New η 1 -C 60 Metal Complex.

C60 [CpRu(CO)2 ]2 is only one transition-metal fullerene complex with pure η1 -coordinated bonds, which was recently synthesized through the reaction between dinuclear Ru complex [CpRu(CO)2 ]2 and C60 . Though new properties can be expected in the η1 -coordinated metal-fullerene complex, its characteristic features are unclear, and the [2 + 2]-type formation reaction is very slow with a very small yield. A density functional theory study discloses that the η1 -coordinated bond is formed by a large overlap between the Ru dσ orbital and C pσ one involved in the lowest unoccupied molecular orbital (LUMO) (π*) of C60 unlike the well-known η2 -coordinated metal-fullerene complex which has a π-type coordinate bond with metal dπ orbital. The binding energy per one Ru-C bond is much smaller than those of η2 -coordinated Pt(PMe3 )2 (C60 ) and IrH(CO)(PH3 )2 (C60 ) because the Ru d orbital exists at low energy. The formation reaction occurs via Ru-Ru bond cleavage on the C60 surface followed by a direction change of CpRu(CO)2 to afford C60 [CpRu(CO)2 ]2 in a stepwise manner via two asymmetrical transition states to avoid a symmetry-forbidden character. The calculated Gibbs activation energy (ΔG°‡ ) is very large and the Gibbs reaction energy (ΔG°) is moderately negative, which are consistent with a very slow reaction rate and very small yield. The charge transfer from CpRu(CO)2 to fullerene CT(Ru → C60 ) is important in the reaction, but it is small due to the presence of the Ru d orbital at low energy, which is the reason for the large ΔG°‡ and moderately negative ΔG°. The use of Li+ @C60 is theoretically predicted to accelerate the reaction and increase the yield of Li+ @C60 [CpRu(CO)2 ]2 , because the CT(Ru → C60 ) is enhanced by the low energy LUMO of Li+ @C60 . It is also predicted that Li+ @C60 [Re(CO)4 (PMe3 )]2 is a next promising target for the synthesis of the η1 -coordinated metal-fullerene complex, but syntheses of C60 [Co(CO)4 ]2 , C60 [Re(CO)5 ]2 , Li+ @C60 [Co(CO)4 ]2 , and Li+ @C60 [Re(CO)5 ]2 are difficult. The use of nonpolar solvent is another important factor for the synthesis of the η1 -coordinated metal complex with Li+ @C60 .