Exohedral η⁵ and η⁶ transition-metal organometallic complexes of C₆₀ and C₇₀: a theoretical study

Abstract

Suitable transition-metal fragments are proposed to overcome the unfavorable interaction arising from the splayed out π-orbitals of the five- and six-membered rings of C₆₀ and C₇₀ in complex formation. Computations carried out at the semiempirical PM3(tm) level on a series of C₆₀MC_nH_n and C₇₀MC_nH_n complexes suggest that it is possible to stabilize η⁶ complexes of C₆₀ and C₇₀ using appropriate transition-metal fragments. Isodesmic equations of the type C_mH_mMC_nH_n + C_k → C_kMC_nH_n + C_mH_m, where k = 60, 70, m = 5, 6, and n = 3-6, indicate that C₃H₃Co and C₃H₃Rh are ideal fragments for stabilizing η⁶-C₆₀ complexes. Model studies at HF/LANL2DZ and B3LYP/LANL2DZ levels on C₆H₆ML_n complexes with C₆H₆ constrained so as to mimic the C₆₀ geometry support these results. Single-point calculations at these same levels on complexes of sumanene (C₂₁H₁₂) using geometry optimized at the PM3(tm) method also lead to similar conclusions. η⁵ complexes are less favorable in comparison; structural modifications such as those in the recently synthesized C60Ph6 should readily help in η⁵ bonding. However, the difficulty of formation of the mononuclear η⁵-C₆₀ML_n complex is reduced considerably if all 12 five-membered rings are used in η⁵ bonding as in C₆₀(NiC₃H₃)₁₂. For C₇₀, however, the stability of the exohedral complexes depends on the position and curvature of the five- and six-membered rings. Capping on the flattest six-membered face leads to maximum stability.