Exohedral eta(5) and eta(6) transition-metal organometallic complexes of C-60 and C-70: A theoretical study

Suitable transition-metal fragments are proposed to overcome the unfavorabl e interaction arising from the splayed out pi-orbitals of the five- and six -membered rings of C-60 and C-70 in complex formation. Computations carried out at the semiempirical PMS(tm) level on a series of C60MCnHn and C70MCnH n complexes suggest that it is possible to stabilize eta(6) complexes of C- 60 and C-70 using appropriate transition-metal fragments. Isodesmic equatio ns of the type CmHmMCnHn + C-k --> CkMCnHn + CmHm, where k = 60, 70, m = 5, 6, and n = 3-6, indicate that C3H3Co and C3H3Rh are ideal fragments for st abilizing eta(6)-C-60 complexes. Model studies at HF/LANL2DZ and B3LYP/LANL 2DZ levels on C6H6MLn complexes with C6H6 constrained so as to mimic the C6 0 geometry support these results. Single-point calculations at these same l evels on complexes of sumanene (C21H12) using geometry optimized at the PM3 (tm) method also lead to similar conclusions, eta(5) complexes are less fav orable in comparison; structural modifications such as those in the recentl y synthesized C60Ph6 should readily help in eta(5) bonding. However, the di fficulty of formation of the mononuclear eta(5)-C60MLn complex is reduced c onsiderably if all 12 five-membered rings are used in eta(5) bonding as in C-60(NiC3H3)(12) For C-70, however, the stability of the exohedral complexe s depends on the position and curvature of the five- and six-membered rings . Capping on the flattest six-membered face leads to maximum stability.