Ability of fullerenes to act as eta(6) ligands in transition metal complexes. A comparative PM3(tm)-density functional theory study

High-quality DFT calculations are employed to estimate the arene exchange e nergies for reactions of general formula: (C6H6)M(CO)(3) + C-n --> CnM(CO)(3) + C6H6 n = 60, 70, 78 or 84, M = Cr n = 60, 70, M = Mo or W (C6H6)Co(CmHm) + C-60 --> C60Co(CmHm) + C6H6 m = 3 or 6 For C-60 and C-70 complexes of Cr(CO)(3), full geometry optimizations at th e DFT level using moderately large basis sets were performed, while for the other, systems a hybrid approach was developed in which the geometries wer e obtained at the PM3(tm) level and the energetics were evaluated at the DF T level. C-70 is shown to be a slightly better arene ligand than C60; howev er, no enhancements of arene-like bonding capabilities are seen for C-78 an d C-84 relative to C-70. Explicit calculation of a series of exchange energ ies at the DFT level using both DFT and PM3(tm) geometries demonstrates tha t PM3(tm) geometries are sufficiently accurate for the calculation of energ etics at a higher theoretical level, but PM3(tm) calculations are inadequat e for a quantitative assessment of exchange energies. (C) 2001 John Wiley & Sons, Inc.