Large differences in secondary metal-arene interactions in the transition-metal dimers ArMMAr (Ar = terphenyl; M = Cr, Fe, or Co): Implications for Cr-Cr quintuple bonding

Quantum mechanical calculations, using both CASPT2 and DFT methods, for the model systems (MeMMMe, PhMMPh, (MeMMMe)(C6H6)(2), ArMMAr, (ArMMAr#)-M-#; M = Cr, Fe, Co; Ar = C6H4-2(C6H5), Ar-# = C6H3-2,6(C6H3-2,6-Me-2)(2)) are described. These studies were undertaken to provide a multireference description of the metal-metal bond in the simple dimers MeMMMe and PhMMPh (M = Cr, Fe, Co) and to determine the extent of secondary metal-arene interaction involving the flanking aryl rings of the terphenyl ligands in quintuply bonded Ar'CrCrAr' (Ar'= C6H3-2,6(C6H3-2,6-Pr'(2))(2)). We show that in the Cr-Cr species the Cr-arene interaction is a feeble one that causes only a small weakening of the quintuple bond. In sharp contrast, in the analogous Fe and Co species strong eta(6)-arene interactions that preclude significant metal-metal bonding are predicted.