Quantum mechanical calculations, using both CASPT2 and DFT methods, for the model systems (MeMMMe, PhMMPh, (MeMMMe)(C₆H₆)₂, Ar^§MMAr^§, Ar^#MMAr^#; M = Cr, Fe, Co; Ar^§ = C₆H₄-2(C₆H₅), Ar^# = C₆H₃-2,6(C₆H₃-2,6-Me₂)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-Prⁱ₂)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 η⁶-arene interactions that preclude significant metal−metal bonding are predicted.

Multiconfigurational quantum chemical methods show that a quintuple bond is present between the two Cr^I units in the model complex [PhCrCrPh]. The Cr—Cr (1.75 Ã…) and Cr—Ph (2.02 Ã…) bonds are shorter than those in the recently reported compound [Ar'CrCrAr'] (Ar'=2,6-(2,6-iPr₂C₆H₃)₂C₆H₃; 1.83 and 2.15 Ã…, respectively). This difference is attributed to the additional Cr—Ar' interactions.