The versatility of pentalene coordination to transition metals: a density functional theory investigation

DFT calculations with full geometry optimization have been carried out on a series of real and hypothetical compounds of the type [CpM(C8H6)], [(CO)3M(C8H6)], [M(C8H6)2], [(CpM)2(C8H6)], [[(CO)3M]2(C8H6)], and [M2(C8H6)2] (M = transition metal). The bonding in all the currently known compounds is rationalized, as well as in the (so far) hypothetical stable complexes. Depending on the electron count and the nature of the metal(s), eta2 (predicted), eta3, eta5, eta8, or intermediate coordination modes can be adopted. In the case of the mononuclear species, the most favored closed-shell electron counts are 18 and 16 metal valence electrons (MVE). In the case of the dinuclear species, an electron count of 34 MVEs is most favored. However, other electron counts can be stabilized, especially in the case of dinuclear complexes. Coordinated pentalene should most often be considered as formally being a dianion, but sometimes as a neutral ligand. In the former case it can behave as an aromatic species made of two equivalent fused rings, as a C5 aromatic ring connected to an allylic anion, or even as two allylic anions bridged by a C7=C8 double bond. In the latter case, it can behave as a bond-alternating cyclic polyene or as a C5 aromatic ring connected to an allylic cation.