Ten-Electron Donor Indenyl Anion in Binuclear Transition-Metal Sandwich Complexes: Electronic Structure and Bonding Analysis

Whereas dinuclear sandwich complexes of pentalene, azulene and naphthalene are not uncommon, their bis-indenyl counterparts are scarce, with only two M-2(indenyl) 2 examples known so far. This paper investigates by the means of DFT calculations the possibility for such compounds to exist for the M= Sc-Ni and Y-(Re)-Pd series. Stable diamagnetic species are predicted for total valence electron counts (TNE) of 34 and 40. The most favored situation corresponds to TNE = 34. For this electron count, which exhibits full ligand hapticity, no formal metal-metal bond exists, but only weak non-bonding attraction. Adding supplementary electrons (TNE > 34) results in partial ligand decoordination. When TNE < 34, metal-metal bonding is predicted to compensate electron deficiency, with the largest formal bond order for TNE = 26. However, in the case of the series of the first-row metals, the metal-metal interaction is rather weak, thus favoring the existence of low-lying high-spin states. On the other hand, all the second-row (and Re) complexes are computed to be diamagnetic. It is concluded that there is no specific reason for M-2(indenyl) 2 complexes not being isolable.