Metal–Metal Bonding in Late Transition-Metal [M₂L₅] Complexes: Exploring the Limits of the Isolobal Analogy between the CO and AlCp* Ligands

Late dinuclear transition-metal (especially group 10 and 11) homoleptic carbonyl complexes are elusive species and have so far not been isolated. A typical example is the 30-electron species [Ni₂(CO)₅], the structure and bonding of which is still debated. We show that, by using the AlCp* ligand (isolobal to CO), it is possible to isolate and fully characterize [Ni₂(AlCp*)₅] (1), which inspired us to revisit by DFT calculations, the bonding situation within [Ni₂L₅] (L = CO, AlCp*) and other isoelectronic species. The short Ni–Ni X-ray distance in 1 (2.270 Å) should not be attributed to the existence of a typical localized triple-bond between the metals, but rather to a strong through-bond interaction involving the three bridging ligands via their donating lone pairs and accepting π* orbitals. In contrast, in the isostructural 32-electron [Au₂(AlCp*)₅] (2) cluster an orbital with M–M antibonding and Al...Al bonding character is occupied, which is in accordance with the particularly long Au–Au distance (3.856 Å) and rather short Al...Al contacts between the bridging ligands (2.843 Å). This work shows that, unlike late transition-metal [M₂(CO)_x] species, stable [M₂(AlCp*)_x] complexes can be isolated, owing to the subtle differences between CO and AlCp*. We propose a similar approach for rationalizing the bonding in the emblematic 34 electron species [Fe₂(CO)₉].