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].

晚期的双核过渡金属（尤其是第10组和第11组）均相羰基配合物为难以捉摸的物种，迄今为止尚未实现分离。一个典型的例子是30电子物种[Ni2(CO)5]，其结构和键合至今仍存在争议。本研究通过使用AlCp*配体（与CO等孤立），成功实现了[Ni2(AlCp*)5](1)的分离和全面表征，这激发我们通过DFT计算重新审视[Ni2L5](L = CO, AlCp*)及其他同电子物种的键合情况。在化合物1中，Ni-Ni的短X射线距离（2.270 Å）不应归因于金属间典型局部三键的存在，而应归因于涉及三个桥接配体的强键合相互作用，这些配体通过其捐赠孤对电子和接受π*轨道。相比之下，在结构相同的32电子[Au2(AlCp*)5](2)簇中，一个具有M-M反键和Al...Al键合特性的轨道被占据，这与特别长的Au-Au距离（3.856 Å）以及桥接配体之间相对较短的Al...Al接触（2.843 Å）相一致。这项工作表明，与晚期的过渡金属[M2(CO)x]物种不同，稳定的[M2(AlCp*)x]配合物可以通过CO与AlCp*之间的微妙差异实现分离。我们提出了一种类似的方法来解释标志性34电子物种[Fe2(CO)9]中的键合。