Unraveling the Major Differences between the Trinuclear Cyclopentadienylmetal Carbonyl Chemistry of Cobalt and That of NickelA Theoretical Study

The geometries and energetics of the trinuclear cyclopentadienylmetal carbonyls Cp3M3(CO)n (Cp = η5-C5H5); M = Co, Ni; n = 3, 2, 1, 0) have been investigated by density functional theory. The cobalt and nickel systems are found to be rather different owing to the different electronic configurations of the metal atoms. For cobalt, the small calculated energy separation of 5.0 kcal/mol between the two lowest-energy singlet Cp3Co3(μ3-CO)(μ-CO)2 and Cp3Co3(μ-CO)3 tricarbonyl structures accounts for the experimental results of both isomers as stable species that can be isolated and structurally characterized by X-ray crystallography. The corresponding Cp3Ni3(CO)3 species in the nickel system are predicted not to be viable owing to exothermic CO dissociation to give the experimentally observed very stable Cp3Ni3(μ-CO)2, which is found to be the lowest-energy isomer by a substantial margin of ∼25 kcal/mol. In all of the low-energy Cp3M3(CO)n (n = 2, 1) structures, including that of the experimentally known triplet spin state Cp3Co3(μ3-CO)2, all of the carbonyl groups are face-bridging or face-semi-bridging μ3-CO groups bonded to all three metal atoms of the M3 triangle. In the lowest-energy carbonyl-free Cp3M3 (M = Co, Ni) structures, agostic C–H–M interactions are found using hydrogens of the Cp rings. In addition, the lowest-energy Cp3Ni3 is the only structure among all of the low-energy Cp3M3(CO)n (M = Co, Ni; n = 3, 2, 1, 0) structures in which each Cp ring is a bridging rather than terminal ligand.

本研究采用密度泛函理论（density functional theory），对三核环戊二烯基金属羰基化合物Cp₃M₃(CO)ₙ（其中Cp代表η⁵-环戊二烯基，即η⁵-C₅H₅；M为Co、Ni；n=3、2、1、0）的几何结构与能量学特征开展了系统研究。由于金属原子的电子构型存在差异，钴系与镍系体系呈现出显著不同的行为特征。对于钴系体系，两种最低能量单重态Cp₃Co₃(μ₃-CO)(μ-CO)₂与Cp₃Co₃(μ-CO)₃三羰基结构之间的计算能量差仅为5.0 kcal/mol，这一结果完美解释了两种异构体均可作为稳定物种被分离，并通过X射线晶体学（X-ray crystallography）进行结构表征的实验观测结果。而镍系中对应的Cp₃Ni₃(CO)₃物种则被预测无法稳定存在，因其会发生放热的CO解离反应，生成实验中已观测到的稳定性极高的Cp₃Ni₃(μ-CO)₂；该物种被证实为镍系的最低能量异构体，能量优势高达约25 kcal/mol。在所有低能Cp₃M₃(CO)ₙ（n=2、1）结构中，包括实验上已确证的三重自旋态Cp₃Co₃(μ₃-CO)₂，所有羰基均为面桥或面半桥型μ₃-CO基团，与M₃三角形的全部三个金属原子形成配位键。在能量最低的无羰基Cp₃M₃（M=Co、Ni）结构中，观测到了源自Cp环氢原子的agostic C-H-M相互作用。此外，在所有低能Cp₃M₃(CO)ₙ（M=Co、Ni；n=3、2、1、0）结构中，能量最低的Cp₃Ni₃是唯一一种每个Cp环均作为桥联配体而非端基配体存在的结构。