A Class of Halide-Supported Trinuclear Nickel Clusters [Ni₃(μ₃-L)(μ₃-X)(μ₂-dppm)₃]<i>ⁿ</i>⁺ (L = I^-, Br^-, CO, CNR; X = I^-, Br^-; <i>n</i> = 0, 1; dppm = Ph₂PCH₂PPh₂): Novel Physical Properties and the Fermi Resonance of Symmetric μ₃-η¹ Bound Isocyanide Ligands

The dihalide capped trinuclear nickel clusters Ni3(μ3-X)2(μ2-dppm)3 (X = I (1a), Br (1b); dppm = Ph2PCH2PPh2) were synthesized and converted to their respective monocations [Ni3(μ3-X)2(μ2-dppm)3]+ (X = I (1a+), Br (1b+)) via a single electron oxidation. Clusters 1a and 1a+ were characterized by X-ray crystallography. Displacement of a triply-bridging iodide ligand in Ni3(μ3-I)2(μ2-dppm)3 (1a) by π-acceptor ligands produces a class of 48-electron trinuclear nickel clusters of the general formula [Ni3(μ3-L)(μ3-I)(μ2-dppm)3]+ (L = CO (2a); CNR, R = CH3 (3a), 2,6-(CH3)2C6H3 (4a), i-C3H7 (5), C6H11 (6), t-C4H9 (7), CH2C6H5 (8), C6H5 (9), p-C6H4I (10), p-C6H4Br (11), p-C6H4Cl (12), p-C6H4F (13), p-C6H4CH3 (14), p-C6H4CF3 (15), p-C6H4OCH3 (16), and p-C6H4CN (17)). A similar but less extensive study was conducted with the dibromide capped cluster 1b. The X-ray crystal structure of cluster 2a, as the PF6- salt, was also obtained. Clusters 2−17 possess strikingly similar spectroscopic and electrochemical properties. This is ascribed to the lack of interaction between the a2 LUMO of clusters 2−17 and the molecular orbitals of the capping π-acceptor ligands. The unexpected appearance of two ν(C⋮N) bands in the FT-IR spectra of clusters 3−17 was demonstrated to be the result of a Fermi resonance involving the ν(C⋮N) fundamental and the first overtone of the ν(N−C(alkyl)) fundamental of the capping isocyanide. In addition, molecular orbital calculations on 1−17 provide insights into the differences in the physical properties and reactivities of clusters of this class capped by π-donor (1) or π-acceptor ligands (2−17).