Reflectance infrared and UV-visible spectroelectrochemical studies on a series of [Ni-3(mu(3)-L)(mu(3)-I)(mu(2)-dppm)(3)][PF6] (L=CO; CNR, R equals alkyl, aryl; dppm equals Ph(2)PCH(2)PPh(2)) clusters

The trinuclear nickel clusters [Ni-3(mu(3)-L)(mu(3)-I)(mu(2)-dppm)(3)](+) (L = CO (1); CNR, R = CH3 (2), i-C3H7 (3), C6H11 (4), t-C4H9 (5), CH2C6H5 (6), C6H5 (7), p-C6H4I (8), p-C6H4F (9), p-C6H2CH3 (10), p-C6H4CF3 (11), p-C6H4OCH3 (12), p-C6H4CN (13), 2,6-(CH3)(2)C6H3 (14); dppm = Ph(2)PCH(2)PPh(2)) all contain a triply bridging pi-acceptor (carbonyl or isocyanide) ligand. Compounds 1-14 all undergo single electron reductions over a relatively narrow range of E(1/2)(+/0) (-1.08 V to -1.18 V vs. SCE in acetonitrile) and are known electrocatalysts for the reduction of carbon dioxide. Specular reflectance infrared spectroelectrochemical (SEC) measurements on 1-14 indicate that the capping isocyanide or carbonyl ligand remains triply bridging (mu g,eta(1)) upon single electron reduction. The magnitude of the nu(C=O) or nu(C=N) absorption band shift upon reduction is related to both the electronic and steric properties of the capping pi-acceptor ligand. Spectroelectrochemical studies with UV-visible detection revealed a hypsochromic shift upon reduction of the clusters. The SEC cell and spectrometer utilized are extremely versatile and allow for data from 600 to 22 000 cm(-1) to be acquired without modifying the SEC cell and making only minor configuration changes to the spectrometer.