Trinuclear nickel clusters, [Ni3(μ3-L)(μ3-I)(μ2-dppm)3]n+ (L = I−, CO, CNCH3, CN-2,6-Me2C6H3, CN-i-C3H7, CN-t-C4H9, CN-n-C4H9, NO+) and the “dimer of trimer” hexanuclear nickel clusters {[Ni3(μ3-I)(μ2-dppm)3]2(μ3,μ3′,η1,η1′- CN-R-NC)}2+, (R = −(CH2)6−, p-C6H4): characterization of large clusters by mass spectrometry

Structural, Spectroscopic, and Electrochemical Studies of the Complexes [Ni2(μ-CNR)(CNR)2(μ-dppm)2]n+ (n = 0, 1, 2): Unusual Examples of Nickel(0)−Nickel(I) and Nickel(0)−Nickel(II) Mixed Valency

Reaction of Ni(COD)(2) (COD = cyclooctadiene) with dppm (dppm = bis(diphenylphosphino) methane) followed by addition of alkyl or aryl isocyanides yields the class of nickel(0) dimers Ni(2)(mu-CNR)(CNR)(2)(mu-dppm)(2) (R = CH(3) (1), n-C(4)H(9) (2), CH(2)C(6)H(5) (3), i-C(3)H(7) (4), C(6)H(11) (5), t-C(4)H(9) (6), p-IC(6)H(4) (7), 2,6-(CH(3))(2)C(6)H(3) (8)). The cyclic voltammograms of the dimers exhibit two sequential single electron oxidations to the +1 and +2 forms. Specular reflectance infrared spectroelectrochemical (IRSEC) measurements demonstrate reversible interconversions between the neutral Ni(0) dimers and their +1 and +2 forms. Bulk samples of the +2 forms are prepared by chemical oxidation using [FeCp(2)][PF(6)], while the +1 forms are prepared by the comproportionation of neutral and +2 forms. The neutral complexes 6 and 8 were characterized by X-ray diffraction as symmetric, locally tetrahedral binuclear Ni(0) complexes. The +2 forms of these complexes, 6(2+) and 8(2+), have asymmetric structures with one locally square planar and one locally tetrahedral metal center, evidence for a Ni(II)-Ni(0) mixed valence state. The X-ray structural characterization of 6(+) is symmetrical and qualitatively similar to that of the neutral complex 6. The +1 forms all exhibit intense near IR electronic absorptions that are assigned as intervalence charge transfer (IVCT) bands. On the basis of structural, spectroscopic, and electrochemical data, the +1 forms of the complexes, 1(+)-8(+), are assigned as Robin-Day class III, fully delocalized Ni(+0.5)-Ni(+0.5) mixed valence complexes.

Reflectance infrared and UV–visible spectroelectrochemical studies on a series of [Ni3(µ3-L)(µ3-I)(µ2-dppm)3][PF6] (L = CO; CNR, R = alkyl, aryl; dppm = Ph2PCH2PPh2) clusters

The trinuclear nickel clusters [Ni3(µ3-L)(µ3-I)(µ2-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-C6H4CH3 (10), p-C6H4CF3 (11), p-C6H4OCH3 (12), p-C6H4CN (13), 2,6-(CH3)2C6H3 (14); dppm = Ph2PCH2PPh2) all contain a triply bridging π-acceptor (carbonyl or isocyanide) ligand. Compounds 1–14 all undergo single electron reductions over a relatively narrow range of E1/2( +/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 (µ3,η1) upon single electron reduction. The magnitude of the ν(C≡O) or ν(C≡N) absorption band shift upon reduction is related to both the electronic and steric properties of the capping π-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. Key words: nickel, cluster, carbonyl, isocyanide, spectroelectrochemical.