Synthesis and characterisation of v3-octahedral [Ni36Pd8(CO)48]6- and [Ni35Pt9(CO)48]6- clusters displaying unexpected surface segregation of Pt atoms and molecular and/or crystal substitutional Ni/Pd and Ni/Pt disorder

The synthesis and structure, as well as the chemical and electrochemical characterisation of two new nu(3)-octahedral bimetallic clusters with the general [Ni44-xMx(CO)(48)](6-) (M = Pd, x = 8; M = Pt, x = 9) formula is reported. The [Ni35Pt9(CO)(48)](6-) cluster was obtained in reasonable yields (56% based on Pt) by reaction of [Ni-6(CO)(12)](2-) with 1.1 equivalents of Pt-II complexes, in ethyl acetate or THF as the solvent. The [Ni36Pd8(CO)(48)](6-) cluster was obtained from the related reaction with Pd-II salts in THF, and was isolated only in low yields (5-10% based on Pd), mainly because of insufficient differential solubility of its salts. The unit cell of the [NBu4](6)[Ni35Pt9(CO)(48)] salt contains a substitutionally Ni-Pt disordered [Ni-24(Ni14-xPtx)Pt-6 (CO)(48)](6-) (x = 3) hexaanion. A combination of crystal and molecular disorder is necessary to explain the disordering observed for the Ni/Pt sites. ne unit cell of the corresponding [Ni36Pd8(CO)(48)](6-) salt contains two independent [Ni-30(Ni8-xPdx)Pd-6(CO)(48)](6-) (x = 2) hexaanions. The two display similar substitutional Ni-Pd disorder, which probably arises only from crystal disorder. The structure of [Ni36Pd8(CO)(48)](6-) establishes the first Ni-Pt disordered [Ni-24(Ni14-xPtx)Pt-6- (CO)(48)](6-) (x = 3) hexaanion. A combination of crystal and molecular disorder is necessary to explain the disordering observed for the Ni/Pt sites. ne unit cell of the corresponding [Ni36Pd8(CO)(48)](6-) salt contains two independent [Ni-30(Ni8-xPdx)Pd-6(CO)(48)](6-) (x = 2) hexaanions. The two display similar substitutional Ni-Pd disorder, which probably arises only from crystal disorder. The structure of [Ni36Pd8(CO)(48)](6-) establishes the first similarity between the chemistry of Ni-Pd and Ni-Pt carbonyl clusters. A comparison of the chemical and electrochemical properties of [Ni35Pt9(CO)(48)](6-) with those of the related [Ni38Pt6(CO)(48)](6-) cluster shows that surface colouring of the latter with Pt atoms decreases redox as well as protonation propensity of the cluster. In contrast, substitution of all internal Pt and two surface Ni with Pd atoms preserves the protonation behaviour and is only detrimental with respect to its redox aptitude. A qualitative rationalisation of the different surface-site selectivity of Pt and Pd, based on distinctive interplays of M-M and M-CO bond energies, is suggested.