SYNTHESIS AND CHEMICAL BEHAVIOR OF THE [CO6NI2C2(CO)16]2- CLUSTER - ELECTROGENERATION OF THE [CO6NI2C2(CO)16]N- (N=1, 3, 4) CLUSTER ANIONS - CRYSTAL AND MOLECULAR-STRUCTURES OF [NET4]2[CO6NI2C2(CO)16] AND [NET4]2[CO6NI2C2(CO)14(CH3CN)2]

The synthesis, the redox behaviour of the [Co6Ni2C2(CO)16]2- anion and the preparation of the new [CO6Ni2C2(CO)14(CH3CN)2]2- cluster are reported. The [CO6Ni2C2(CO)16]2- anion was selectively obtained by redox condensation between Co3(CO)9CCl and [Co3(CO)9CCl and [Co3Ni9C(CO)20]3- or [Ni6(CO)12]2-, Electrochemical and chemical studies evidenced the rather extended redox behaviour of [Co6Ni2C2(CO)16]2-which exhibits the ability to add, sequentially and reversibly, two electrons as well as to lose one electron reversibly. In tetrahydrofuran solution, the formal electrode potentials (vs. SCE) for the redox changes are as follows: E-degrees'(1-/2-) = -0.04 V, E-degrees'(2-/3-) = -1.24 V, E-degrees'(3-/4-) = -1.48 V. All these [Co6Ni2C(CO)16]n- (n = 1, 3, 4) congeners are not indefinitely stable in solution; however, they are sufficiently long-lived to allow their characterization by ESR spectroscopy. By way of contrast, chemical reduction affords only one relatively stable compound which could be isolated in the solid state but escaped complete characterization. The carbonyl-substituted [Co6Ni2C2(CO)14(CH3CN)2]2- dianion has been obtained on attempting the crystallization of this reduced species in acetonitrile solution, The parent [Co6Ni2C2(CO)16]2- and the carbonyl-substituted [Co6Ni2C2(CO)14(CH3CN)2]2- dianions have been isolated as tetraethylammonium salts, and their structural characterization has been carried out by X-ray diffraction studies. [NEt4]2[Co6Ni2C2(CO)16] crystallizes in the monoclinic space group P2(1)/c, with unit cell dimensions a = 9.015(4) angstrom, b = 20.215(5) angstrom, c = f 3.120(3) angstrom, beta = 113.10(2)-degrees, and Z = 2; R = 0.029 and R(w) = 0.033 for 1578 observed reflections. [NEt4]2[Co6Ni2C2(CO)14(CH3CN)2] crystallizes in the orthorhombic space group Pbca, with unit cell dimensions a = 12.817(2) angstrom, b = 21.306(5) angstrom, c = 16.492(7) angstrom, and Z = 4; R = 0.055 and R(w) = 0.056 for 1076 observed reflections. The structures of the two compounds are very similar and their metal frameworks are based on two fused trigonal prisms sharing a square face. The two trigonal prismatic cavities lodge the carbon atoms which display a rather short C-C interatomic separation of 1.49 angstrom. Selected average distances (angstrom) are: metal-metal 2.603 and 2.605; metal-carbide 1.993 and 1.998, for [Co6Ni2C2(CO)16]2- and [Co6Ni2C2(CO)14(CH3CN)2]2-, respectively.