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The binary cyanide, Cu(CN)2, has never been synthesised. However, by using Ni(CN)42- as a structural synthon, copper(II) can be stabilised in a cyanide-only environment in CuNi(CN)4 and in the solid-solution CuxNi1+x(CN)2 (0 ≤ x < 1/4). The atomic structure of the layers in CuNi(CN)4 and the stacking relationship between nearest-neighbour layers have been determined from total neutron diffraction studies at 10 and 300 K. The structure consists of flat layers of square-planar, ordered [Ni(CN)4] and [Cu(NC)4] units. (NB it is very unusual to find Cu(II) in a genuine square-planar environment within an extended solid). The layered structure of this new material is closely related to those of the Group 10 cyanides, Ni(CN)2, Pd(CN)2.xNH3 and Pt(CN)2.xH2O, except that these are generated from vertex-sharing square-planar metal-cyanide units with head-to-tail disorder of the C≡N groups. The overall appearance of the powder X-ray diffraction pattern of CuNi(CN)4, including the unusual peak shapes of the observed Bragg reflections, has been successfully explained using models incorporating stacking disorder between next nearest neighbour layers. CuNi(CN)4 forms less extended sheets than Ni(CN)2 [1, 2], but larger sheets than those found in Pd(CN)2.xNH3 and Pt(CN)2.xH2O, which are nanocrystalline [3]. CuNi(CN)4, like Ni(CN)2 [1], shows interesting thermal expansion behavior i.e. intralayer negative thermal expansion (αa = -9.7 MK-1) and interlayer positive thermal expansion (αc = +89 MK-1). CuNi(CN)4 forms as an anhydrous material from aqueous solution, unlike Ni(CN)2, which can form hydrates such as Ni(CN)2.nH2O (n = 3, 3/2, 2, 1). However, on investigating the Cu-Ni-CN phase diagram, it is found that nickel-rich compounds, CuxNi1+x(CN)2 (0 ≤ x < 1/4), can be formed via hydrated phases, CuxNi1+x(CN)2.3H2O, and readily rehydrate. Attempts to form copper-rich phases results in partial reduction of Cu(II) with the formation of copper(I) cyanide, CuCN, in addition to CuNi(CN)4, suggesting that Cu(II) ions are stable in a cyanide environment only when connected to the nitrogen ends of bridging C≡N ligands. Hence the non existence of Cu(CN)2 can now be explained.
