Local and average structure in zinc cyanide: toward an understanding of the atomistic origin of negative thermal expansion.

Neutron diffraction at 11.4 and 295 K and solid-state (67)Zn NMR are used to determine both the local and the average structures in the disordered, negative thermal expansion (NTE) material, Zn(CN)2. Solid-state NMR not only confirms that there is head-to-tail disorder of the C≡N groups present in the solid, but yields information about the relative abundances of the different Zn(CN)4­n(NC)n tetrahedral species, which do not follow a simple binomial distribution. The Zn(CN)4 and Zn(NC)4 species occur with much lower probabilities than are predicted by binomial theory, supporting the conclusion that they are of higher energy than the other local arrangements. The lowest energy arrangement is Zn(CN)2(NC)2. The use of total neutron diffraction at 11.4 K, with analysis of both the Bragg diffraction and the derived total correlation function, yields the first experimental determination of the individual Zn­N and Zn­C bond lengths as 1.969(2) and 2.030(2) Å, respectively. The very small difference in bond lengths, of ~0.06 Å, means that it is impossible to obtain these bond lengths using Bragg diffraction in isolation. Total neutron diffraction also provides information on both the average and the local atomic displacements responsible for NTE in Zn(CN)2. The principal motions giving rise to NTE are shown to be those in which the carbon and nitrogen atoms within individual Zn­C≡N­Zn linkages are displaced to the same side of the Zn···Zn axis. Displacements of the carbon and nitrogen atoms to opposite sides of the Zn···Zn axis, suggested previously in X-ray studies as being responsible for NTE behavior, in fact make negligible contributions at temperatures up to 295 K.

Mixed copper, silver, and gold cyanides, (M(x)M'(1-x))CN: tailoring chain structures to influence physical properties.

Binary mixed-metal variants of the one-dimensional MCN compounds (M = Cu, Ag, and Au) have been prepared and characterized using powder X-ray diffraction, vibrational spectroscopy, and total neutron diffraction. A solid solution with the AgCN structure exists in the (Cu(x)Ag(1-x))CN system over the range (0 ≤ x ≤ 1). Line phases with compositions (Cu(1/2)Au(1/2))CN, (Cu(7/12)Au(5/12))CN, (Cu(2/3)Au(1/3))CN, and (Ag(1/2)Au(1/2))CN, all of which have the AuCN structure, are found in the gold-containing systems. Infrared and Raman spectroscopies show that complete ordering of the type [M-C≡N-M'-N≡C-](n) occurs only in (Cu(1/2)Au(1/2))CN and (Ag(1/2)Au(1/2))CN. The sense of the cyanide bonding was determined by total neutron diffraction to be [Ag-NC-Au-CN-](n) in (Ag(1/2)Au(1/2))CN and [Cu-NC-Au-CN-](n) in (Cu(1/2)Au(1/2))CN. In contrast, in (Cu(0.50)Ag(0.50))CN, metal ordering is incomplete, and strict alternation of metals does not occur. However, there is a distinct preference (85%) for the N end of the cyanide ligand to be bonded to copper and for Ag-CN-Cu links to predominate. Contrary to expectation, aurophilic bonding does not appear to be the controlling factor which leads to (Cu(1/2)Au(1/2))CN and (Ag(1/2)Au(1/2))CN adopting the AuCN structure. The diffuse reflectance, photoluminescence, and 1-D negative thermal expansion (NTE) behaviors of all three systems are reported and compared with those of the parent cyanide compounds. The photophysical properties are strongly influenced both by the composition of the individual chains and by how such chains pack together. The NTE behavior is also controlled by structure type: the gold-containing mixed-metal cyanides with the AuCN structure show the smallest contraction along the chain length on heating.

Structures of Pd(CN)2 and Pt(CN)2: intrinsically nanocrystalline materials?

Analysis and modeling of X-ray and neutron Bragg and total diffraction data show that the compounds referred to in the literature as "Pd(CN)(2)" and "Pt(CN)(2)" are nanocrystalline materials containing small sheets of vertex-sharing square-planar M(CN)(4) units, layered in a disordered manner with an intersheet separation of ~3.44 Å at 300 K. The small size of the crystallites means that the sheets' edges form a significant fraction of each material. The Pd(CN)(2) nanocrystallites studied using total neutron diffraction are terminated by water and the Pt(CN)(2) nanocrystallites by ammonia, in place of half of the terminal cyanide groups, thus maintaining charge neutrality. The neutron samples contain sheets of approximate dimensions 30 Å × 30 Å. For sheets of the size we describe, our structural models predict compositions of Pd(CN)(2)·xH(2)O and Pt(CN)(2)·yNH(3) (x ≈ y ≈ 0.29). These values are in good agreement with those obtained from total neutron diffraction and thermal analysis, and are also supported by infrared and Raman spectroscopy measurements. It is also possible to prepare related compounds Pd(CN)(2)·pNH(3) and Pt(CN)(2)·qH(2)O, in which the terminating groups are exchanged. Additional samples showing sheet sizes in the range ~10 Å × 10 Å (y ~ 0.67) to ~80 Å × 80 Å (p = q ~ 0.12), as determined by X-ray diffraction, have been prepared. The related mixed-metal phase, Pd(1/2)Pt(1/2)(CN)(2)·qH(2)O (q ~ 0.50), is also nanocrystalline (sheet size ~15 Å × 15 Å). In all cases, the interiors of the sheets are isostructural with those found in Ni(CN)(2). Removal of the final traces of water or ammonia by heating results in decomposition of the compounds to Pd and Pt metal, or in the case of the mixed-metal cyanide, the alloy, Pd(1/2)Pt(1/2), making it impossible to prepare the simple cyanides, Pd(CN)(2), Pt(CN)(2), or Pd(1/2)Pt(1/2)(CN)(2), by this method.

Bis[bis(methoxycarbimido)aminato]copper(II) 1-methylpyrrolidin-2-one disolvate.

The title compound, [Cu(C(4)H(8)N(3)O(2))(2)].2C(5)H(9)NO, consists of a neutral copper complex, in which the Cu(II) centre coordinates to two bis(methoxycarbimido)aminate ligands, solvated by two molecules of 1-methylpyrrolidin-2-one. The complex is planar and centrosymmetric, with the Cu(II) centre occupying a crystallographic inversion centre and adopting approximately square-planar geometry. N-H...O hydrogen-bonding interactions exist between the amine NH groups of the ligands and the O atoms of the 1-methylpyrrolidin-2-one molecules. The associated units pack to form sheets.

Dicopper(II) trihydroxide cyanoureate dihydrate.

The title compound, poly[[mu-cyanoureato-tri-mu-hydroxido-dicopper(II)] dihydrate], {[Cu(2)(C(2)H(2)N(3)O)(OH)(3)].2H(2)O}(n), is a new layered copper(II) hydroxide salt (LHS) with cyanoureate ions and water molecules in the interlayer space. The three distinct copper(II) ions have distorted octahedral geometry: one Cu (symmetry 1) is coordinated to six hydroxide groups (4OH + 2OH), whilst the other two Cu atoms (symmetries 1 and 1) are coordinated to four hydroxides and two N atoms from nitrile groups of the cyanoureate ions (4OH + 2N). The structure is held together by hydrogen-bonding interactions between the terminal -NH(2) groups and the central cyanamide N atoms of organic anions associated with neighbouring layers.