Dicaesium pentacyanotricuprate(I), Cs 2 Cu 3 (CN) 5

Cs2Cu3(CN)5 has a layered structure consisting of [Cu3(CN)5]2− sheets stacked in an ABAB fashion along the c axis, with Cs+ cations lying between the sheets. The sheets are generated by linking –(CuCN)– chains, in which the C≡N groups are ordered, via [Cu(CN)3]2− units. The two bridging cyanide groups of each [Cu(CN)3]2− unit show partial `head-to-tail' disorder of C and N, whilst the third C≡N group is terminal and ordered with C bonded to Cu.


Comment
Copper(I) cyanide frameworks, like those of other transitionmetal cyanides, can be viewed as constructed from M(CN) x structural building blocks. For copper(I), a range of potential building blocks are known, including simple species, such as linear [Cu(CN) 2 ] À , trigonal [Cu(CN) 3 ] 2À and tetrahedral [Cu(CN) 4 ] 3À units, and larger fragments, such as -(CuCN)chains. These units have well defined geometries and can be assembled to form new solids by combining with themselves, in association with charge-balancing species where necessary, or with other complex metal ions or organic species, e.g. Lewis bases such as amines, to generate one-, two-and threedimensional frameworks.

Figure 1
A projection of the crystal structure along the c axis, showing layers stacked as ABAB with Cs + cations between the layers. Key: Cu atoms are black, Cs orange, C green, N blue and Z (C or N of a disordered cyanide group) cyan.
The present work is a continuation of our investigations of copper(I) cyanide materials prepared in the presence of alkalimetal cations (Chippindale et al., 2004;Pohl et al., 2006). Cs 2 Cu 3 (CN) 5 reported here has the same layer structure as K 2 Cu 3 (CN) 5 , prepared previously in acetonitrile under solvothermal conditions (Pohl et al., 2006).
The layer structure of Cs 2 Cu 3 (CN) 5 can be described in terms of -(Cu2CN)-chains running along the b axis and linked through bridging [Cu1(CN) 3 ] 2À units to generate a network of (CuCN) 8 rings within the layers. The layers stack in an ABAB fashion along the c axis ( Fig. 1). Cs + cations lie between the layers bonded to 12 cyanide groups, with Cs-C/ N distances in the range 3.11 (2)-3.58 (3) Å .
There are two crystallographically distinct Cu atoms, both of which have approximately trigonal-planar coordination (Fig. 2). Atom Cu1, on a special position of site symmetry 2, is bonded to two equivalent bridging cyanide groups, Z3 Z4, through the Z4 ends of the groups. The Z3 Z4 unit shows partial 'head-to-tail' disorder, as determined by refinement, with Z3 having occupancy 0.78 (4) for C3 and 0.22 (4) for N3 and Z4 having occupancy 0.22 (4) for C4 and 0.78 (4) for N4. The coordination around Cu1 is completed by a third cyanide group, C1 N1, bonded as a terminal group to Cu1 through C1. Atom Cu2, sited on a general position, bonds directly to C2, N2 and Z3 and is also approximately trigonal planar, although the geometry around Cu2 is less regular than that found for Cu1. The refinement of site occupancies for the cyanide group C2 N2 indicates that the C and N atoms are fully ordered. The greater deviation from linearity of the Cu2-N2 C2 angle compared with the Cu2-C2 N2 angle in Cu2-C2 N2-Cu2 iii (symmetry code as in Table 1) confirms this assignment: stronginteractions between a metal and the C end of a cyanide usually result in a smaller deviation from linearity of the M-C-N angle than the M 0 -N-C angle (Vahrenkamp et al., 1997).

Crystal data
Z denotes a disordered cyanide group.  The orientations of the three distinct C N groups were investigated as follows. Each C N was modelled as Zx Zy with starting values for the occupancies of both Zx and Zy set to (0.5 C + 0.5 N). The site occupancies were then refined subject to the constraints that the total occupancy for each site was 1.00 and the displacement parameters of C and N on the same site were equal. Cyanide groups C1 N1 and C2 N2 were found to be fully ordered and the occupancies of these groups were fixed in subsequent refinements. The occupancies in the remaining bridging Z3 Z4 group have refined values for Z3 of 0.78 (4) for C3 and 0.22 (4) for N3, and for Z4 of 0.22 (4) for C4 and 0.78 (4) for N4.