The Structure of Tris(ethy]enediamine)chromium(III) Pentacyanonickelate(II) Sesquihydrate, [Cr(NH₂CH₂CH₂NH₂)₃][Ni(CN)₅] 1.5H₂O

The crystal and molecular structure of tris(ethylenediamine)chromium(III) pentacyanonickelate(II) sesquihydrate, [Cr-(NH₂CH₂CH₂NH₂)₃] [Ni(CN)₅] • 1. 5H₂O, has been determined from three-dimensional X-ray data collected by counter methods. The structure has been refined on F to a final conventional R factor of 9.4% and a weighted R factor of 6.9% for the 2758 observed reflections. The salt crystallizes in space group C_2h⁵-P2₁/c of the monoclinic system, with eight formula units in a cell of dimensions a = 14.883 (4), b = 15.748 (4), c = 16.464 (5) Å, β = 94.88 (1)°. The calculated density of 1.55 g/cm³ agrees well with the observed density of 1.56 ± 0.02 g/cm³. The structure consists of discrete Cr(C₂H₈N₂)₃^{3 +} and Ni(CN)₅^3- ions linked by intermolecular hydrogen bonds. The waters of crystallization participate in some of these bonds. There are two crystallographically independent Ni(CN)₅^3- ions in the compound: one is a regular square pyramid and the other is a distorted trigonal bipyramid. The square pyramid has axial and average equatorial Ni-C bond lengths of 2.168 (14) and 1.862 (6) Å, respectively. The average C-Ni-C angle between the opposing basal carbon atoms is 159.5 (4)°, so that the nickel atom is 0.34 Å above the basal plane of carbon atoms. The distorted trigonal-bipyramidal Ni(CN)₅^3- group has Ni-C axial bonds that are significantly shorter than the Ni-C equatorial bonds. The average axial Ni-C bond length is 1.837 (9) Å. The C-Ni-C angle between the axial carbon atoms is 172.8 (5)°. There are two equivalent equatorial Ni-C bonds of average length 1.907 (9) Å and one longer equatorial bond of 1.992 (14) Å. This longer bond and the large C-Ni-C angle, 141.2 (5)°, between the other two equatorial carbon atoms are the primary deviations from a regular trigonal-bipyramidal geometry. The observed C₂ geometry is apparently not due to crystal-packing interactions but is rather an energy minimum of the free ion. The isolation of both geometries in the same crystal demonstrates the very small energy difference between them. The conformations of the two tris(ethylenediamine)chromium cations are different. For the Λ configuration of each, one is ΛΛλ∂ and the other is Λ∂∂λ.