[Cr(NH3)6] [ZnCI4]CI, trigonal, R-3c; a= 7.5415 (5), c=44.959 (3) A (on hexagonal axes); Z = 6, Do = 1.78, De= 1.785 g cm -3. The compound was prepared in an attempt to produce the trigonal-bipyramidal [ZnCIs] 3anion. The structure consists of [Cr(NH3)6] 3 +, [ZnC14] zand CIions. The [ZnCI4] 2ion is severely distorted from a regular tetrahedral geometry by the close approach of the CIion in the incipient formation of a fifth Zn-CI bond. [Cr(NH3)6] [CoCI4]CI is isostructural. Introduction. Initial cell dimensions for hexagonal axes were determined from precession photographs (Mo Ka radiation). Systematic absences (hkl absent unless h + k + l= 3n, hhl absent unless l= 2n) indicated space group R3c (No. 167) or R3c (No. 161). A yellow crystal displaying point-group symmetry 3m, with faces of the forms {001} and {104}, was sealed in a Lindemann glass tube and mounted on a Hilger & Watts Y290 four-circle diffractometer in a general orientation. Unit-cell parameters and the orientation matrix were refined by least squares from 12 reflexions (20>38 °, MoK~ radiation, 2=0.71069 A, Zr filter) (Busing & Levy, 1967). Intensities were collected for two equivalent sets of reflexions with 20<55 ° (0-20 scan mode, 4s count at each of 80 steps of 0.01 ° in 0, background counts of 80s at each end of the scan). Three standard reflexions showed < 2 % intensity decay during the collection; linear decay functions were fitted and applied. The crystal was measured with a travelling microscope and absorption corrections applied by a Gaussian integration method (/z= 33 cm -1, 312 grid points, transmission factors range from 0.411 to 0.716). 987 reflexions with positive net recorded intensity were merged to give 521 unique reflexions, with an internal consistency index ~(w~+w2)(F~F2)2/2~(wtF2+ w2F22)=30% [ w = 1/a2(F), based on counting statistics]. The statistical distribution of E values indicated the centrosymmetric space group R3e. This would require the Cr and Zn atoms to lie on special positions with site symmetries 3($6) and 32(D3), which implies a regular trigonal-bipyramidal [ZnCIs] 3anion. A Fourier synthesis phased by the metal atoms, however, showed the Zn atom to be disordered over two equal-weight symmetry-related sites close to the 32 position. All attempts to find a solution in lower-symmetry space groups were unsuccessful. With a twofold disorder model for Zn in R'3c, the CI and N atoms were located, and the structure refined by full-matrix least squares to a minimum value of Y.wd 2 (A = IFolIFcl). Complex scattering factors for Cr a+, Zn 2+, CI-, N and H were those of Cromer & Waber (1974) and Cromer & Ibers (1974). With anisotropic thermal parameters, R was reduced to 9.12 % and Rw[=(~wA2/~wlFol2) 1/2] to 8.27%. It was then possible to resolve the 'equatorial' CI atom [C1(2)] into two half-weight symmetry-related components. Further refinement (R=7.15, Rw=5.90%) followed by a difference map revealed the positions of the H atoms. These were also refined, with individual isotropic temperature factors, and the final values of R and Rw were 4.95 and 3.77%. The number of parameters refined was 41. All shift/e.s.d, ratios in the final cycle were < 0.005. The weighting scheme was shown to be consistent by an analysis of variance (Hamilton, 1974). Atomic parameters are shown in Table 1. It was not possible to resolve CI(1) into disorder components, and the reasonable U values indicate that any disorder for CI(1) is insignificant. A final difference synthesis showed no significant features.* Discussion. Although [CuC15] 3(Raymond, Meek & [bers, 1968), [CdCIs] 3(Long, Herlinger, Epstein & * A list of structure factors has been deposited with the British Library Lending Division as Supplementary Publication No. SUP 31844 (5 pp.). Copies may be obtained through The Executive Secretary, International Union of Crystallography, 13 White Friars, Chester CH1 1NZ, England. Table 1. Atomic coordinates