Peter W. Smith

Structure systematics in A3Mo2X9, X = Cl, Br, I, from Rietveld refinement of X-ray powder data

Abstract The structures of the compounds A3Mo2Cl9, A = K, NH4, Rb, Cs, Me4N; A3Mo2Br9, A = K, Rb, Cs, Me4N; Cs3Mo2I9, and Cs3W2Cl9 have been refined by Rietveld analysis of powder X-ray diffraction data. The results have been used to analyze the effect of change of the A cation on the geometry of the (M2X9)3− complex anion, which comprises two octahedra sharing a common face. The observed increase in the M-M separation with increasing size of the A cation is interpreted in terms of a balance between attractive M-M bonding forces, which draw the metals together, and strong covalent M-terminal halogen bonds which, together with elongation of (M2X9)3− due to packing effects, cause a net lengthening of M-M. In the series A3Mo2Cl9, for example, the MoMo separation increases from 2.524(8) to 2.778(8) A when the A cation changes from K to Me4N.

The structure of Na6Zn3(AsO4)4 · 3H2O and its relationship to the garnet and other Ia3d-derived structures

Abstract The hydrated sodium zinc arsenate, Na 6 Zn 3 (AsO 4 ) 4 · 3H 2 O, has cubic symmetry, space group P 2 1 3, a = 12.243(3) A, Z = 4. The structure was refined to wR ( F 2 ) = 0.028 using 548 independent reflections from a microtwinned crystal. Both zinc and arsenic atoms are tetrahedrally coordinated. The ZnO 4 tetrahedra share all vertices with the basal vertices of the AsO 4 tetrahedra to give a 3-D framework that is closely related to the [B 7 O 12 ] 3− framework of corner-linked BO 4 and BO 3 in boracite. The sodium atoms (CN = 6) and water molecules occupy [110] channels in the framework. The structural relationships with the garnet structure and related Ia 3 d -derived structures are discussed.

Notes. Electronic spectrum and metal–ligand bonding parameters of the V(H2O)63+ ion

The electronic spectrum of a crystal of NH4V(H2O)6(SO4)2·6H2O is reported and the transition energies interpreted in conjunction with previously reported magnetic susceptibility data using the angular overlap model. Satisfactory agreement with experiment can be obtained only if the π bonding in the plane of each water molecule is significantly weaker than that perpendicular to this plane.

Structure, vibrational and electronic spectra, and bonding in trans-diaquabis(oxalato)vanadate(III) complex salts, A[V(ox)2(H2O)2]·xH2O (A = Cs, K, or NH3Me), and the X-ray crystal structure of the potassium salt (x= 3)

The X-ray crystal structure of potassium diaquabis(oxalato)vanadate(III), K[V(ox)2(H2O)2]·3H2O, has been determined [monoclinic, space group P2/c, with unit-cell parameters a= 7.971(4), b= 5.691(2), c= 14.167(5)A, β= 108.97(3)°, and Z= 2]. The structure has been refined to an R value of 0.032 using 560 independent reflections. In comparison with the corresponding Cs+ and NH3Me+ salts, the K+ salt exhibits a significantly longer bond distance for axially co-ordinated water and this is manifested in band shifts of 500–1 000 cm–1 in the visible region. For all three salts vibrational spectra and single-crystal polarised electronic spectra are reported together with assignments. Analysis of the ligand-field spectra is presented in terms of the angular overlap model. It is shown that the band shifts occurring in the electronic spectrum of the K+ salt can be reproduced by a reduction in the σ-bonding capacity of the co-ordinated water together with a slight increase in the σ bonding of the oxalate ligand, both of which are consistent with structural differences existing between the K+ and the other two salts. Significant anisotropy exists in the metal–oxalate π interaction with the in-plane contribution relatively small. In addition, anomalous temperature-dependent bands resulting from vibronic coupling with internal O–H stretching vibrations are present in the electronic spectra of both the Cs+ and NH3Me+ salts.

Unusual features in the polarised single crystal electronic spectrum of (NH4)2[V(H2O)6](SO4)2: Analysis of Fano antiresonance structure and bonding in the [V(H2O)6]2+ complex

Abstract A detailed analysis of the electronic spectrum of the V 2+ Tuttons salt (NH 4 ) 2 V(H 2 O) 6 (SO 4 ) 2 and its deuterate is presented. The spectra exhibit a number of unusual features including some not previously noted in earlier studies of this compound. In particular, there is structure superimposed on the 4 A 2g → 4 T 2g band envelope ascribed to Fano antiresonances, which is due to the interaction of the 2 E g and 2 T 1g spin forbidden states with the 4 T 2g state. The duplication of this structure in a much less intense band ∼ 3200 cm −1 to higher energy, suggests that the latter is induced by coupling of the 4 T 2g state to an OH stretching frequency, and this is confirmed by deuteration. A similar band coupled to the 4 T 1g (F) state is observed at ∼ 22000 cm −1 . From the spectral assignments, angular overlap model ligand field parameters are obtained and the bonding in the [V(H 2 O) 6 ] 2+ complex is discussed.

Crystal and molecular structure of chromium(II) sulphate pentahydrate and single-crystal electronic spectra and bonding of CrSO4·5H2O, CuSO4·5H2O, and CuSO4·5D2O

The crystal and molecular structure of CrSO4·5H2O is reported; this compound is isostructural with the analogous copper(II) salt. The two independent molecules in the unit cell have quite similar, tetragonally elongated ligand co-ordination geometries, the distortion from regular octahedral arrangements being marginally smaller than those in the copper(II) compound. The average Cr–O distance is ca. 0.07 A longer than the average Cu–O bond length, this difference being more marked for the in-plane than the axial bonds. The low-temperature single-crystal electronic spectrum of CrSO4·5H2O shows bands in the region 10 000–15 000 cm–1 assigned as d–d transitions coupled to metal–ligand vibrations and a weak peak at 18 450 cm–1 thought to be due to coupling with water O–H stretching vibrations. The spectrum of CuSO4·5H2O is also reported and is similar to that of the chromium(II) compound except that the bands are ca. 2 000 cm–1 lower in energy. For the copper(II) complex the assignment of the peak due to coupling with O–H stretches of the water molecules was confirmed by measuring the electronic spectrum of the corresponding deuteriated compound. An analysis of the transition energies using the angular overlap model suggests that the ligand interaction with the d orbitals is ca. 20% higher in the chromium(II) compound than in the copper(II) derivative. In both compounds the dz2 orbital is depressed in energy compared with the predictions of the simple bonding model, and better agreement with the observed transition energies is obtained if the π-bonding interaction involving the water molecules is approximately isotropic about the metal–ligand bond axes.