Hexaaqua Cation Research Articles

Paramagnetism of caesium titanium alum and the Jahn–Teller interaction

The magnetic properties of [Ti(OH2)6]3+ within CsTi(SO4)2⋅12H2O and doped into a range of isostructural diamagnetic lattices, has been reinvestigated using electron paramagnetic resonance (EPR) and magnetic susceptibility measurements. Whereas previous attempts to rationalise the variation of the magnetic susceptibility with temperature have employed a Ag2 trigonal ground term, the data are interpreted within a framework used to describe other tervalent hexa-aqua cations whereby the trigonal field is considerable and of a sign such that the eg orbital set is lower lying, resulting in a Eg2 trigonal ground term. The magnetic susceptibility is then modeled using a novel approach based on the numerical diagonalisation of the vibronic Hamiltonian. Hence, it is shown that the singular magnetic properties of CsTi(SO4)2⋅12H2O arise from a delicate balance between spin–orbit and Jahn–Teller coupling. Particular use is made of isotopic substitution to obtain information regarding the nature of the vibronic interaction. An excellent fit to both the susceptibility data and ground state g values is provided by a model in which the Jahn–Teller interaction is dominated by coupling to phonon modes involving librational modes of sulphate and water coordinated to titanium(III). This interpretation is further supported by Raman data on the corresponding compounds. This analysis provides the first satisfactory explanation of the magnetic properties of [Ti(OH2)6]3+ in the β-alum lattice.

Water Molecule Dispositions in the Cesium Sulfate α- and β-Alums: Single-Crystal Neutron Diffraction Studies of CsM(SO4)2.12H2O (M=V, Rh)

Room-temperature single-crystal neutron diffraction studies are recorded for two alums, Cs( Rh /V)(SO4)2.12H2O [cubic, Pa3, a 12.357(5) ( Rh ), 12.434(1)Ǻ (V)], residuals 0.037 and 0.068 for 328 and 164 'observed' reflections, with the intention of defining water molecule hydrogen atom orientations. Whereas the two tervalent hexaaqua cations are similar in size [ rM -O = 2.010(6)Ǻ (M = V) and 2.006(2)Ǻ (M = Rh )] the vanadium salt adopts the β alum modification while rhodium gives an α alum. Significantly, the water coordination geometry is different in the two cases with the tilt angle between the plane of the water molecule and the M-O bond vector being 1° (M = V) and 35° (M = Rh ). The tilt angle for water coordinated to rhodium in CsRh (SeO4)2.12H2O is inferred from the unit cell dimensions to be similar to that of the corresponding sulfate salt and not that which generally pertains for caesium selenate alums. Significant differences in the H-O-H bond angle are found for trigonal planar and trigonal pyramidal water coordination, suggesting that differences in the metal(III)-water interaction are a determinant of the geometry of the coordinated water molecule in the caesium sulfate/ selenate alum lattices.

Single-crystal Raman spectroscopy of the α alums CsM(SO4)2· 12H2O (M = Co or Ir) between 275 and 1200 cm–1

Low-temperature polarised single-crystal Raman spectra of CsM(SO4)2·12H2O (M = Co or Ir) have been recorded between 275 and 1200 cm–1. A full assignment of the spectra has been completed, including all the Raman-active internal modes of the tervalent hexaaqua cation [ν1(MO6) 548 (Co) and 554 (Ir), ν2(MO6) 509 (Co) and 540 (Ir) and ν5(MO6) 332 av. (Co) and 314 av. cm–1(Ir)]. The relationship between the wavenumber of the ν1(MO6) mode and the metal–oxygen bond length is significantly different for the tervalent cations of the second- and third-row transition metals than for those of first-row transition metals and Group 13 metals. Solution Raman spectra of [Ir(OH2)6]3+ have enabled determination of ν1(MO6)aq(536 cm–1); the relationship between this value and that obtained at low temperature in the solid state closely follows that found previously for the chromium(III) and rhodium(III) aqua ions. The external modes of water co-ordinated to the tervalent cation are highly sensitive to the alum type, the metal–ligand bond length, and the stereochemistry of water co-ordination and provide additional insight into the metal–water interaction. This, in turn, casts light on the assignment of the external modes to particular molecular motions.

Relationship between the electronic and molecular structure of tervalent aqua ions: low-temperature neutron diffraction structure of CsCr(SO4)2·12H2O

The alum CsCr(SO4)2·12H2O has been investigated by single-crystal neutron diffraction at 15 K. The structure, which conforms to the β modification, was refined using a total of 651 unique reflections to give a weighted R value of 0.041. The stereochemistry of water co-ordination to chromium(III) is trigonal planar, with the Cr–O bond vector making an angle of 0.8(6)° with the plane of the co-ordinated water molecule. The angle between the plane of the co-ordinated water molecule and the CrO6 framework, φ, is –19.0(4)°, similar to that found for the corresponding iron salt [–19.4(3)°], but smaller than found for the corresponding ruthenium [–22.0(3)°] and vanadium [–22(1)°] salts. The relationship between φ and the splitting of the t2g(Oh) orbital energies is described using the angular overlap model. Accordingly, the preferred value of φ depends on the occupancy of the t2g orbitals. Using this approach the φ values found for the caesium sulphate β alums are explained, and the electronic structures of the titanium and vanadium hexaaqua cations are discussed. The general features of the site of the tervalent cation in the alum lattice are described, these being important owing to the extensive use of the alum lattice for the spectroscopic study of tervalent aqua ions.

Low-temperature neutron-diffraction structure of [Ru(OH2)6]3+in the caesium sulphate alum lattice CsRu[SO4]2·12H2O

The salt CsRu[SO4]2·12H2O has been investigated by single-crystal neutron diffraction at 15 K. The crystals belong to the cubic space group Pa, Z= 4, with a= 12.371 (6)A. The structure, which conforms to the β alum modification, was refined using 894 inequivalent reflections to give a weighted R factor of 0.026. The stereochemistry of the water co-ordination to RuIII is trigonal planar, with the angle between the metal (III)–water bond and the plane of the co-ordinated water molecule being 0.8(6)°. The angle between the plane of the co-ordinated water molecule and the RuO6 octahedron is 22.0(6)°, midway between the positions appropriate for Th and all-horizontal D3d symmetry of [Ru(OH2)6]3+. The anisotropic thermal ellipsoids of the oxygen atom co-ordinated to RuIII are large and of a shape similar to that of its co-ordinated hydrogen atoms. This observation is ascribed to a softening of the potential about the water co-ordinated to RuIII. Single-crystal Raman spectra of CsRu[SO4]2·12H2O exhibit features which are assigned to soft modes and which indicate that the potential softens on cooling below ca. 90 K. In contrast, neither the low-temperature structure nor the single-crystal Raman spectra of the isomorph CsFe[SO4]2·12H2O exhibits features consistent with softening of the potential about the water co-ordinated to MIII. Since the sizes of the hexa-aqua cations of RuIII and FeIII are similar it is concluded that the electronic structure of RuIII is implicated in the softening of the potential about its co-ordinated water molecule. The trigonal-field splitting of the 2T2g(Oh) ground term is discussed in terms of the overall stereochemistry of [Ru(OH2)6]3+.