Spectroscopic, Magnetochemical, and Crystallographic Study of Cesium Iron Phosphate Hexahydrate: Characterization of the Electronic Structure of the Iron(II) Hexa-aqua Cation in a Quasicubic Environment Graham Carver, † Christopher Dobe, † Thomas B. Jensen, † Philip L. W. Tregenna-Piggott,* ,†,‡ Stefan Janssen, ‡ Eckhard Bill, § Garry J. McIntyre, | and Anne-Laure Barra ⊥ Department of Chemistry and Biochemistry, UniVersity of Bern, Freiestrasse 3, 3012 Bern, Switzerland, Laboratory for Neutron Scattering, ETH Zu ¨rich and Paul-Scherrer Institute, CH-5232 Villigen PSI, Switzerland, Max-Planck-Institut fu ¨r Bioanorganische Chemie, Stiftstrasse 34-36, D-45470 Mu ¨lheim an der Ruhr, Germany, Institut Laue-LangeVin, BP156, 38042 Grenoble Cedex 9, France, and High Magnetic Field Laboratory, CNRS/MPI, B.P. 166, 38042 Grenoble, France Received February 2, 2006 Spectroscopic, magnetochemical, and crystallographic data are presented for CsFe(H 2 O) 6 PO 4 , a member of a little-known isomorphous series of salts that facilitates the study of hexa-aqua ions in a quasicubic environment. Above 120 K, the deviations from cubic symmetry are minimal, as shown by the first example of an iron(II) Mo ¨ ssbauer spectrum that exhibits no measurable quadrupole splitting. Two crystallographically distinct [Fe(OH 2 ) 6 ] 2+ complexes are identified from inelastic neutron-scattering (INS) experiments conducted between 2 and 15 K. The data are modeled with the ligand-field Hamiltonian, H ˆ) λL ˆ S ˆ + B(kL ˆ + 2S ˆ ) + ∆ tet {L ˆ z 2 - (1/3)L(L + 1)}+ ∆ rhom {L ˆ x 2 - L ˆ y 2 }, operating in the ground-term 5 T 2g (O h ) basis. An excellent reproduction of INS, Mo ¨ ssbauer, HF-EPR, and magnetochemical data are obtained in the 2 and 15 K temperature regimes with the following parameters: λ ) -80 cm -1 ; k ) 0.8; site A ∆ tet ) 183 cm -1 , ∆ rhom ) 19 cm -1 ; site B ∆ tet ) 181 cm -1 , ∆ rhom ) 12 cm -1 . The corresponding zero-field-splitting (ZFS) parameters of the conventional S ) 2 spin Hamiltonian are as follows: site A D ) 12.02 cm -1 , E ) 2.123 cm -1 ; site B D ) 12.15 cm -1 , E ) 1.37 cm -1 . A theoretical analysis of the variation of the energies of the low-lying states with respect to displacements along selected normal coordinates of the [Fe(OH 2 ) 6 ] 2+ , shows the zero-field splitting to be extremely sensitive to small structural perturbations of the complex. The expressions derived are discussed in the context of spin-Hamiltonian parameters reported for the [Fe(OH 2 ) 6 ] 2+ cation in different crystalline environments. 1. Introduction Aqua ions are the most fundamental species that exist in aqueous solution. The study of their structure, bonding, and reactivity is necessary not only for an understanding of solu- tion chemistry but also for a comprehension of the physical and chemical properties of the elements themselves. Many experimental studies have focused on the solid state, where the aqua ions are held rigidly in a crystalline environment, thus facilitating their characterization; the drawback being the difficulty in assessing the role that the host plays in dictating their electronic and molecular structures. 1-7 Theo- retical predictions of the most stable conformations of aqua * To whom correspondence should be addressed. E-mail: philip.tregenna@psi.ch. † University of Bern. ‡ ETH Zu ¨rich and Paul-Scherrer Institute. § Max-Planck-Institut fu ¨r Bioanorganische Chemie. | Institut Laue-Langevin. ⊥ High Magnetic Field Laboratory. (1) Simmons, C. J.; Hitchman, M. A.; Stratemeier, H.; Schultz, A. J. J. Am. Chem. Soc. 1993, 115, 11304. (2) Best S. P.; Forsyth, J. B. J. Chem. Soc., Dalton Trans. 1991, 1721. (3) Cotton, F. A.; Fair, C. K.; Lewis, G. E.; Mott, G.; Moss, F. K.; Schultz, A. J.; Williams, M. J. J. Am. Chem. Soc. 1984, 106, 5319. (4) Bussie `re, G.; Beaulac, R.; Cardinal-David, B.; Reber, C. Coord. Chem. ReV. 2001, 219-221, 509-543. (5) Tregenna-Piggott, P. L. W.; Andres, H. P.; McIntyre, G. J.; Best, S. P.; Wilson, C. C.; Cowan, J. A. Inorg. Chem. 2003, 42, 1350. (6) Ptasiewiczbak, H.; Olovsson, I.; McIntyre, G. J. Acta Crystallogr. B 1997, 53, 325-336. (7) Telser, J.; van Slageren, J.; Vongtragool, S.; Dressel, M.; Reiff, W. M.; Zvyagin, S. A.; Ozarowski, A.; Krzystek, J. Magn. Reson. Chem. 2005, 43, 130-139. Inorg. Chem. 2006, 45, 4695-4705 10.1021/ic0601889 CCC: $33.50 © 2006 American Chemical Society Inorganic Chemistry, Vol. 45, No. 12, 2006 4695 Published on Web 05/16/2006