Hydrate, crystal symmetry

The cases listed in Tables 7 and 19 give general support to this idea. We may also draw attention to three cases where failure to make use of crystal symmetry (which would be chemically feasible) is associated with hydrogen bonds that are significantly longer than those in Type A structures sodium bicarbonate (Sect. XV), potassium hydrogen oxalate (XVI B) and sodium hydrogen oxalate hydrate (XVI C). 

The structures of the primary hydration sphere in solution are similar to those reported for the hydrated crystals (Albertson and Elding 1977), i.e. for the lighter cations n = 9 with tricapped trigonal prism (TCTP) structure and D31, symmetry, and for the heavier cations n = 8 with square prismatic structure and D2h symmetry. The geometric parameters (Ln HjO and H2O H2O distances) were taken from the X-ray data on solutions from Habenschuss and Spedding (1979b, c, 1980). 

The hydrated pentanitrato complexes all have a dodecacoordinated central ion [R(N03)5(H20)2] . An interesting case is the ammonium complex (NH4)2La(N03)5 /1H2O ( = 3 or 4) (Eriksson et al., 1982). The two hydrates are structurally closely related because the removal of water does not bring about the collapse of the structure and the coordination geometry and crystal symmetry are preserved. The only significant change is the contraction of the unit cell when there is one water molecule less. The ammonium ions are involved in the network of hydrogen bonds with the complex anions and the noncoordinated water molecules. 

The largest protonated cluster of water molecules yet definitively characterized is the discrete unit lHi306l formed serendipitously when the cage compound [(CyHin)3(NH)2Cll Cl was crystallized from a 10% aqueous hydrochloric acid solution. The structure of the cage cation is shown in Fig. 14.14 and the unit cell contains 4 [C9H,8)3(NH)2aiCUHnOfiiai- The hydrated proton features a short. symmetrical O-H-0 bond at the centre of symmetry und 4 longer unsymmetrical O-H - 0 bonds to 4... 

Crystal field splitting parameter, 2, 309 Crystal field theory, 1, 215-221 angular overlap model, 1, 228 calculations, 1, 220 generality, 1,219 low symmetry, 1,220 /-orbital, 1, 231 Crystal hydrates, 2, 305,306 bond distances, 2, 307 Crystals... 

One of the most promising techniques for studying transition metal ions involves the use of zeolite single crystals. Such crystals offer a unique opportunity to carry out single crystal measurements on a large surface area material. Suitable crystals of the natural large pore zeolites are available, and fairly small crystals of the synthetic zeolites can be obtained. The spectra in the faujasite-type crystals will not be simple because of the magnetically inequivalent sites however, the lines should be sharp and symmetric. Work on Mn2+ in hydrated chabazite has indicated that there is only one symmetry axis in that material 173), and a current study in the author s laboratory on Cu2+ in partially dehydrated chabazite tends to confirm this observation. 

Crystals, however, are not always so perfectly ordered. Atomic mobility exists within the crystal lattice however, it is greatly reduced relative to the amorphous state. Partial loss of solvent from the lattice can result in static disorder within the lattice where the atomic positions of a given atom can deviate slightly within one asymmetric unit of the unit cell relative to another. Lattice strain and defects occur for many reasons. Solvents can be present within channels of the lattice in sites not described by the symmetry of the crystal structure itself, resulting in disordered solvent molecules or incommensurate structures and potentially nonstoichiometric solvates or hydrates. 

---