Centrosymmetric coordination site

Taking into account all of the factors influencing intensities of crystal field spectra discussed so far, the following generalizations may be made. Transitions of 3d electrons within cations in octahedral coordination are expected to result in relatively weak absorption bands. Intensification occurs if the cation is not centrally located in its coordination site. In tetrahedral coordination, the intensities of crystal field transitions should be at least one-hundred times larger than those in octahedrally coordinated cations. Spin-forbidden transitions are usually about one-hundred times weaker than spin-allowed transitions in centrosymmetric, octahedrally coordinated cations, but become... 

The valence and coordination symmetry of a transition metal ion in a crystal structure govern the relative energies and energy separations of its 3d orbitals and, hence, influence the positions of absorption bands in a crystal field spectrum. The intensities of the absorption bands depend on the valences and spin states of each cation, the centrosymmetric properties of the coordination sites, the covalency of cation-anion bonds, and next-nearest-neighbour interactions with adjacent cations. These factors may produce characteristic spectra for most transition metal ions, particularly when the cation occurs alone in a simple oxide structure. Conversely, it is sometimes possible to identify the valence of a transition metal ion and the symmetry of its coordination site from the absorption spectrum of a mineral. 

June alexandrite (chrysoberyl) Al2Be04 red/green Cr3+ Crystal field transitions in Cr+ concentrated in non-centrosymmetric distorted six-coordinated site. 

The polarized spectra of staurolite, Fe2Al9Si4023(0H), accomodating tetrahe-drally coordinated Fe2+ ions (point symmetry Cm mean Fe-0 = 200.8 pm) and consisting of absorption bands spanning the 5,000 to 7,000 cm-1 region (Bancroft and Bums, 1967a Dickson and Smith, 1976), are illustrated in fig. 4.6 and discussed in 4.4.3. These bands completely mask any contributions from Fe2+ ions that might be present in centrosymmetric octahedral sites in the staurolite structure. The A, and CFSE parameters of tetrahedral Fe2+ ions in staurolite are estimated to be about 5,300 cm-1 and 3,700 cm-1, respectively. The spectra of the cobaltian staurolite, lusakite, illustrated in fig 4.7, indicates that tetrahedrally coordinated Co2+ ions have A, and CFSE values of about 6,500 cm-1 and 7,800 cm-1, respectively. 

A number of other molecular squares were reported, in which all nonbridging coordination sites on the metal ions are blocked by capping ligands. The family of complexes [(bpy)2Fe°(CN)2]2[M°(bpy)2]2 (PF6)4 [M = Fe, Co (141), Ru (142)] was reported by Oshio and co-workers (143). Note that the presence of two cis bidentate bpy ligands imparts chirality to each octahedrally coordinated metal center. Moreover, due to steric requirements of the bpy ligands on adjacent metal centers, all the metal ions within one square adopt the same chirality, which makes the square itself chiral, with either a AAAA or a AAAA configuration [Fig. 22(a)]. The overall crystal stmcture, however, is centrosymmetric, since both enantiomers are present in the unit cell. 

XAS data comprises both absorption edge structure and extended x-ray absorption fine structure (EXAFS). The application of XAS to systems of chemical interest has been well reviewed (4 5). Briefly, the structure superimposed on the x-ray absorption edge results from the excitation of core-electrons into high-lying vacant orbitals (, ] ) and into continuum states (8 9). The shape and intensity of the edge structure can frequently be used to determine information about the symmetry of the absorbing site. For example, the ls+3d transition in first-row transition metals is dipole forbidden in a centrosymmetric environment. In a non-centrosymmetric environment the admixture of 3d and 4p orbitals can give intensity to this transition. This has been observed, for example, in a study of the iron-sulfur protein rubredoxin, where the iron is tetrahedrally coordinated to four sulfur atoms (6). 

The erbium ion is coordinated to eight carboxyhc oxygens [2.362—2.415 A)] from four oxalate (acid oxalate) moieties which forms a square antiprism around Er(III). A water molecule forms the cap (Er—OH2 =2.441 A) above the large square face of the antiprism. The acid oxalates (HOOCCOO) and oxalate ions occupy the crystallographic sites at random. The statistically averaged oxalate groups are centrosymmetric and planar. A very short H-bond (2.43 A) has been observed between two water molecules in two equivalent molecules, but the physical significance is difficult to assess because these waters are disordered in the molecule. 

The synthesis of the X2M(Et2Dtc) complexes (X = Cl, Br, I M = As, Sb, Bi) is accomplished by the reaction of the M(Et2Dtc)3 complexes with MX3 in a 1 2 molar ratio in CHC13 (170). The structure of the Br2 As(Et2 Dtc) complex, which is monomeric and a nonelectrolyte in solution, in the solid state shows a loosely held centrosymmetric dimer with bridging bromide ions. The coordination geometry of the five-coordinate As(III) is intermediate between a square pyramid and a trigonal bipyramid, with a stereochemically active lone pair occupying a site of a distorted octahedral coordination sphere (Fig. 9). 

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