Transition metal ion coordinative environment

DMPP itself is not a reactive diene in Diels-Alder reactions,but it is activated by coordination to transition metal ions. Complex 198 contains a labile perchlorato ligand that is easily displaced by the dienophile, which possesses a coordinating atom (O, S, As or P) in the group E. The cycloaddition reaction occurs intramolecularly in a highly organised environment, which leads to the coordinated exo cycloadduct 199 exclusively. A standard decoordination step affords the desired enantiopure ligands 200. Only the exo-syn isomers are formed, which bear the lone pair at the phosphorus atom and the dienophile functionalities at the same side of the molecule. ... 

Coordinative Environment. The coordinative environment of transition metal ions affects the thermodynamic driving force and reaction rate of ligand substitution and electron transfer reactions. FeIIIoH2+(aq) and hematite (a-Fe203) surface structures are shown in Figure 3 for the sake of comparison. Within the lattice of oxide/hydroxide minerals, the inner coordination spheres of metal centers are fully occupied by a regular array of O3- and/or 0H donor groups. At the mineral surface, however, one or more coordinative positions of each metal center are vacant (15). When oxide surfaces are introduced into aqueous solution, H2O and 0H molecules... 

Carbonate has proved to be a versatile ligand. Its coordination mode expands from 1 to 6, the last one is visualized when each atom binds two metal ions simultaneously. In recent years, significant progress has been made in the synthesis, structure, and magnetic properties of polynuclear carbonato complexes of transition metal ions. Such studies have also been extended to lanthanides and actinides. The speciation studies of these metal ions in aquatic environments in the presence of carbonate have resulted in significant... 

A prototypical example of a molecular probe used extensively to study the mineral adsorbent-solution interface is the ESR spin-probe, Cu2+ (Sposito, 1993), whose spectroscopic properties are sensitive to changes in coordination environment. Since water does not interfere significantly with Cu11 ESR spectra, they may be recorded in situ for colloidal suspensions. Detailed, molecular-level information about coordination and orientation of both inner- and outer-sphere Cu2+ surface complexes has resulted from ESR studies of both phyllosilicates and metal oxyhydroxides. In addition, ESR techniques have been combined with closely related spectroscopic methods, like electron-spin-echo envelope modulation (ESEEM) and electron-nuclear double resonance (ENDOR), to provide complementary information about transition metal ion behaviour at mineral surfaces (Sposito, 1993). The level of sophistication and sensitivity of these kinds of surface speciation studies is increasing continually, such that the heterogeneous colloidal particles in soils can be investigated ever more accurately. 

Further resolution of the 3d orbital energy levels takes place within a transition metal ion when it is located in a low-symmetry site, including non-cubic coordination environments listed in table 2.4 and polyhedra distorted from octahedral or cubic symmetries. As a result, the simple crystal field splitting parameter, A, loses some of its significance when more than one energy separation occurs between 3d orbitals of the cation. 

Transition metal ions most susceptible to large Jahn-Teller distortions in octahedral coordination in oxide structures are those with 3d4, 3d9 and low-spin 3(f configurations, in which one or three electrons occupy eg orbitals. Thus, the Cr2+ and Mn3+, Cu2+, and Ni3+ ions, respectively, are stabilized in distorted environments, with the result that compounds containing these cations are frequently distorted from type-structures. Conversely, these cations may be stabilized in distorted sites already existing in mineral structures. Examples include Cr2+ in olivine ( 8.6.4) and Mn3+ in epidote, andalusite and alkali amphiboles ( 4.4.2). These features are discussed further in chapter 6. 

As noted in 2.11, ligands forming high-symmetry coordination polyhedra (i.e., regular octahedra, tetrahedra, cubes and dodecahedra) about central transition metal ions are rare. Such highly idealized coordinations, nevertheless, do exist in the periclase (octahedra), cubic perovskite (octahedra, dodecahedra) and spinel (tetrahedra) structures. The more important rock-forming oxide and silicate minerals provide, instead, low-symmetry coordination environments. These include trigonally distorted octahedra in the corundum, spinel and gar-... 

Chapter 5 summarizes the crystal field spectra of transition metal ions in common rock-forming minerals and important structure-types that may occur in the Earth s interior. Peak positions and crystal field parameters for the cations in several mineral groups are tabulated. The spectra of ferromagnesian silicates are described in detail and correlated with the symmetries and distortions of the Fe2+ coordination environments in the crystal structures. Estimates are made of the CFSE s provided by each coordination site accommodating the Fe2+ ions. Crystal field splitting parameters and stabilization energies for each of the transition metal ions, which are derived from visible to near-infrared spectra of oxides and silicates, are also tabulated. The CFSE data are used in later chapters to explain the crystal chemistry, thermodynamic properties and geochemical distributions of the first-series transition elements. 

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