Trigonal complexes chemistry

A large number of rare-earth adducts have been synthesized and characterized (see Gmelin Handbook of Inorganic Chemistry, Part D4, 1986 for a general review or Cunha et al. (1992) for a review on europium trigonal complexes). However, the molecular structure of only a few compounds has been established by means of single-crystal X-ray diffraction. In table 16, we compare the coordination numbers in the solid state and in solution for the solvates for which extensive data are at hand, that is, adducts with DMSO. Solid solvates R(N03)3 nDMSO display a CN change from 10 to 9 between Eu and Tb. X-ray diffraction data for 1M solutions of Er(N03)3 in DMSO yield the same coordination number (CN=9) and similar distances as in the crystalline phase (Johansson et al. 1991). 

The coordination chemistry of Zn" and Cd", although much less extensive than for preceding transition metals, is still appreciable. Neither element forms stable fluoro complexes but, with the other halides, they form the complex anions [MX3] and [MX4] , those of Cd" being moderately stable in aqueous solution. "" By using the large cation [Co(NH3)6] + it is also possible to isolate the trigonal bipyramidal [CdCls] "... 

Of particular importance in structural chemistry is the concept of hybridization, that is, the construction of linear combinations of atomic orbitals that transform according to the symmetry of the structure. For the present, a simple illustration is provided by the hybridization of atomic orbitals in a molecule or complex ion of trigonal structure. 

A review17 with 25 references of five-coordination in palladium(II) and platinum(II) chemistry is presented. The complexes have invariably a trigonal bipyramidal geometry with the bidentate ligand and the alkene in the equatorial plane. 

Metalla-j3-diketones or metalla-/3-diketonate anions react with trigonal boron compounds, BX2Y, in which X = Y = halogen or X = Cl and Y = Ph, to afford neutral (metalla-/3-diketonato)B(X) (Y) complexes. Complexes of this type are known with mangana-, rhena-, and ferra-/3-diketonato ligands. The earlier chemistry of these complexes has also been previously reviewed (J). 

The structural chemistry of the organotin halides is dominated by their Lewis acid properties and their propensity to form five- and six-coordinate complexes. Self-association may give oligomers or polymers in the solid state, which usually dissociate in solution. The structure of tricyclohexyltin chloride in the crystal is temperature-dependent. At 108 K, it has the form of a rod-like polymer with distorted trigonal-bipyramidal tin and Sn-Cl separations of 245.6(7) and 300.77(7) pm, but at 298 K, the structure is best regarded as consisting of near-tetrahedral discrete molecules.3... 

We hope that this review of chiral sulfur compounds will be useful to chemists interested in various aspects of chemistry and stereochemistry. The facts and problems discussed provide numerous possibilities for the study of additional stereochemical phenomena at sulfur. As a consequence of the extent of recent research on the application of oiganosulfur compounds in synthesis, further developments in the field of sulfur stereochemistry and especially in the area of asymmetric synthesis may be expected. Looking to the future, it may be said that the static and dynamic stereochemistry of tetra- and pentacoordinate trigonal-bipyramidal sulfur compounds will be and should be the subject of further studies. Similarly, more investigations will be needed to clarify the complex nature of nucleophilic substitution at tri- and tetracoordinate sulfur. Finally, we note that this chapter was intended to be illustrative, not exhaustive therefore, we apologize to the authors whose important work could not be included. 

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