Shape five coordination

The symmetry of five-coordinate zinc complexes has been analyzed using continuous symmetry measures. This approach and the computational tool allow the degree of polyhedricity to be evaluated quantitatively. The methodology identifies the minimal distance of a given structure to a desired general shape with the same number of vertices.27... 

At this stage it could be useful to make a comment on the structural features of complexes 4-17. The coordination number of the metal affects strongly the shape of the calix[4]arene fragment. In the case of five-coordinate and, eventually, four-coordinate metals, the calix[4]arene moiety displays a cone, which changes to an elliptical conformation in the case of a six-coordinate metal.4,10 Such conformation changes, which appear in the solid state, are also detectable in solution, as in the... 

For five-coordination, the usual geometry is trigonal bipyramidal. This varies from fully symmetrical, as in Pt SnF, to extremely distorted. At the limit the shape can equally well be described as capped tetrahedral or distorted square pyramid. 

There is naturally an overriding interest in the geometry of the five-coordinate intermediate, or activated complex. General considerations of the shape in which there will be least mutual... 

For NP2 and NP3 at pH 7.5, the shift in Soret band positions of the NO complexes for the two oxidation states is somewhat larger—8-10 nm, from 421-423 to 413 nm for the Fe(III) and Fe(II) complexes, respectively (50). However, in contrast to NPl-NO (49) and NP4-NO (50), at pH 5.5 NP2-NO and NP3-NO show very different spectral shifts upon electrochemical reduction, as shown in Fig. 6c for NP3-NO. The Soret band shifts to 395 nm, and both the wavelength maximum and shape of the Soret band are typical of five-coordinate heme-NO centers, including guanylyl cyclase, upon binding NO (53, 54). The reduced forms of both NP2-NO and NP3-NO exhibit similar pH dependence of the absorption spectra, whereas NPl-NO and NP4-NO do not show any pH dependence of their absorption spectra over the pH range 5.5-7.5 (50). 

As outlined in Section 50.5.4.6 the tripod-shaped tetradentate ligands have the most favourable geometry for the formation of five-coordinated complexes with trigonal bipyramidal geometry. However, the presence of different donor groups in the donor set may give rise either to coordination distortion or to stereochemistries other than trigonal bipyramidal. 

The heavier atoms of Group 13 appear not to form p -p bonds. Five- and six-coordinate structures (schemes (4) and (5)) are almost entirely restricted to the heavier atoms, although at least one complex containing five-coordinate boron is known. According to VSEPR theory, scheme (4) structures should be trigonal bipyramidal. However, InClf- and TlClf are found to be square pyramidal in crystalline solids. As noted in Section 8.2, this shape (also adopted by MnClf-) is possibly favoured by crystal packing requirements. 

Figure 11.1. Schematic representation of typical silver (central atom) coordination environments with different coordination numbers and geometries (a) linear two-coordinate (b) triangular three-coordinate (c) T-shaped three-coordinate (d) square planar four-coordinate (e) tetrahedral four-coordinate (f) trigonal bipyramidal five-coordinate (g) tetragonal pyramidal five-coordinate (h) octahedral six-coordinate (i) trigonal prism six-coordinate (j) seven-coordinate (k) tetragonal prism eight-coordinate.

The VSEPR model works at its best in rationalizing ground state stereochemistry but does not attempt to indicate a more precise electron distribution. The molecular orbital theory based on 3s and 3p orbitals only is also compatible with a relative weakening of the axial bonds. Use of a simple Hiickel MO model, which considers only CT orbitals in the valence shell and totally neglects explicit electron repulsions can be invoked to interpret the same experimental results. It was demonstrated that the electron-rich three-center bonding model could explain the trends observed in five-coordinate speciesVarious MO models of electronic structure have been proposed to predict the shapes and other properties of non-transition element... 

Opening of the diphenylene ring also led to a number of unusual five-coordinate complexes with a distorted structure intermediate between T- and Y-shape. The high trans effect of the phenylene rings may be part of the reason for the low-coordination number for Ir(III) (equation 7). ... 

The five-coordination compounds show some more exotic possibilities. PF5 has a trigonal bipyramidal shape with inequivalent axial and equatorial positions. The lone pair in SF4 chooses an equatorial position since it can do less damage there— it make a 90° angle with two F atoms, whereas an axial position would make 90° angles with three F atoms. This leaves SF4 with a shape resembling a distorted. seesaw. The two lone pairs in CIF3 are most stable when located in two equatorial positions, separated by 120". This leaves Cll 3 in a distorted tee shape. The eomplex that forms between 1 and l> in iK iieous solution is a linear ion. 

Indium(III) phthalocyanine complexes show a rich structural diversity. Compounds like PcInI are monomeric, with a five-coordinate, square-pyramidal geometry at indium. The indium atom is located out of the N4 plane, and the phthalocyaninato ring forms a dome shape.In general. 

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