Square pyramidal configuration

Simple nickel salts form ammine and other coordination complexes (see Coordination compounds). The octahedral configuration, in which nickel has a coordination number (CN) of 6, is the most common stmctural form. The square-planar and tetrahedral configurations (11), iu which nickel has a coordination number of 4, are less common. Generally, the latter group tends to be reddish brown. The 5-coordinate square pyramid configuration is also quite common. These materials tend to be darker in color and mostiy green (12). 

These complexes may be either six-coordinate with an octahedral configuration or five-coordinate with a square-pyramidal configuration, in which the organo ligand occupies the apical position a few form dimers through the interaction of each cobalt with a coordinated atom of the equatorial ligand of the other half (see Section In virtually all groups... 

Fig. 7. Structures of five-coordinate Cu2+ from first principles molecular dynamics. A Berry twist mechanism for the interconversion of the two structures is shown (from left to right) the reorientation of the main axis of a square pyramidal configuration by pseudo-rotations via a trigonal bipyramidal configuration. The grey atoms in the plane of the trigonal bipyramid are all candidates for becoming apical atoms in a square pyramid.

I also have a comment about the trigonal bipyramid vs. square pyramid configurations. Little physical motion of atoms is required to go from one to the other. There can be, I think, a tendency to take symmetry arguments too literally as black and white situations. The full symmetry of these complexes is considerably less than the symmetry one derives just from the arrangements of atoms next to the central ion. I do think it is possible to depend too much on exact symmetry designations. 

The halogen pentafluorides and the [XeF5]+ cation have a square pyramidal configuration and any weak secondary bonds are found below the base of the pyramid and situated to avoid the axial, lone pair position. These contacts are much more significant for the xenon compounds than for the interhalogens, where they are so weak as to be virtually indistinguishable from normal intermolecular contacts, as seen in the structure of IF5.40... 

Nickel, on the other hand, on alumina and on silica supports was found to have only five nearby sulfurs (square pyramidal) with Ni-Mo coordination numbers from 1 to 1.5. Ni-Mo-S supported on carbon was observed to have Ni-S coordination numbers of 6 in a trigonal-prismatic configuration. In addition, Ni (at low Ni concentrations) was found to have one nearby Ni, which could indicate that, in some catalysts, Ni is present as pairs on the MoS2 surface. The overall structure of the Ni-Mo-S was believed to be similar to that of millerite (i.e., Ni is located in the center of the MoS edge in a square-pyramidal configuration, with one sulfur extending perpendicular to the surface) (62-64). 

Complexes ZnL(ClC>4)2 and ZnLCl(C104) (L = tetramethylcyclam) have been isolated.227 The latter contains the five-coordinate ZnLCl+ cation for which NMR results indicate a square pyramidal configuration with the chlorine in the apical position and all four methyl groups on the same side of the plane. 

Vlv in VO-NaY shows anisotropic and eight equally spaced hyperfine ESR splittings indicative of V,v in a square pyramidal configuration. Upon coordination with bpy the in-plane coordination becomes more covalent, while V=0 shows decreased covalency. All this is evidence for the existence of square pyramidal VO(bpy)2 complexes with tetragonal planar distortion. 

The H NMR spectra of the complexes show the ligands to be labile. There is the further possibility that the cation is fluxional, having trigonal bipyramid and square pyramid configurations as limiting forms. 

The square-pyramidal configuration has never been observed experimentally. Althongh theoretically possible, it is considered unlikely and is nsnally eliminated Irom active consideration in stereochemical work. The method is summarized in Table 3. 

These results suggest that distorted trigonal bipyramidal configurations are most prevalent in these compounds, with the exception of a complex hydroxy-aluminosilicate, vesuvianite (Phillips et al. 1987), in which the Al site is closer to a square pyramidal configuration. 

For specificity, let us consider the application of these ideas to a-CraOa. Its crystal structure can be represented as a hexagonal close-packed lattice of oxide ions (the closed-packed layers of oxide ions alternate ababab. ..) in which two-thirds of the octahedral holes are filled with Cr3+ ions in a systematic fashion. Suppose the crystal to be cleaved in a close-packed plane in the presence of water. To preserve electrical neutrality, the oxide ions in this plane must be equally divided between the surfaces of the faces being formed. As a result, each Cr3+ in the layer below these oxide ions would be five-coordinate and in a square pyramidal configuration. Each ion would react with a molecule of water following which a proton would move from each adsorbed water molecule to an adjacent oxide ion. Thus, the outer face would consist of a close-packed layer of hydroxide ions. This is shown in Fig. 2. The basic point is that electrical neutrality and six-coordination can both be preserved by replacing what would be a plane of oxide ions in bulk by an equivalent plane of hydroxide ions at the surface. Similar ideas obtain on alumina. 

Molecules XX5. The square pyramidal configuration expected for molecules of this type is confirmed by an X-ray study of crystalline BrFj (at-120°C) which shows that the Br atom lies just below the plane of the four F atoms. One Br-F bond (1-68 A) was found to be shorter than the other four (1-75-1-82 A). 

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