Equitorial position

When hydrogens are replaced with other, bulkier groups, it becomes apparent that the axial positions are less energetically favored than the equitorial positions. That means that, if given a choice, bulkier groups will tend to bond to cyclohexane in equitoral positions, as this reduces their steric hinderance and potential energy. 

XeF2 Linear 5 3 5 electron pairs forms trigonal bypyramid with three lone pairs at equitorial positions XeF4 Square planar 6 2 6 electron pairs form an octahedron with two position occupied by lone pairs XeF6 Distorted octahedron 7 1 Pentagonal bipyramid with one lone pair... 

The properties of Type 2 copper in blue-copper oxidases contrast sharply with those of Type 1 copper in this class of enzymes. The optical (weak visible absorbance) and EPR (0.015 cm Aj <0.020 cm ) spectra clearly resemble those encountered for Cu(II) copper in low molecular weight tetragonal complexes. Despite its prosaic spectroscopy, Type 2 copper chemistry appears to be richer than that of Type 1 copper. Type 2 copper will bind exogeneous ligands at both axial and equitorial positions and is accessible to water in the solvent milieu. In blue copper oxidases, this lability to ligand exchange is used to stabilize an intermediate in the course of O2 reduction (see below). 

Axial bonds are those in the vertical plane. Equitorial bonds are those at an angle to the horizontal plane. Substituents larger than hydrogen are more stable in equitorial positions. This makes conformation II more stable than I since II has the large Cl in the less stable axial position while I has the smaller H in the axial position. Chlorocyclohexane consists primarily of conformation II. 

Arts. Conformation I since the methyl group is in the more stable equitorial position. 

Morishima et al. (171) measured NMR contact shifts for Ni (2-I-) (acac)2 complexes of piperidine(I), 4-methylpiperidine (II), iV-methyl-piperi-dine(III), 1,4-dimethylpiperidine(IV), and quinuclidene(V). The P carbon atom of the ligand in complexes (III) and (IV) show an attenuation of the contact shift relative to this same carbon in (I) and (II). The authors rationalized this discrepency on the basis of the orientation of the lone pair on nitrogen. For complexes (I) and (II) the lone pair prefers an equitorial position, whereas, in (III) and (IV) it prefers an axial position. For (I) and (II) a zig-zag path of distribution is necessary. Apparently (see data in Table LVII) the zig-zag path is more favorable. For quinuclidene (V) the y carbon shows a downfield shift (positive spin density) (opposite to I-IV). The downfield shift may be accounted for by spin delocalization through space involving the lone-pair electrons. 

Figure 5 X-band EPR spectra of complexes formed upon CO adsorption on reduced Ni/Si02 catalysts under a pressure of (A) 10 torr CO followed by evacuation at 340 K, (B) 10 torr, (C) 100 torr, (D) 400 torr. Subscript a represents axial and subscript e represents equitorial positions. Reprinted with permission from Dyrek K and Che M (1997) EPR as a tool to investigate the transition metal chemistry on oxide surfaces. Chemical Reviews 97 . 303-331 Copyright 1997 American Chemical Society.

The final example of ten-coordination to be mentioned is that found in trinitrato-l,2-bis(pyridine-2-aldimino)ethanegadolinium(III) (Smith et al., 1973). The coordination polyhedron approximates a pentagonal bipyramid (considering the nitrate ions to occupy a single coordination position) with four of the five equitorial positions occupied by the nitrogen atoms of the tetradentate amine ligand and the remaining three positions occupied by the three nitrate ions. 

A second important feature of the synthesis is introduction of a MOM group into 630. This positions the anomeric alkoxy group in an equitorial site so that, once again, reduction of the carbonyl group can take place from the less hindered side, introducing complete 3,5-anti stereoselection. 

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