Octahedral complexes stereochemistry

The coordination number of Ni rarely exceeds 6 and its principal stereochemistries are octahedral and square planar (4-coordinalc) with rather fewer examples of trigonal bipyramidal (5), square pyramidal (5), and tetrahedral (4). Octahedral complexes of Ni arc obtained (often from aqueous solution by replacement of coordinated water) especially with neutral N-donor ligands such as NH3, en, bipy and phen, but also with NCS, N02 and the 0-donor dimethylsulfoxide. dmso (Me2SO). 

There have been few studies of substitution in complexes of nickel(II) of stereochemistries other than octahedral. Substitution in 5-coordinated and tetrahedral complexes is discussed in Secs. 4.9 and 4.8 respectively. The enhanced lability of the nickel(II) compared with the cobalt(II) tetrahedral complex is expected from consideration of crystal field activation energies. The reverse holds with octahedral complexes (Sec. 4.8). 

Only octahedral complexes have been discussed, mainly because much less is known about conformational effects in other configurations. In some tetrahedral complexes, for example, [Be(OCH2CH20)2]2 or [B(OCH2 CH20)2]-, the ligands may be sufficiently inert to dissociation to be examined with respect to their stereochemistry. Similar remarks apply to planar and other structures. 

Tetrahedral stereochemistry is well known for FeUI and gives rise to the 5T2 -5E transition at 4500 cm-1, e 100, which is nearer the lower values found for octahedral complexes in other systems. In some solids, strong Jahn—Teller effects are present.114"117... 

The influence of the nature of the metal ion on the stereochemistry of its environment in octahedral complexes is not well known. Most of the studies have been performed with compounds of cobalt(III). It would be useful to get more information about the influence of crystal field parameters, charge and ionic radii on the stereochemistry of metal complexes. 

The discovery by Bijvoet (22) of the method of anomalous dispersion of X-rays gave a means of establishing the absolute configurations of particular enantiomers, and, in 1955, the first report of the absolute stereochemistry of an octahedral complex appeared. This concerned the important ion (+)-[Co(en)3]3+ which had the configuration (10) shown in (II). 

Whereas chirality in tetrahedral compounds of carbon requires four different groups to be bonded around the tetrahedral carbon atom, this is not necessarily the case for other central atoms with other stereochemistries. For example, octahedral complexes have more relaxed rules. Whereas a chiral tetrahedral organic compound is, as a consequence of the rule for chirality, asymmetric (or totally lacking in symmetry), chiral octahedral complexes need not be asymmetric, but may have axes of rotation (they are then dissymmetric). The common rule for chirality is simple - a compound must have non-superimposable mirror images. For an octahedral complex, this can occur even when three different pairs of monodentate ligands are coordinated, as discussed later. We shall look a little more closely at four- and six-coordinate complexes below. 

Discussion of square planar complexes in terms of mechanistic concepts developed in more detail for octahedral complexes serves to illustrate that the concepts devised initially for octahedral geometry can be transferred in large part to other coordination numbers and stereochemistries. In doing so, of course, it is necessary to consider the most likely way a reaction may occur, as exemplified above for square planar geometry. 

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