Substitution Octahedral complexes

Although in the previous section the basic concepts related to substitution reactions were explained with reference to octahedral complexes, substitution reactions are also common in square planar complexes. Studies on these complexes have resulted in a great deal of knowledge of the mechanisms of these reactions, so a brief description of the topic is presented next. 

Although the Bailar inversion (an octahedral complex substitution D L conversion, where d represents a right-handed screw relative to the C2 symmetry axis of an octahedral complex ion of the type cis-[Co(en)2Cl2] in which en = ethylenediamine, as shown in Fig. 1) was first reported by John C Bailar, Jr. and R. W. Auten in 1934, the exact mechanism of this type of reaction is still unknown. Originally, the reaction was considered to be due to the reaction of Ag2C03 with the [Co(en)2Cl2] ion... 

This is the most common and stable state of chromium in aqueous solution. The Cr ion, with 2d electrons, forms mainly octahedral complexes [CrX ], which are usually coloured, and are kweticallv inert, i.e. the rate of substitution of X by another hgand is very slow consequently a large number of such complexes have been isolated (see below, under chromium(III) chloride). 

However, high spin (P and (P species, which possess 4, 5, and 4 unpaired electrons, respectively, are labile, as are (P through (P octahedral complexes. In addition to the inert (P systems, low spin (P and (P complexes are inert to rapid substitution. The (P species are the least labile of the configurations classed as labile. 

Planar-octahedral equilibria. Dissolution of planar Ni compounds in coordinating solvents such as water or pyridine frequently leads to the formation of octahedral complexes by the coordination of 2 solvent molecules. This can, on occasions, lead to solutions in which the Ni has an intermediate value of jie indicating the presence of comparable amounts of planar and octahedral molecules varying with temperature and concentration more commonly the conversion is complete and octahedral solvates can be crystallized out. Well-known examples of this behaviour are provided by the complexes [Ni(L-L)2X2] (L-L = substituted ethylenediamine, X = variety of anions) generally known by the name of their discoverer I. Lifschitz. Some of these Lifschitz salts are yellow, diamagnetic and planar, [Ni(L-L)2]X2, others are blue, paramagnetic, and octahedral, [Ni(L-L)2X2] or... 

Several authors have suggested that the pathway may prove to be the most common mechanism in substitution reactions of octahedral complexes generally. However, the D path can be clearly demonstrated in some cases including at least two examples from Co(III) chemistry. The path (I - III - IV, Fig. 7) through the fivecoordinate intermediate would lead, in the case of rate studies in the presence of excess anionic ligand, to observed first-order rate constants governed by equation (13)... 

Figs. 11 and 12 show typical mo diagrams for square planar and octahedral complexes. Inspection reveals that the metal orbital (z is the axial direction) in a square planar complex is involved in the n bonding system and available for a bonding in the transition state. This is a feature shared by nucleophilic substitution at square planar complexes with the spectacularly associative nucleophilic aromatic substitutions. The octahedral complexes discussed in this chapter... 

Beattie and Basolo have investigated the reactions of the substitution-inert octahedral complexes of Pt(IV) with tris(bipyridine)chromium(II). A rapidmixing, stopped-flow apparatus was made use of in the majority of experiments. 

A hexadentate N202S2 ligand (116) has been synthesized as a dithiaalkyl-substituted triazene-1-oxide and forms an unusual hexacoordinated zinc complex with thioether donors. Both the phenyl and the methyl derivatives form structurally characterized octahedral complexes.896... 

Under (i) the square and pyramidal complexes are often easier to substitute than the octahedral complexes for the obvious reason that they have open residual coordination sites, looking upon all the complexes as derived from an octahedron. The mechanism of substitution can then be the typical organic Sn2 attack. More usually the reactions of complex ions proceed by predissociation, SnI, so that the important consideration is that c and d should be at least relatively good leaving groups. 

If one end of a chelate ring on an octahedral complex is detached from the metal, the five-coordinate transition state can be considered as a fluxional molecule in which there is some interchange of positions. When the chelate ring reforms, it may be with a different orientation that could lead to racemization. If the chelate ring is not symmetrical (such as 1,2-diaminopropane rather than ethyl-enediamine), isomerization may also result. For reactions carried out in solvents that coordinate well, a solvent molecule may attach to the metal where one end of the chelating agent vacated. Reactions of this type are similar to those in which dissociation and substitution occur. 

Explain the difference with respect to the size of the neighboring groups on substitution in an octahedral complex by associative and dissociative mechanisms. 

For studies of substitution reactions of octahedral complexes, many... 

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