Stereochemistry of octahedral substitutions

An object which is not superimposable upon its mirror image is chiral (like our right and left hands cheir = hand in Greek). We can say that the general property of handedness is chirality. Two components that differ in handedness are called enantiomers (from Greek, meaning opposite forms). Stereo- 

When ethylenediamine chelates to a metal atom, a five-membered ring is formed  

In this ring, the M-N bonds are longer than the C-C and C-N bonds, which causes a strain in the ring. The strain can be relieved by ring puckering. There are two different puckered forms, 8 and X, which are enantiomers. 

Since there are three ethylenediamine ligands, it may be shown that there is a set of eight possible configurations. In the crystals of [Co(en)3] only one of the eight possible forms is usually present. Why this is so is a question that has not been fully answered yet. Additional information can be found in special literature.  

Of special interest is the fact that base hydrolysis leads to substantial stereo changes  

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Although Werner s interpretation of the cause of stereochemical changes during substitution and how they take place adequately explains the experimental facts, it has almost no predictive value. This is not intended as a criticism for we will see in the discussion which follows that we are still largely unable to predict the steric course of a substitution reaction of an octahedral metal complex. This is true despite the availability of much more experimental data and more sophisticated theories of bonding. Excellent reviews (27) have been written on the stereochemistry of octahedral substitution reactions. The discussion that follows deals almost exclusively with cobalt (III) complexes, but the principles involved are generally applicable to other octahedral systems. 

This promise has been only partially fulfilled because of the difficulty of interpreting anation mechanisms where second order kinetics, first order in entering anion and first order in complex, are often found because of ion association which contributes a term in anion concentration to the rate law. A further difficulty, emphasised by Archer in his recent review on the stereochemistry of octahedral substitution reactions, is found in cobalt(III) chemistry because of the difficulty in isolating trans solvent-containing species. This results in continued doubt in the study of such systems as ... 

G. R. Dobson, Inorg. Chem., 1980, 19, 1413. The stereochemistry of ligand-substitution reactions of octahedral metal carbonyls under kinetic control. 

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). 

Any detailed description of the mechanism of an octahedral substitution must also account for the stereochemical changes that accompany reaction. Werner recognized this and made use of it in his discussions of the stereochemistry of reactions of cobalt(III) complexes. The available experimental results can be explained on the basis of possible molecular rearrangements and some cautious predictions can even be made. The base hydrolysis of cobalt III)ammines appears to be unique in that it often occurs with rearrangement it also affords the few known examples of optical inversion. These results can be explained by formation of a 5-coordinated species with a trigonal bipyramidal structure. Optically active metal complexes racemize by either an intramolecular or an in-termolecular process. Substitution reactions of platinum metal complexes often occur with retention of configuration. 

C tereochemistry has played a major role in the development of chemistry, and it continues to be most significant. Werner made extensive use of the information available to him on the stereochemistry of metal complexes in developing his coordination theory. He made the first meaningful attempt to understand the mechanisms of substitution reactions of these systems on the basis of the stereochemical changes accompanying such reactions. The paper 49) he wrote in 1912 is a real milestone and should be read by anyone interested in octahedral substitution reactions. It is valuable because of the large amount of experimental data it contains on reactions of cis and [Pg.408]

The planar quinquedentate ligand 2,6-diacetylpyridine bis(semicarbazone), D APSC, has been synthesized and used to prepare the complex [(H20)2(DAPSC)Cr]-0H(N03)2,H20. The two water molecules occupy axial positions in the pentagonal bipyramid. It is suggested that the characterization of this stereochemistry for chro-mium(iii) may imply a possible role for seven-co-ordinate species as intermediates in octahedral substitution reactions. ... 

The terminology and notation that have been used to describe coordination compounds have been derived with one notable exception from the terms and symbols developed to describe the stereochemistry of carbon compounds. The terms ois, trans endo, exo dextro, d, D, (+) and leva, l, L (-) all have been used to describe the stereochemistry of coordination compounds in a close analogy with organic compounds (see Figure 1). As the descriptions of the chemistry and structures of coordination systems have become more varied and complex, the meanings of these terms have become less precise, as in the example of a ois or trans tricarbonyl octahedral compound (see Figure 2). The terms fao and mer were coined to indicate the facial and meridional disposition of substituted octahedral structures. 

The dissociative mechanism tends to be most favored in TBP d , followed by tetrahedral and then d octahedral. For example, d Co2(CO)s has a half-life for CO dissociation of a few tens of minutes at 0 , but for d Mn2(CO)io at room temperature the half-life is about 10 years This order is consistent with the relative stabilities of the stereochemistries of the starting material and of the intermediates in each case, as predicted by crystal field arguments (Section 1.4). Substitution rates tend to follow the order 3rd row < 2nd row > 1st row. For example, at 50 , the rate constants for CO dissociation in M(CO)5 are Fe, 6 x 10 " Ru, 3 x 10 Os, 5 x 10 . The rate for Fe is exceptionally slow, perhaps because Fe(CO)4, but not the Ru or Os analog, is high-spin and less stable, leading to a higher activation energy. 

The tetrahedron is important in organic chemistry as representing the stereochemistry of the saturated carbon atom. It was first used in organic chemistry by Pasteur when he summarized his studies in 1862 on optical rotation of tartaric acid in solution. At about the same time Butlerov applied the tetrahedron concept to the carbon atom in connection with an assumed structure of ethane. Butlerov s paper influenced Kekule in the development of a tetrahedral carbon modeP useful for visualizing the links in acetylene H-C=CH and hydrogen cyanide, H-C=N. A few decades later Alfred Werner considered inorganic coordination compounds in a way analogous to that for carbon compounds. The octahedron was the key to a major portion of Werner s work which involved octahedral cobalt(III) ammines such as Co(NH3)6 + and their substitution products. 

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