Racemization, octahedral complexes

The first resolution of an octahedral complex into its enantiomers was achieved in 1911 by A. Werner, who got the Nobel Prize in 1913, with the complex [Co(ethylenediamine)(Cl)(NH3)] [10]. Obviously, resolution is to be considered only in the case of kinetically inert complexes whose enantiomers do not racemize quickly after separation. This is a very important remark since, as noted above, the interesting complexes are those containing exchangeable sites required for catalytic activity and thus more sensitive to racemization. We will not discuss here the very rare cases of spontaneous resolution during which a racemic mixture of complexes forms a conglomerate (the A and A enantiomers crystallize in separate crystals) [11,12]. 

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. 

These parameters often parallel one another since they are related to similar characteristic of the system (ehange in number of particles involved in the reaction etc.). The catalyzed hydrolysis of CrjO by a number of bases is interpreted in terms of a bimolecular mechanism, and both AS and AK values are negative. In contrast the aquation of Co(NH2CH3)5L (L = neutral ligands) is attended by positive AS and AK values. The steric acceleration noted for these complexes (when compared with the rates for the ammonia analogs) is attributed to an mechanism.There is a remarkably linear AK vs AS plot for racemization and geometric isomerization of octahedral complexes when dissociative or associative mechanisms prevail, but not when twist mechanisms are operative (Fig. 2.15). For other examples of parallel AS and AF values, see Refs. 103 and 181. In general AK is usually the more easily understandable, calculable and accurate parameter and AK is... 

Octahedral Complexes 345 Tkble 7.5 Activation Parameters for Racemization and Ligand Dissociation of MCAA)" Ions at 25 °C... 

The bimolecular racemization of ethylenedinitrilotetraacetatocobalt-ate(III) 29j 36) should be noted. This racemization suggests that bimolec-ularity should not be excluded in mechanistic considerations of octahedral complexes. Base hydrolysis studies of other complexes without ionizable protons would be of considerable value, provided they are of intermediate field strength. 

Ni— N, 215 pm) and square planar bis chelates [NiL2]S04. TTie former have been easily obtained in H20/Me0H solution and the latter in MeOH solution under anhydrous conditions. Analogous complexes can be obtained with the ligand l-amino-2-propanol. It has been found that the coordination of either the racemic or the optically active isomer of the latter ligand in trans octahedral complexes Ni(NCS)2L2 makes the difference in the complex stability very smalP (bond distances Ni—O, 209-211 Ni—N, 207, 208 pm). 

Thus, chromatography on Sephadex ion-exchangers is very effectively applied to multivalent complex cations. Jensen and Woldbye have reviewed the optical activity of coordination compounds resolutions of racemic octahedral transition metal complexes through both diastereoisomer and chromatographic techniques are summarized. 

The reactions of CN with nickel(II) compounds of the C-racemic ligand, (8), have been followed kinetically. " The square-planar complex exists in three forms a, with hydrogens 1, 8 up and 4, 11 down with all hydrogens down and 7, with 4, 8 up and 1, 11 down. Transformation between the isomers is dependent on [OH j. All three isomers rapidly equilibrate with one CN" to form [Ni(tetraL)CN], and addition of the second CN is rate determining. Though the final product is [Ni(CN)4], trans addition of the second CN" produces an unreactive octahedral complex [Ni(tetraL)(CN)2]. The final product is believed to be produced via cis addition of the second cyanide, following a hydrogen-bond interaction with one of the NH units (Scheme 6). 

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