Racemization, of octahedral complexes

Kinetically labile and inert complexes Dissociation, association and interchange Activation parameters Substitution in square planar complexes Substitution in octahedral complexes Racemization of octahedral complexes Electron-transfer processes... 

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... 

The jS-diketones are a versatile class of ligand with many known modes of coordination, some of which are shown in Figure 3. 0,0 -chelation (11) of the monoanion is by far the most common mode, being found for almost all of the metallic and metalloid elements. Compounds of the stoichiometry M(dike)3 and M(dike)2 are the most common. The former generally have an almost octahedral distribution of the six oxygen atoms when M is a main group or transition metal. These compounds have been used extensively in studies of the mechanism of the racemization of trischelate complexes. 

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. 

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. 

Fig. 2.15 Plot of AS (J K- mol ) vs AK (cm moK ) for racemization and geometrical isomerization of a variety of octahedral metal complexes. Only a few entries are selected from the 27 reactions tabulated in Ref. 180. The deviation of (four) Cr(IlI) complexes represented by Cr(phen) + (3) from the linear plot (best fit for 23 complexes) may indicate that these recemize by twist, and not dissociative, mechanisms. Racemization of Cr(C204)3 -(l), Co(Ph2dtc)3(2), Cr(phen)3 (3), Ni(phen)f+(4). Geometrical isomerization of trans-Cr(C204)2(H20)2 (5), trans-Co(en)2(H20) +(6), (3-Co(edda)en+(7).

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

Oxalate (ox, 204 ) complexes of Cr° have been known since the very beginning of coordination chemistry. Thus, the resolution of chiral [Cr(ox)3] with strychninium counterion by Werner in 1912 prodnced the first optically active anionic coordination compound. There also exists a series of bis(oxalato) complexes of the type [Cr(ox)2X2] + , where X can be any of a variety of nentral donors (e g. H2O, NH3, etc.) or anionic ligands (e g. SCN, N3, etc.). These compounds have been used to study the mechanism of cis/trans isomerization and racemization of optically active octahedral coordination compounds. 

An X-ray crystallographic analysis revealed that 64 is a 2 -metallacryptand ( = triple helicate) (Figure 29). Each of the two iron centers is octahedrally surrounded by six oxygen atoms. In the chiral, racemic complex 64, both iron centers are identically coordinated. Therefore, the 2 -metallacryptand is either a (A,A)- c or A,A)-fac triple helicate. The crystals obtained are composed of the homochiral 2 -metallacryptand 64. The donating power within the interior of the complex is insufficient for the complexation of alkali cations. Above all, the three phenyl hydrogen atoms in 64 are directed inward towards the empty cavity in the center. 

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