Octahedral complexes isomerization reactions

With inert octahedral complexes, these reactions appear to be intramolecular. This can be demonstrated by allowing the reaction to proceed in the presence of the uncomplexed isotopically labeled ligand. A form of linkage isomerism that involves 0-atom transfer has been observed in hKNOXNOj) and M(CO)(C02) systems. 

These results are consistent with active sites consisting of highly distorted octahedral WOx clusters on Zr02. Acid sites formed by these octahedral WO surface species are more effective isomerization sites than previously reported tetrahedral WOx species on AI2O3 [17], possibly because of the ability of WOx clusters to form metastable proton-containing complexes during catalytic isomerization reactions. 

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... 

On the other hand, Werner was aware of and did investigate substitution reactions of octahedral complexes which yield isomer mixtures (97-102). Furthermore, he knew that isomerization sometimes occurred without apparent reaction. In fact, his contemporary, S. M. J0rgensen, observed the isomerization of [CoCl2(en)2] before the turn of the century (62). Therefore, the stereomobility of substitution reactions of inert, octahedral species has long been of interest to coordination chemists. 

Most other inorganic reactions have been carried out using ETC catalysis isomerization of octahedral complexes [39 1], disproportionation [42], metal-metal bond cleavage and formation [43, 44], CO extrusion in formyl complexes [11]. Although many studies involve electrochemical initiation, the use of a chemical oxidant is also often shown to work. It is possible to use a photoexcited state as the initiator given its enhanced redox power [45]. 

Two general synthetic paths to Rh polyamine complexes have emerged extension of the procedure for the synthesis of [Rh(en)2X2] (reaction of RhCls SHjO or [RhCl ] " with the HCl salt of the appropriate amine), or the reaction of t/ fl -[Rhpy4Cl2] with the appropriate amine. The [Rh(trien)Cl2] ion was first prepared by Basolo and Johnson and later by Gillard and Wilkinson. Three geometric isomers are possible for octahedral complexes with linear tetraden-tates (49), and while isomeric assignment was not made in these initial studies, later work showed... 

In metal complexes, ligands may occupy different positions aroimd the central atom. Since the ligands in question are usually either next to one another cis) or opposite each other (trans), this type of isomerism is often also referred to as cis-trans isomerism. Such isomerism is not possible for complexes with coordination numbers of 2 or 3 or for tetrahedral complexes. In those systems, all coordination positions are adjacent to one another. However, cis-trans isomerism is very common for square planar and octahedral complexes, the only two types to be discussed here. Methods of preparation and reactions of some of these compounds are described in Chapter 4. 

The use of activation volumes in the diagnosis of mechanism has continued to provide much valuable information. Activation volumes for substitution at octahedral complexes have formed the subject of a well-referenced review,in which the importance both of intrinsic and of solvation contributions is recognized. The topics of most relevance to this chapter include isomerization and racemization reactions of cobalt(III) complexes, aquation of cobalt(III) and of iron(II) complexes, and base hydrolysis of cobalt(III) complexes. Merbach s continuing investigations into the effects of pressure on rates of solvent exchange at 2-h and 3+ transition metal cations, while not being always strictly... 

This chapter deals with substitution reactions, including aquation, base hydrolysis, formation, and ligand exchange and replacement, and isomerization of inert metal complexes in which the metal has a co-ordination number of five or more. In fact the great majority of the references reported are concerned with octahedral complexes references to complexes of other coordination numbers have been collected together at the end of this chapter (Section 10). 

Density functional theory (B3LYP) has been used to investigate the mechanism of the cis-trans isomerization in Ir(H)2(CO)(C6H3 CH2P(H2) 2).300 The preferred mechanism involves two consecutive trigonal twists in which the complex passes through a distorted octahedral intermediate, as shown in reaction Scheme 19. 

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