Isomerism coordination-sphere

This compound has an octahedral coordination sphere slightly distorted towards a trigonal prism. Oxidation by air leads to Ru2(S2CNEt2)5 which exists in two isomeric forms (Figure 1.52) [138]. 

The isomerization mechanism is clearly established by labeling experiments. The rearrangement of a to c via a 7i-allyl hydride complex b in the coordination sphere of the metal is a key step in this cayalytic cycle (Scheme 54) [174, 175]. hi case of polyunsaturated derivatives, formation of a stable q" complexes (Scheme 55) is preferred over the rearrangement (a c). 

Besides dissociation of ligands, photoexcitation of transition metal complexes can facilitate (1) - oxidative addition to metal atoms of C-C, C-H, H-H, C-Hal, H-Si, C-0 and C-P moieties (2) - reductive elimination reactions, forming C-C, C-H, H-H, C-Hal, Hal-Hal and H-Hal moieties (3) - various rearrangements of atoms and chemical bonds in the coordination sphere of metal atoms, such as migratory insertion to C=C bonds, carbonyl and carbenes, ot- and P-elimination, a- and P-cleavage of C-C bonds, coupling of various moieties and bonds, isomerizations, etc. (see [11, 12] and refs, therein). 

Several interesting observations have been made on this reaction. First, the rate of isomerization was found to be the same as the rate of dehydration. All attempts to dehydrate the starting complex by conventional techniques were found to lead to isomerization. On the basis of this and other evidence, the mechanism proposed involves the aquation in the complex followed by anation. In this process, water first displaces Cl- in the coordination sphere and then is displaced by the Cl-, possibly by an SN1 mechanism. A trigonal bipyramid transition state could account for the Cl- reentering the coordination sphere to give an cis product. The rate law for this reaction is of the form... 

Rates of reaction and the course of each step depended upon the structures of the species. With a-methylstyrene and with methallyl chloride, steps (b) and (c) were not detectable perhaps (a) did not happen and the product was that of direct addition with no exchanges or isomerization. With 1-hexene, 3-methyl-1-butene-1,3-methyl-2-butene, and cyclohexene the exchanges and isomerization were much faster than step (d). By step (d) the product left the coordination sphere of the metal and no longer participated in any of the processes. 

The energy minima between the energy barriers for the monomer coordination and insertion correspond to alkene-bound intermediates of the kind simulated by our molecular mechanics calculations (Figures 1.7 and 1.9). The possible dissociation of the monomer coordinated with the wrong enantioface can lead back to the alkene-free intermediate or, directly, to the alkene-bound intermediate with the right enantioface (through some isomerization mechanism, for which the monomer does not leave the coordination sphere of the metal). 

Furthermore, isomerization of the heptadentate XXXI, coordinated to Ni(II) via transmetallation (91), occurs again aimed at its adaptation to a octahedral coordination sphere around ions with d8 configuration. In the case of the more flexible pentadentate ligands, XXXII and its analogue with three methylene groups (L2) (92), formation of monomeric (with XXXII) and dimeric (with L2), but also of polymeric (with L2) Ni(II) complexes with an octahedral environment around the metal is possible (94). 

N-, 0-, and S-heterocyclic ligands also form [Os(NH3)5 t)2-(C,C)-L ]2+ complexes [L = 2,6-lutidine, 2,6-lutidinium, pyridinium, N-methylpyridinium, and lV-methyl-4-picolinium (85, 167), NJV -dimethylimidazolium (90), pyrrole (90, 179), IV-methylpyrrole (90, 179), thiophene (90,179), furan (90,179), and 1,3-dimethyluracil (72, 73)]. On oxidation to Os(III), arene ligands are rapidly lost from the coordination sphere, or in the case of the substituted arene ligands with good a donors, rapid linkage isomerization reactions occur (Section V,D). 

The crystal structures of two isomeric 15-membered ring macrocycles containing the donor groups ON2S2 (70) and OS2N2 (71) have been determined as their silver thiocyanate salts.468 469 In the first, the coordination sphere around the Ag1 ion embedded in the cavity was approximately square pyramidal, with a weak interaction between the Ag ion and the O atom (Ag—O 288.3 pm). In the second, a square pyramidal structure was again obtained, although in this case there was no interaction with the O atom (Ag—O 371.9 pm). 

Ionization, hydrate and coordination isomerism are classifications of constitutional isomerism that originated with Werner.27,28 Ionization and hydrate isomerism (equation 1) apply to cases in which there is a ligand exchange between primary and outer coordination spheres, whereas coordination isomerism (equation 2) arises in systems containing at least two metal ions, so that alternative primary coordination spheres are available. 

If the electronic spin state change were the critical determinant of the dynamics of spin equilibria, then the AS = 1 equilibration between planar and tetrahedral nickel(II) isomers would occur more rapidly than equilibration of the octahedral AS = 2 spin states. This is not observed. Even though the AS = 1 transition is most likely adiabatic, the large coordination sphere reorganization energy requirement causes these nickel(II) isomerizations to occur relatively slowly, with relaxation times of the order of a microsecond. 

Just as expansion of the coordination sphere by transition from the low-spin to the high-spin state enhances isomerization and racemiza-tion, so too should it enhance the rate of ligand substitution. Unpublished observations on the ligand substitution of [Fe(pyim)3]2+, which is coupled with its spin equilibrium, indicate that it is the high-spin state which preferentially undergoes the substitution reaction, as expected (117). 

The introduction of more than one C- or jV-substituted en ring into the coordination sphere considerably increases the isomeric complexity. Many of these ligands are now unsymmetrical and their complexes may exhibit geometric isomerism dependent upon the end-for-end orientation.257 Thus there are 24 distinct configurational and conformational forms expected for Co(R,5-pn)3+, 258 Table 6 lists a variety of C- and JV-substituted ethylenediamine type ligands that have been investigated. 

This C—H substitution process results in a Markovnikov orientation, with the H that is allylic to the more substituted end of the alkene preferentially abstracted. The stereochemistry of the resulting ir-allyl complex does not represent the stereochemistry of the starting alkene, as the complexes are capable of isomerization under the conditions in which they are formed. Typically, a thermodynamic mixture is obtained, with the syn form of the complex predominating over the anti form (equation 1). The syn form is more stable due to unfavorable steric interactions that the anti form encounters with the coordination sphere of the palladium. 

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