Octahedral complexes mechanism

The mechanism of octahedral complex formation by labile metal ions. D. J. Hewkin and R. H. Price, Coord. Chem. Rev., 1970, 5, 45-73 (177). 

Now look at octahedral complexes, or those with any other environment possessing a centre of symmetry e.g. square-planar). These present a further problem. The process of violating the parity rule is no longer available, for orbitals of different parity do not mix under a Hamiltonian for a centrosymmetric molecule. Here the nuclear arrangement requires the labelling of d functions as g and of p functions as m in centrosymmetric complexes, d orbitals do not mix with p orbitals. And yet d-d transitions are observed in octahedral chromophores. We must turn to another mechanism. Actually this mechanism is operative for all chromophores, whether centrosymmetric or not. As we shall see, however, it is less effective than that described above and so wasn t mentioned there. For centrosymmetric systems it s the only game in town. 

Several authors have suggested that the pathway may prove to be the most common mechanism in substitution reactions of octahedral complexes generally. However, the D path can be clearly demonstrated in some cases including at least two examples from Co(III) chemistry. The path (I - III - IV, Fig. 7) through the fivecoordinate intermediate would lead, in the case of rate studies in the presence of excess anionic ligand, to observed first-order rate constants governed by equation (13)... 

Under (i) the square and pyramidal complexes are often easier to substitute than the octahedral complexes for the obvious reason that they have open residual coordination sites, looking upon all the complexes as derived from an octahedron. The mechanism of substitution can then be the typical organic Sn2 attack. More usually the reactions of complex ions proceed by predissociation, SnI, so that the important consideration is that c and d should be at least relatively good leaving groups. 

Explain the difference with respect to the size of the neighboring groups on substitution in an octahedral complex by associative and dissociative mechanisms. 

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... 

Since we shall not obtain the comparable amount of detailed information on the mechanisms of substitution in octahedral complexes from the studies of more complicated substitutions involving chelation and macrocycle complex formation (Secs. 4.4 and 4.5) it is worthwhile summarizing the salient features of substitution in Werner-type complexes. 

There have been extensive studies of the influence of an entering ligand on its rate of entry into a Pt(ll) complex.The rate constants for reaction of a large number and variety of ligands with trans-Pt(py)2C 2 have been measured (Table 4.13). The large range of reactivities is a feature of the associative mechanism and differentiates it from the behavior of octahedral complexes. The rate constants may be used to set up quantitative relationships. For a variety of reactions of Pt complexes in different solvents (Sec. 2.5.4) ... 

Water exchange rates on [Fe(L)(H20)x]" chelates of trivalent iron have been studied in the group of Rudi van Eldik. With the exception of EDDS -ligand, activation volumes are small and positive, indicating an Id mechanism for water exchange on these complexes which are supposed to be seven-coordinate. [Fe(EDDS)(H20)] is probably an octahedral complex and the... 

Dr. Halpern This could be used in stabilizing, say an activated complex. The point about the hydrolysis observation is that this refers to the octahedral complex, whereas the explanations that have been offered for the effect of amide in the conjugate base mechanism are concerned, not with weakening of the binding, but with stabilizing a five-coordinated intermediate. I wondered if the role of the hydroxide in promoting water substitution might be of the same nature. 

Substitution reactions in octahedral complexes may proceed by D, Idt Ia, or A mechanisms and it is often difficult to distinguish between them because the rate law by itself does not allow the distinction to be made. Consider the replacement of water by ligand L under neutral conditions ... 

Because of the inertness of Co(III) and Cr(III) complexes, their substitution reactions were the first among those of octahedral complexes to be extensively studied. Most evidence supports the fd mechanism for substitution in Co(fll) complexes. First, there is little dependence of reaction rates on the nature of the incoming ligand, if bond making were of significant importance, the opposite would be expected. Data are presented in Table 13.4 for the anation reaction of penta-ammineaquacobaltdll) ... 

It was the optical resolution of [Co(en)2(NH3)Cl]2+ that firmly established Werner s theory and which initiated the study of the optical activity of complex ions. The realization that some octahedral complexes are chiral evidently did not occur to Werner until several years after he published his theory of coordination. He then realized that the demonstration of this property would furnish an almost irrefutable argument in favor of his theory, and he and his students devoted several years to attempts to effect such resolution. Had he but known it, the problem could have been easily solved, for cis-[Co(en)2(N02)2]X (X = Cl, Br) crystallizes in hemihedral crystals which can be separated mechanically, just as Pasteur separated the optical isomers of sodium ammonium tartrate. 

The addition and dissociation of pyridine and substituted pyridine molecules to a planar nickel(II) complex with a quadridentate N202 ligand have been studied by the microwave temperature-jump technique in chlorobenzene solvent (38). The data were interpreted with the assumption of mechanism C (Fig. 7), i.e., that k65 is the smallest rate constant. Subsequently, however, 14N NMR was used to measure the rate of pyridine exchange from the octahedral complex (138). The rates are the same for the two different experiments within a factor of two. This observation excludes mechanism B and is consistent with either mechanism A or C. The rate constants have consequently been presented in Table VI as k64. 

Although direct complex formation is observed kinetically (stopped flow) and spectrophotometrically, where X = Br or Cl, the reaction with I results in an oxidation of the halide. The reactions are rapid and there is the question of inner- or outer-sphere electron transfer, for the [14]aneN4 complex. However, further studies (140) using ligand substituted (dimethyl) complexes reveal that for the rac-Me2[14]aneN4 isomer, two processes are observed, k = 2.9 x 104 M-1 sec-1 and a subsequent redox step, krci = 5.5 x 103 M-1 sec-1, both of which are iodide dependent. The mechanism proposed involves the formation of an octahedral complex which further reacts with a second mole of I- in the redox step ... 

Octahedral six-coordination is especially favoured by the low-spin d6 configuration. This can be understood in terms of simple CFSE considerations. For M(III) (M = Co, Rh, Ir) and Pt(IV), hardly any complexes other than octahedral ones are known. These complexes are kinetically fairly inert, in the sense that they undergo ligand exchange reactions slowly. For this reason, much of our knowledge of kinetics and mechanism in transition element chemistry has come from studies of low-spin d6 octahedral complexes (see Sections 9.4 and 9.5). 

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