Substitution Reactions of Tetrahedral Complexes

Tetrahedral metal complexes are very frequent. Such complexes can be formed by almost all elements with p electrons in the outer shell. F, Te, Bi, Po, At and the noble gases are exceptions. Almost all transition metals, except Nb, Ta, Sc, Y, La and Ac, build tetrahedral complexes. Substitution mechanisms of the tetrahedral complexes of Si, Ge, Sn and P have been most extensively studied. These substitutions are of the A or type, with an intermediate of the coordination number 5, their possible geometries being a square pyramid or a trigonal bipyramid. 

The substitution of X by Y of the silicon L SiX complex can proceed via a trigonal bipyramid, like in organic chemistry, with configuration inversion (scheme 2.9.1)  

In the case of a cis attack, the substitution will proceed with the retention of configuration (scheme 2.9.2)  

The high spin tetrahedral complexes of the first transition period are very labile. Their substitution reactions are associative in nature. Such complexes are, e.g., [Co Cy -, [Co ClsPy] , and [Coi Clapya]. 

spectroscopy has been used to study enantiomeric inversions of a series of tetrahedral chelates of zinc(ii), cadmium(ii), and lead(ii). The rates of inversion depend on the nature of the metal, the donor atoms, and the structure and substituents of the chelate rings. An intramolecular diagonal-twist mechanism is suggested. Rapid complex formation kinetics between zinc(ii) and the glycine zwitterion and pada (pyridine-2-azo-p-dimethylaniline) have been studied using relaxation methods. 

Associative mechanisms for ligand exchange in tetrahedral complexes of transition metals have been reported in several cases. Studies of tetrahedrally distorted copper and nickel complexes have already been mentioned above (see Chapter 2, refs. 131 and 133). Activation parameters for the solvent exchange in the tetrahedral nickel(ii) solvento complex in hexamethylphosphoramide, determined using n.m.r. spectroscopy, are compatible with an associative 

Complex formation between cobalt(ii) and pyridine-2-aldoxime has previously been reported to be a slow process. The reaction has now been reinvestigated, and the new study shows that the mechanism is not so simple as was believed. It probably involves redox processes, in which finally cobalt(iii) is formed. Ligand exchange of peroxide between hydrogen peroxide and tetraperoxo-chromate(v) ion, [Cr(02)4] , has been studied and gives relevant information for the mechanisms of tetraperoxochromate decomposition reactions.  

Possible mechanisms for substitutions in tetranuclear clusters [Fe4S4(SR)4] , which contain tetrahedrally co-ordinated iron, have been discussed, but quantitative kinetic data are not available.  

The relevance of square-planar-tetrahedral rearrangements for reaction mechanisms of square-planar complexes was mentioned in Chapter 2 (references 15 and 16). 

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Explain why substitution reactions of tetrahedral complexes ofBe2+ are slow. 

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