Ligand exchange reactions pseudohalide complexes

Ligand exchange reactions leading to chelated complexes have been described in Section 8.02.3.1. In general, halide and pseudohalide exchange reactions are facile processes. The kinetics and the thermodynamics of the displacement of halide by SGN have been investigated in binuclear A-frame complexes by two-dimensional EXSY... 

For the pseudohalide complexes, in addition to T1 NMR spectra, NMR spectra of the exchanging ligand could be recorded, which made it possible to detect the following exchange pathway, in addition to reaction (17) ... 

A series of anionic monodentate A-donor ligands such as azido, isocyanato, isothiocyanato, and nitro groups have been used to form neutral pseudohalide complexes. They are usually obtained by exchange reactions between a chloride precursor and the appropriate sodium or silver salt of the ligand. Representative examples include the following ... 

LMCT excitation of Co (NH3)5X2 with X = halide or pseudohalide yields the complex fragment Co (NH3)52+ which is kinetically very labile [2,5,94]. Co(II)-ammine complexes decompose in aqueous solution to Co + and ammonia with a rate constant k > lO s [95]. The LMCT photolysis of PtCl52- and other Pt(IV) complexes leads to the formation of Pt L5 intermediates [111-113] which are also kinetically labile and undergo facile ligand substitutions. Since these Pt(III) radicals can efficiently exchange electrons with Pt(IV) they catalyze the substitution of inert Pt(IV) complexes in a chain reaction [2,10]. The overall quantum yield for photosubstitution of PtClg - was shown to exceed unity. 

One successful strategy for ligand substitution by ion exchange involves the use of halide or pseudohalide metal complexes as starting materials in reactions with silver salts that contain the anion that is to be introduced as a new ligand. Since the solubility of silver halides in nonpolar, aprotic solvents is low (see Table 3.1), ion exchange in solvents of poor donor ability typically occurs according to Eq. 5.1 ... 

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