Coordinated ligands Hydrolysis

In aqueous solution the An02" ions (An = Pa Am) may be formed, at least in the absence of strongly coordinating ligands. They are linear cations like An02 + but are less persistent and, indeed, it is probable that Pa02" should be formulated as [PaO(OH)2]" " and [PaO(OH)]. Hydrolysis is extensive in aqueous solutions of Pa and colloidal hydroxo species are formed which readily lead to precipitation of... 

Trimethylphosphate, despite being neutral, is known to form the simple complex [Co(NH3)5(OP(OMe)3)]3+ this reacts with nucleophiles X" S2C)32, I and SCN in that order of reactivity to produce [Co(NH3)5(02P(OMe)2)]3+ and MeX up to 150-fold faster than in the absence of coordination.1008 Both hydroxide and water are ineffective in this reaction, with simple ligand hydrolysis occurring more rapidly than any reaction. 

Hydrolysis of coordinated ligands is a special case of nucleophilic attack. Two examples involving inorganic ligands have already been given in Section II. A on aquation of cobalt(III) complexes. Many further examples will be found in the following Section VII.B on catalysis of hydrolysis of organic substrates by metal ions and complexes. 

Anions can promote hydrolysis of complex cations by producing ion pairs of enhanced reactivity (see 2.178). Usually however, ligands accelerate the removal of a coordinated ligand by entering the metal coordination sphere with it and thereby labilizing it towards hydrolysis. We have already seen the effect of coordinated OH on the enhanced labilities of Fe(III) and Cr(III). Dissociative mechanisms and considerable acceleration are promoted by CFlj, CN , SOj and other groups on inert Cr(III), Co(III) and Pt(IV) complexes. Nitrate ions, for example, reduce the half-life for replacement of water in Cr(H20)g by dmso from —380 h to 10 s ... 

Metal complexing can subject a ligand to severe internal strain or, alternatively, it can relieve strain or it can freeze the conformation of the coordinated ligand. These modifications often lead to enhanced reactivity by reducing the energy difference between ground and transition state. The subjection of the cyano group to strain when 2-cyanophenanthroline is coordinated to a metal ion, and its resultant accelerated hydrolysis, has been referred to previously (Sec. 

One of the earliest reported reactions of coordinated ligands involves the hydrolysis of acetonitrile coordinated to platinum(II) and the formation of acetamide.93 Such hydration is general... 

Finally, reactions of coordinated ligands can produce amines. These include oxidation of coordinated thiocyanate (equation 23) and hydrolysis of coordinated cyanate (equation 24). ... 

The energies for structures along the reaction coordinate for hydrolysis of the bicyclic phosphinate [54] are detailed in Scheme 15. Analysis of the TBP intermediates and SP transition states for exocyclic cleavage in the base-catalysed hydrolysis of the cyclic urea phosphonate [24a] takes account of the increased effective electronegativity of the methylene ligand in this system (Scheme 16). The calculations for [24] vividly illustrate that what is perceived as a disallowed pseudorotation is shown to be energetically favourable by careful analysis and application of relative apicophilicities. 

The coordinating ligands to Zn in /3-lactamases (Scheme 9) are essentially similar to those of CA (Scheme 11). In addition to acting as a CA model, Zm -cyclen (34) was found to catalyze the hydrolysis of A-benzylpenicillin (Scheme 36). Interestingly, (34) also catalyzed epimerization of the hydrolyzed product, (57 )-benzylpenicilloate, to (5 S)-benzylpenicilloate, unlike the enzymatic reaction. 

Chan and Tobe > investigated the reaction between trans-[CaC 2 pa) and cyanide and found that the concentrated aqueous solution was disproportionated to [Co(en>3] and some unidentifiable products, whereas in dilute solution hydrolysis of the cyanide ion occurred. This su ests that the design to replace a coordinated ligand such as Cl" in a complex by CN" is undesirable. However, mixed cyano complexes were obtained in the 1960 s. 

This process actually represents a reaction of a coordinated ligand rather than a substitution reaction. A similar result has been found for the acid hydrolysis of the carbonato complex, [Co(NH3)5C03], which also takes place without breaking the Co—O bond. 

Coordinated ligands often exhibit their own characteristic reactions. For example, the hydrolysis (38) and fluoiination (39) of PCI3 can be accomplished while PCI 3 itself functions as a ligand. 

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