Hydrolysis of coordinated ligands

The ability of metal ions to accelerate the hydrolysis of a variety of linkages has been a subject of sustained interest. If the hydrolyzed substrate remains attached to the metal, the reaction becomes stoichiometric and is termed metal-ion promoted. If the hydrolyzed product does not bind to the metal ion, the latter is free to continue its action and play a catalytic role. The modus operandi of these effects is undoubtedly as a result of metal-complex formation, and this has been demonstrated for both labile and inert metal systems. Reactions of nucleophiles other than HjO and OH will also be considered. 

The discovery that metal ions such as Cu catalyzed the hydrolysis of amino acid esters was first reported nearly 40 years ago. Table 6.3 shows that in the absence of direct interaction of the ester grouping with the metal ion, the primary cause of any increased rates experienced 

There is a major problem in the interpretation of the results when labile metal ion complexes are used as promoters. It is difficult to characterize the reactant species and therefore to distinguish between the following modes of attack by OH (and HjO)  

coordination through O polarizes the C = O bond producing the incipient carbonium ion, which is more susceptible to nucleophilic attack. In 6, there is intramolecular attack by coordinated OH on the N-bonded monodentate amino acid. In 7, only a small rate enhancement would be anticipated from intermolecular attack on the N-bonded monodentate. In these representations, X is OR, but anticipating the next section, it might also be NHR. These interpretive problems are illustrated in the hydrolysis of complexes of the type 8, where again any of three mechanisms corresponding to 5-7 could apply. 

For 11 and 12 both OH -unassisted and -assisted paths exist for hydrolysis with ring closure. 

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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. 

There are a few documented examples of studies of ligand effects on hydrolysis reactions. Angelici et al." investigated the effect of a number of multidentate ligands on the copper(II) ion-catalysed hydrolysis of coordinated amino acid esters. The equilibrium constant for binding of the ester and the rate constant for the hydrolysis of the resulting complex both decrease in the presence of ligands. Similar conclusions have been reached by Hay and Morris, who studied the effect of ethylenediamine... 

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 hydrolysis of coordinated nitriles has recently attracted some attention. A very facile hydrolysis of nitriles to the corresponding amides at platinum(IV) centers has been described.The complex [Cu(H2NCOCH2CONHNH2)Cl] is formed from the reaction NCCH2CONHNH2 the copper(II) both promotes the hydrolysis and is reduced to copper(I). The hydrolysis of 2-cyanopyridine to 2-pyridinecarboxamide is accelerated several hundred times by the copper(II) complexes of the ligands (21) and (22). In the case of the (22) some picolinic acid was formed, resulting from the intramolecular attack of alkoxide to yield an intermediate iminoester. ... 

Nucleophilic attack at remote positions of coordinated ligands is observed when these are activated by strongly electron-withdrawing groups. For instance, PEts displaces a fluoride atom from complex 131 affording a phospho-niovinyl complex (Equation (66)). A related reaction is the hydrolysis of coordinated 2,4,6-trifluoropyrimidine, to afford a 4,6-difluoropyrimidin-2-one complex. An unusual reaction is the formal insertion (probably electrophilic attack) of heterocumulenes (GS2 or CO2) into a coordinate Ni-P bond of 70 (Equation (67)). 

The increase in electrophilicity of coordinated ligands commonly leads to more favorable hydrolysis reactions. There are many reports on hydrolysis reactions of coordinated carboxamides, halogenated alkylamines, Schiff bases, thioamides, nitriles, etc. For example, thiourea and thioacetamide complexes of platinum metals decompose on heating in basic solution with formation of the corresponding metal sulfides, Eq. 1.26 and 1.27 ... 

The formation of solvento-complexes as a result of redox transformations of coordinated ligands is not r tricted to N-donor ligands reactions with other ligands are well known. For example, oxidation of the dimethyl sulfoxide ligand in [Co(NH3)5(DMSO)]3+ can be effected by treatment with KMn04 or CI2 in aqueous solution. The sulfoxide oxidizes to the sulfone which exhibits a much lower donor ability than Me2SO and, as a result of hydrolysis, is expelled from the complex generating the aqua-complex. 

Molecular examples of trivalent molybdenum are known in mononuclear, dinuclear, and tetranuclear complexes, as illustrated in Figure 5. The hexachloride ion, MoCk (Fig- 5a) is generated by the electrolysis of Mo(VI) in concentrated HCl. Hydrolysis of MoCP in acid gives the hexaaquamolybdenum(III) ion, Mo(H20) g, which is obtainable in solution of poorly coordinating acids, such as triflic acid (17). Several molybdenum(III) organometaUic compounds are known. These contain a single cyclopentadienyl ligand (Cp) attached to Mo (Fig. 5d) (27). 

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... 

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