Acid-catalysed aquation

A slightly different aspect of catalysis of aquation of complexes is that of aquation of, for instance, azides by nitrous acid. This has already been mentioned in connection with the generation of intermediates of the type [Co(NH3)5] + in the first section of this chapter. A similar system is nitrous acid catalysed aquation of pyridiomethyl- or benzyl-pentacyano-cobaltate(iii), where aquation proceeds by S l attack at the ligand. ... 

Values of AH, AS, and AV for the acid-catalysed aquation of cis-[Cr(LL)2(OH2)2]- ions (LL=ox or mal) are collected in Table 14 (see Introduction, ref. 11). The small positive AV values for the aquation reactions are consistent with a mechanism in which proton attack at the co-ordinated uniden-... 

This appears to be the first kinetic study of catalysis by a soft cation of aquation of a chloro-complex of a square-planar metal centre many analogous examples are of course known for octahedral complexes. Interestingly, whereas Hg + is very effective both in octahedral and in square-planar systems, Cd + is a very feeble catalyst in octahedral systems (c/., for example, aquation of the [ReClJ anion ). Another type of aquation catalysis well established for octahedral complexes is that of acid catalysis of aquation of nitro- and azido-complexes. Kinetics of comparable reactions have now been described for acid-catalysed aquation of [PdX(LLL)]+ cations, where LLL = dien, Et4dien, or the A -methyl derivative of the latter, and X = N3 or NO2, in methanol. These reactions were conducted in the presence of chloride, and the complete rate law established as... 

Table 8 Rate parameters for the acid-catalysed aquation reactions of a- and / -[Co(edda)C03]" ions at 298.1 K and /= 2.0 mol dm (KNO3) [reactions (1)—(3) data from ref. 79 values in parentheses from ref 80 at 7=1.0 mol dm" (Naa04>]...

Rate data are reported for the acid-catalysed aquation of the [M(NH3)5COal+ ions (M = Rh or Ir ), as well as the rates of formation of these carbonato-com-plexes from [M(NH3)50H] + and carbon dioxide. At 298.1 K and p = 0.5 mol 1 , the aquation rate constants are 1.13 and 1.45 s and the formation rate constants 470 and 5901 mol S for the rhodium(m) and iridium(m) complexes respectively. The close similarity of these rate constants for both metal ions indicates carbon-oxygen bond fission for the aquation reactions. 

Ruthenium(II) [Ru(NH3)50H2] can be most efficiently prepared by zinc amalgam reduction of [RuCl(NH3)5]Cl2 in aqueous solution. More recently, an alternative route avoiding Zn + and Cl" ion has been developed based on the aquation of electrochemically reduced [Ru(03SCF3)(NH3)5] " . Alternative routes include the photolysis and acid-catalysed hydrolysis of [Ru(NH3)g] and the reduction of [Ru(NH3)5(OH2)]. The lability of the aqua ligand in these systems makes [Ru(NH3)jOH2] an excellent starting material for the synthesis of substituted pentaammine complexes, and for the study of their kinetics of formation. 

Hydrolysis of complexes of the type MFe is catalysed not only by acid but also by a series of metal ions. Kinetic data have been obtained for catalysis of hydrolysis of PFe, AsFg", AsFgCOH)- and also of BF4-, by beryllium(ii), aluminium(iii), zirconium(iv), and thorium(rv). Again this may be seen as an extension of studies on cation catalysis of hydrolysis of transition-metal complexes, e.g. the numerous studies of mercury(ii)-catalysed aquations of cobalt(iii)-ammine-halide complexes, or the recent study of metal ion catalysis of chloro(ethylenediaminetriacetato)-cobaltate(m). ... 

Aquation of /ra 5 -[CoCl(N3)en2]+, which results in loss of azide, is acid catalysed, but the limiting rate depends on the acid used. A study of rates as functions of added acid and added salts indicates that this limiting rate behaviour can be ascribed to the formation of ion-pairs of varying lability. Relevant to acid catalysis of aquation of azides is the characterisation of the perchlorate of the proposed protonated form in aquation, [Co(N3H)(NHg)5] +. Complexes of HP04 can be considered as equivalent to protonated forms of POi " complexes kinetic parameters for aquation of [Co(P04)(NH3)s] and of [Co(P04H)(NH8)5]+ have been obtained. ... 

Aquation of [Co(NCO)(NH3)5] + is also acid catalysed, indeed rates are proportional to hydrogen ion concentration. The products are [Co(NH3)6] + and carbon dioxide, so here the mechanism involves not cobalt-nitrogen but rather intraligand nitrogen-carbon bond fission. ... 

Aquation of [Rh(N3)(NH3)5] " is, like that of [Co(N3)(NH3)5] +, acid catalysed. From the pH-rate profile the pifa of the co-ordinated azide in the rhodium compound has been estimated as — 2-2, ... 

A mechanism of aquation is proposed in which the bidentate ligand becomes unidentate-bound in a pre-equilibrium and is then lost in an acid-catalysed process with a small contribution from an acid-independent path. 

Acid-catalysed hydrolysis of cw-[Co(en)2(L)(N02)] + ions (L=NHa, PhNHa, NCS, or HgO) and of fra/w-[Co(en)2(NCS)(N02)] ion has been investigated in perchloric acid over a wide acidity range. Protonation and loss of the co-ordinated nitro-group is involved. The effect of organic solvents on the reaction rates was also investigated. For the aquation of c -[Co(en)2(L)I] + ions (L=NHs, MeNHj, BuNHj, allylamine, PhNHa, or py) an nI reaction is proposed for all cases except L=py for which an Sn2 mechanism is proposed. Perhaps covalent hydrate formation is involved with the pyridine complex (for a discussion of this topic, see the Introduction). 

Aquation of [V0(02)2L] ions (L=ox , bipy, phen, 3,4,7,8-tetramethylphen, picolinate, or isoquinolate) has been studied in HC104/LiC104 solutions at 25 °C and 7=1.0 M. The rate of the reaction is inversely dependent on [HaOj] for L = ox , bipy, or phen, but independent of [HjOg] for the rest. The reactions are acid catalysed. ... 

Catalysed Aquation.— Acid catalysis occurs whenever the leaving group is the anion of a weak base (e.g. F, Ng-, CN, and many oxoanions) or for multidentate leaving groups which often protonate when only partially chelated. Examples appear in earlier sections and will not be considered specifically here. Catalysis by micelles, by electrophiles other than protons (e.g. metal ions or NO+), or by attack at coordinated ligands (e.g. oxidation and reduction) will be discussed here. Base hydrolysis, which usually involves conjugate-base formation, is dealt with in a later section. 

Table 17 Comparison of acid-catalysed rate constants ki [equation (50)] for the aquation /[RuCNHaljL] ions at 298.1 K...

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