Hydrolysis mononuclear

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). 

Similarly, the cyanide, acetate and carbonate are unstable in aqueous solution. Hydrolysis of the halides and other salts such as the nitrate and sulfate is incomplete but aqueous solutions are acidic due to the ability of the hydrated cation [AI(H20)6] to act as proton donor giving [A1(H20)5(0H)]-+, (AI(H20)4(0H)2]+, etc. If the pH is gradually increased this deprotonation of the mononuclear species is accompanied by aggregation via OH bridges to give species such as... 

The study of plutonium hydrolysis is complicated by the formation of oligomers and polymers once the simple mononuclear hydrolysis species start forming. The relative mono-oligomer concentrations are dependent on the plutonium concentration - e.g. the ratio of Pu present as (Pu02)2(0H)22 to that as PuO2(0H)+ is 200 for [Pulx = 0.1 M, 5.6 for 10-1 M and 0.05 for 10 8 M. 

In complex 8, also prepared by Czamik and coworkers [34], the two (cyclen)Co(III) units are linked by a different spacer. Models predicted that the complex is prohibited from forming an intramolecular ji-hydroxo dimer. Furthermore, the mononuclear subunits are more rigidly held compared with complex 6. Whereas no rate increase over 7 is achieved for BNPP hydrolysis, cleavage of the monoester NPP by 8 is 10 times faster than by (cyclen)Co in twofold concentration. 31P NMR data indicate that a four-membered chelate may be involved in the cleavage mechanism, as observed in the case of 5. 

Even more efficient bimetallic cooperativity was achieved by the dinuclear complex 36 [53]. It was demonstrated to cleave 2, 3 -cAMP (298 K) and ApA (323 K) with high efficiency at pH 6, which results in 300-500-fold rate increase compared to the mononuclear complex Cu(II)-[9]aneN at pH 7.3. The pH-metric study showed two overlapped deprotonations of the metal-bound water molecules near pH 6. The observed bell-shaped pH-rate profiles indicate that the monohydroxy form is the active species. The proposed mechanism for both 2, 3 -cAMP and ApA hydrolysis consists of a double Lewis-acid activation of the substrates, while the metal-bound hydroxide acts as general base for activating the nucleophilic 2 -OH group in the case of ApA (36a). Based on the 1000-fold higher activity of the dinuclear complex toward 2, 3 -cAMP, the authors suggest nucleophilic catalysis of the Cu(II)-OH unit in 36b. The latter mechanism is comparable to those of protein phosphatase 1 and fructose 1,6-diphosphatase. 

The above complexes have been shown to mimic the second step of RNA hydrolysis as well, i.e. the-efficient cleavage of ribonucleoside 2, 3 -cyclic monophosphates [55] with bell-shaped pH-rate profile. With these substrates 37 showed much higher bimetallic cooperativity the dime/2 m0nomer ratios range between 64 and 457 for the different 2, 3 -NMPs used, while for 38 this ratio varies between 1 and 26. Since the mononuclear complexes have nearly the same activity toward the different 2, 3 -NMPs, these kinetic data indicate a notable base-selectivity of the dimer complexes. Since no correlation was observed with the size,... 

A new imidazole-functionalized calix[4]arene ligand, able to form a dinuclear Cu2+ complex, has been reported to hydrolyze HPNP and ethyl p-nitropheny lphosphate [70]. The dinuclear complex was found to be 22-and 330-fold more reactive than the corresponding monomer towards the above substrates, respectively. Dinuclear Cu2+ complexes of linked triazacyclononane ligands are reported to promote the hydrolysis of the monoribonucleotide GpppG, a model for the 5 -cap structure of mRNA [71]. The dinuclear complexes offer some 100-fold higher reactivity compared to the mononuclear Cu2+-triazacyclononane system. 

Type A trinuclear carboxylates are synthesized in general by basic hydrolysis of mononuclear salts.(3) The synthesis of 3B marked the introduction of the stepwise approach described in Scheme 1. The [Fe20Cl5]2- anion was used as the dinuclear component but, since it has a half-life of 1.3 hours in acetonitrile(8b), it can also be a source for mononuclear Fe + building blocks. Aqueous hydrolysis of Fe(0H)3 was used to synthesize 4E, while 8 was prepared by hydrolysis of the mononuclear salt, (C6Hi5N3)FeCl3 in the presence of NaBr. The basic hydrolysis of the dinuclear... 

Wall et al. built a binuclear copper(II) complex 43 in order to see acceleration of phosphodiester cleavage (52). With the substrate (50 p.M) shown, the reaction might be considered as a model for the first step of the hydrolysis of RNA, in which the alcohol function of the side chain intramolecularly attacks the Cun-activated phosphate as a nucleophile for a ring closure reaction. Compared to an analogous mononuclear complex 44 (at 1 mM), a rate constant ca. 50 times larger for 43 (at 1 mM) was observed at 25°C and pH 7, implying that the two metal ions probably cooperate. An analogous zinc(II) complex 45 was reported only as a structural model for the active site of phospholi-... 

Gallium hydroxide is amphoteric, and is a much stronger acid than aluminum hydroxide. For Ga(OH)3 the first acid dissociation constant is 1.4 x 10-7 [for Al(OH)3 the value is 2 x 10-11].1 Polymerization occurs in aqueous Ga3+ solutions to which OH- is added,533 but this tendency is less than in the case of aluminum solutions (Section 25.1.5.1). The formation constants of mononuclear hydroxo complexes of Ga, including Ga(OH)4, and the hydrolysis constants of gallium ions have been measured by a competing ligand technique.534... 

The interconversions between monobridged dinuclear Cr111 ammines with their traditional names are illustrated in Scheme 35.302,361 Inter-relations exist between the mono- t-hydroxo and di-jU-hydroxo species in acid solution, and these are outlined in Scheme 36. Acid hydrolysis of the mono-bridged dinuclear cations to mononuclear species is slow and salts of (94) and (95) as well as of the long-established ju-diols are known. In ammonia buffer [(OH)(NH3)4-Cr(OH)Cr(NH3)4(OH)]3+ forms.362... 

Polynuclear complexes with hydroxide (or oxide) as bridging ligands constitute an important class of complexes. They are formed by hydrolysis of mononuclear aqua complexes of most metal ions and they therefore constitute an important aspect of the hydrolytic chemistry of metal ions. They display a chemistry which is interesting in itself, but which is also relevant in relation to applied chemistry and to biochemistry, as mentioned in Section XIII. 

Hydrolysis is strong acid in which all hydroxo bridges are cleaved, followed by identification of the various mononuclear species and a determination of their molar ratios, may provide extremely valuable information. A straightforward example is the cleavage of the tetra-nuclear species Cr4(NH3)12(OH)66+, which yields Cr(H20)63+ and cis-Cr(NH3)4(H20)23+ in a ratio of 1 3 (40). Since it could be demonstrated at the same time that the polynuclear cation does not exhibit acid base properties in the pH region for terminally coordinated water, it was concluded that the only possible structure was 6 in Fig. 1, as later confirmed by a crystal-structure analysis (41). 

A very large number of dihydroxo-bridged chromium(III) and cobalt(III) complexes have been synthesized from the parent mononuclear species by aqueous hydrolysis, as shown in Eq. (1) for a cationic species, but also neutral and... 

The yields of the dihydroxo-bridged complexes vary and are often relatively low, probably owing to the formation of different isomers and higher polynuclear complexes and to hydrolysis of the ligand L. For many of the complexes listed above, the preparative procedures involved the preparation of the mononuclear species in situ. 

One way to overcome the above problem would be to suppress hydrolysis of the amine ligands by working with an appropriate amine buffer medium. This strategy has been used with great success by Andersen et al. to obtain quantitative equilibrium data for the formation of mononuclear amine complexes (195, 196). Andersen et al. have also studied the formation of polynuclear complexes under similar conditions, but equilibrium was not attained with respect to these species (40, 42, 60, 87). The fact, however, that both thermal hydrolysis and charcoal/chromium(II)-catalyzed hydrolysis in such an amine buffer medium give the same polynuclear species in almost identical ratios would seem to indicate that some degree of equilibration had been achieved. It therefore seems likely that these methods could, in principle, be modified so as to also be applicable for equilibrium studies. Quite a different approach would be to study complexes with macrocy-clic amines such as cyclam, which are known to have a reduced tendency to hydrolysis. However, such systems have not as yet been studied in detail. 

Hydrolysis of polynuclear hydroxo-bridged chromium (III) complexes in concentrated solutions of strong acid yields the corresponding mononuclear species. Such cleavage reactions are fast in comparison with the hydrolysis in dilute acid and proceed with retention of configuration of the mononuclear entities. A few representative examples are shown in Eqs. (46)-(49) (40, 42,161, 252). 

Acid hydrolysis of the dihydroxo-bridged complex frans-(H20)(tacn)-Rh(0H)2Rh(tacn)(H20)4+ yields mononuclear triaqua complex (100). Only one reaction step was observed for [H+] = 0.2-1.0 M, the rate expression being of the form kobs = a + 6[H+], If it is assumed that the first bridge cleavage is faster than the second, as has been found for the corresponding tetraammine and bis(ethylenediamine) complexes, the observed [H+] dependence can be interpreted in terms of Scheme 4, which for large [H+] leads to the approximate expression kobs = 4 + ( 5/I a5)[H+]. Values for k4 and ksIKaS are listed in Table XXVII. 

The kinetics for the acid hydrolysis of A,A/A,A-(phen)2Cr(OH)2-Cr(phen)24+ revealed that the formation of cis diaqua mononuclear species proceeds in one step without significant buildup of mono-hydroxo-bridged intermediates (336). The rate law observed is kobs = a + 6[H+] at low acidities ([H+] = 0.01-0.05 M) and kobs = 6[H+] at... 

The kinetics of the acid hydrolysis of (nta)Co(OH)2Co(nta)2 have been reported in two independent studies (177,178). At [H+] 10 2 M, the hydrolysis yields mononuclear Co(nta)(H20)2 quantitatively and proceeds in two kinetically well-separated steps, a fast step (<1/2 0.04 second at pH 0) followed by a much slower step (about 102 times slower under similar conditions). The occurrence of two steps has, however, been interpreted quite differently in the two studies. Thacker and Higginson assumed that their samples contained one of the two possible isomers contaminated with small amounts of the other isomer and it was suggested that the rapid and slow reactions correspond to hydrolysis of the two isomeric forms (178). 

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