Hydroxo mononuclear

Substitution of water molecules by Cl or Br has been reported for the chal-cogenide clusters [Mo3Y7(H20)6j (Y = S, Se). Finally, dirhodium cyclo-pentadienyl derivatives with -OH bridges have been reported. An unusual five-coordinated hydrido-hydroxo mononuclear derivative of osmium is obtained upon reaction of HOsCl(CO)(PPr 3)2 with KOH in methanol. ... 

Fig. 26.5 Log ttM values concerning the formation of some hydroxo mononuclear complexes...

Cinellu, M.A., Minghetti, G., Pirma, M.V., Stoccoro, S., Zucca, A. and Manassero, M. (2000) Gold(lll) derivatives with anionic oxygen ligands mononuclear hydroxo, alkoxo and acetato complexes. Crystal structure of [Au(bpy)(OMe)2][Pp6]. Journal of the Chemical Society, Dalton Transactions, (8), 1261. 

Dehydrative condensation of transition metal hydroxo complexes is a versatile synthetic method, when the conjugated acid of the incoming ligand (H-A) is a protic compound. There are reports of the synthesis of two types of square-planar hydroxopalladium complexes, mononuclear [TplPr2 x(L)Pd-OH] (X = H, Br) and dinuclear complexes [TplPr2Pd(H20)( -0H)2 (H20)PdTplPr2] (TplPr2 = hydrotris(3,5-diisopropylpyrazolyl)borate), and some dehydrative condensations of them.278... 

In 1996 Stack and co-workers reported an unusual 3 1 (copper 02 stoichiometry) reaction between a mononuclear copper(I) complex of a A-permethylated (lR,2R)-cyclohexanediamine ligand with dioxygen. The end product of this reaction, stable at only low temperatures (X-ray structure at —40 °C) is a discrete, mixed-valence trinuclear copper cluster (1), with two Cu11 and a Cu111 center (Cu-Cu 2.641 and 2.704 A).27 Its spectroscopic and magnetic behavior were also investigated in detail. The relevance of this synthetic complex to the reduction of 02 at the trinuclear active sites of multicopper oxidases4-8 was discussed. Once formed, it exhibits moderate thermal stability, decomposed by a non-first-order process in about 3h at —10 °C. In the presence of trace water, the major isolated product was the bis(/i-hydroxo)dicopper(II) dimer (2). 

However, ageing of these virtually monodisperse systems was shown to promote an increase in width in favour of length. There was no change from hydroxo to oxo bridges, but instead there was an alteration in polynuclear shape. While the internal structure was maintained, it was found that the polynuclears became shorter and wider, as the mononuclear ferric species released from the ends of the needles by acid cleavage redeposited on the centres of the molecules. 

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. 

There are as yet no structural data on the mononuclear hydroxo complexes formed at high pH. Schmidbaur et al. (91) have obtained the structure of a compound that had been crystallized from a solution at pH 13.2. The compound contains the anion [Be4(OH)10]2, which has a structure resembling that of adamantane or P4O10, as shown in Fig. 9. 

The polynuclear hydroxo-anions isolated as solids will also be present in solution. Their existence points to a chemistry that is formally comparable to polynuclear oxy-anion chemistry—compare [Be2(OH)7]3 to Cr20 . In both cases the protonation of a divalent mononuclear anion results in the formation of a binuclear bridged structure. 

The kinetics of reaction of Fe " aq, of FeOH +aq, and of Fe2(OH)2 " "aq with variously proto-nated forms of phosphate, phosphite, hypophosphite, sulfate, and selenite have been investigated, mainly at 283 K. The formation mechanism from the dimer is somewhat complicated, e.g., by formation of mononuclear complexes, probably via /i-hydroxo-/r-oxoanion di-iron intermediates, after the initial 4 complexation step. ... 

Experimental measurement (6, 8) of the solvolysis of Sarin and DFP in the presence of 1 to 1 diamine-metal chelates, such as those listed in Table I, under varying solution conditions showed that the catalytic effect was not proportional to, or a simple function of, the total metal chelate species in solution. A detailed analysis of the variation of rate with composition of the solutions indicated (4, 7) the presence of hydroxo and dihydroxo mononuclear forms of the chelate compound, as well as a binuclear //,-dihydroxo species. If the possible reactivities of all catalytic species are taken into consideration, the rate expression would have the form ... 

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... 

W. N. Perara and G. Hefter, Mononuclear Cyano- and Hydroxo-Complexes of Iron(lII), Inorg. Chem. 2003,42, 5917. 

Recently the stability of silver(I) complexes of iV-methyl-substituted 4-methyldiethylenetriamines has been investigated by potentiometric pH and pAg measurements. Besides mononuclear complexes, polynuclear and protonated complexes were formed. Evidence of hydroxo complexes was also presented.48... 

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). 

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). 

Analysis of the products of these cleavage reactions has often served as proof of the structures of the polynuclear species. Cleavage of hydroxo-bridged complexes of nuclearity higher than two will in most cases yield at least two different mononuclear species. Identification of these species and determination of the relative ratio in which they are formed reduce the number of possible bridged skeletons greatly, and the studies of polynuclear ammine and amine chromium(III) made by Andersen et al. (mentioned in Section IV) provide many examples of this, one of which is shown in Eq. (48) above (see also Section II,A). 

The cleavage of polynuclear hydroxo-bridged rhodium(III) and iridium(III) complexes into the corresponding mononuclear fragments has been reported in only a few instances, but the well-established tendency of mononuclear complexes of these metal ions to undergo substitution reactions with retention of configuration indicates the possibility of analytical and synthetic applications such as described above for chromium (III). 

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... 

Finally, it should be noted that this chemistry may have biological relevance. Several metalloenzymes are believed to contain more than one metal ion bound at the active site. One relevant example is the glucose tolerance factor (GTF) which is important for the metabolic degradation of glucose (398-401). GTF is a low-molecular-weight protein which contains chromium(III). Its structure is not known, but it has been suggested that the active site contains a dinuclear chromium(III) complex (401). The fact that hydroxo-bridged dinuclear chromium(III) complexes exhibit reactions which are often very fast compared with those observed for the parent mononuclear species seems to support such a proposal. 

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