Formation of Polynuclear Complexes

Formation of hydroxo-bridged complexes by hydrolysis in aqueous solution is, not surprisingly, the most common preparative method. As a rule, such reactions give quite complex product mixtures containing species with different nuclearities, each of which may be present in many isomeric forms. The fact that most of the preparative procedures employed lead to the isolation of one single and pure isomer probably more often reflects favorable solubility properties rather than stereospecificity. In some cases ion-exchange chromatography has been used to isolate the polynuclear species, but systematic analysis of hydrolysis mixtures by this technique has been reported for only a few systems. 

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. 

It should be stressed that some of the complexes mentioned above are more easily made by other routes. The complex (H20)4Cr(0H)2(H20)44+ is produced quantatively by the Tl3+ oxidation of aqueous chro-mium(II), but only in low yield by reaction Eq. (1) (30, 31). The complex A,A-(en)2Cr(OH)2Cr(en)24+ is obtained (16) in good yield by reaction Eq. (1), and the yield may be increased by working in methanolic solution (182). However, the solid-state reaction (131) described below (Section IV,B) gives this complex in essentially quantitative yield. 

With a few exceptions, all of the dihydroxo-bridged complexes listed above may exist in several, and often many, isomeric forms. The topic is discussed briefly in Section II, and a few additional comments seem relevant in the present context. 

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Formation of polynuclear complexes by phenolic oximes is also possible with such oxo metal species in high oxidation states, e.g., [(C4H9)4N][Mo205(sal-2H)], which has both a //2-oxo-bridgc between the two metal centers and two Mo-N-O-Mo oximinato bridges formed by the doubly deprotonated salicylaldoxime ligands, sal-2H.145... 

The following sections (53.4.4.2-6) attempt to describe the electronic properties of simple mononuclear complexes of the copper(II) ion,47,48 to show how these are related to the different stereochemistries of the copper(II) ion and how these properties are modified by the formation of polynuclear complexes.17,30 Particular emphasis is placed on the appearance of the different types of electronic property and how they may be used to provide qualitative evidence for the different types of copper-copper interactions, and hence for possible polynuclear structure formation, particularly in the solid state. While the main emphasis will be on the electronic properties in the solid state, where X-ray evidence may be obtained for a single magnetic species,10 the measurement of the electronic properties in solution will also be described, although in solution a mixture of complex species may be present in equilibrium and complicate the interpretation of the electronic properties.584,816,817,824... 

Aqueous solutions containing cadmium and thiocyanate have also been shown to contain the species [Cd(SCN) (H20)I] (n = 1-4) there is no evidence for the formation of polynuclear complexes, although both S- and N-bonded thiocyanate are present.1016... 

The Pd(Il) ions are four-coordinate and planar. An examination of this molecule reveals a triangular array of exposed mercaptido sulfur donors. It is conceivable that this molecule may serve as a donor for other metal ions with the formation of polynuclear complexes. 

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. 

Comparing with photochemically induced formations of polynuclear complexes, photodecompositions of such compounds are more frequent processes [1] and this general phenomenon is fully applicable in the photochemistry of metallotetrapyrroles as well. 

In this it is not a question of actual differences in coordination number but of changes in the relative number of oxygen ions per central ion by the formation of polynuclear complex ions. 

A plot of p([Cu2 ]/[Cu7ot]) 3S a function of pH for three separate titrations fall on a single curve despite up to fivefold differences in measured dissolved copper concentration at a given pH (Figure 2). This behavior of the ratio [Cu2+]/[Cujot] is indicative of the formation of mononuclear hydrolysis species and excludes the possibility that the observed reduction in free cupric ion may have been caused by precipitation of Cu(0H)2 (solid) or the formation of polynuclear complexes. Analysis of data for p[Cu2+], pECujoj] and pH in the pH range 7.7 to 10.8 indicated the presence of two hydrolysis species (CuOH and Cu(0H)2) whose stability constants are given in Table I. Our value of the stability constant for the monohydroxo complex (106.48) falls... 

It has been pointed out that the formation of a precipitate can often be considered the find stage in the formation of polynuclear complexes. Aggregates of ions that form the building stones in the lattice are produced in the solution, and these aggregates combine with other ions to form neutral compounds. 

Values for the thermodynamic stability constants of inoiganic complexes (with HCO, CO3 , S0 , Cl and 0H ) and organic complexes (except for Cu) of trace metals and major ions were chosen principally from the compilation by Zirino and Yamamoto [ 10] Tor Cd, Whitfield and Turner [11 ]for Pb, Van den Berg [9 ]for Cu, and Truesdell and Jones [12] for major ions. The formation of polynuclear complexes has generally been discounted (except for Pb). The activity coefficients of all species have been calculated using the Davies equation [l3 ]. The stability constants of humic complexes of Cu ( Cu-L been calculated in this work. 

Other workers have also investigated the binding mode of inosine, 1-methylinosine, and 5 -inosine monophosphate towards cis-[Pt(NH3)2] and [Pt(en)] " as a function of pH using H n.m.r. and Raman spectroscopy.Stable complexes are formed over a wide range of pH mono- and bis-complexes, co-ordinated at N(7), are formed with inosine and 1-methylinosine at pH <6, while for pH >6 protons are dissociated from N(l) and co-ordination occurs at both N(7) and N(l) with formation of polynuclear complexes indicated from the spectroscopic results. 

There is a trend towards the formation of polynuclear complexes in the solutions of oxocompounds of Group V of the Periodic Table in molten salts,... 

The standard enthalpy of formation was calculated in this review from the careful measurements of [76VAS/LYT]. A value of-(607.4 1.6) kJ-moP was reported by these authors. However, it was necessary to correct their published experimental data for the formation of polynuclear complexes at high Zr concentrations (see Appendix A). 

For the addition of a metal ion, i.e., the formation of polynuclear complexes, the following notation is used, analogous to Eq.(IL5) ... 

Sandstrom M (1977) An X-ray diffraction and Raman study of mercury(II) chloride complexes in aqueous solution. Evidence for the formation of polynuclear complexes. Acta Chem Scand Ser A 31 141-150 Sandstrom M, Persson I, Ahrland S (1978) On the coordination around mercuiy(II), cadmium(II) and zinc(II) in dimethyl sulfoxide and aqueous solutions. An X-ray diffraction, Raman and infrared investigation. Acta Chem Scand Ser A 32 607-625... 

The hydrolysis of Th(IV) at total concentrations larger than 0.1-1 mM is dominated by the formation of polynuclear complexes that have high positive charges, e.g. Thj (OH)f and Th (OH) 5. Hence electrostatic effects are expected to play an important role for the stability of these species as shown by the ionic strength dependence of the reaction ... 

This study does not provide any thermodynamic data but it confirms the formation of polynuclear complexes and that hydroxide bridging is an important stmctnral element both in solution and the solid state. 

This is a potentiometric study made at 25°C and an ionic strength of 0.50 M. The total concentration of thorium varied from 5 to 0.25 mM and the value of from 0.01 to 2.58. The authors demonstrate the formation of polynuclear complex but they have preferred to analyse their data using the core and link model described in [1964H1E/S1L]. The experimental data are consistent with this model, but as this does not provide a proper chemical description of the constitution of the complexes, as discussed in the present review, these data are not accepted by this review. 

The authors note that the slope -5.5 of the initial part of the solubility curve indicates the formation of polynuclear complexes with charge Z > 4, because the species Th(OH) and Th(OH)j would result in a slope of-3 to -2. By using the equilibrium constants proposed by Baes and Mesmer [1976BAE/MES] for Th(OH) and Th(OH)2 and estimating the activity coefficients for the reactants, Ryan and Rai suggest login = -45.9 for the reaction ... 

The liquid-liquid extraction study has been made at [Th]tot below 10 M, typically at 10 M under these conditions the formation of polynuclear complexes and colloids should be minimised. The experimental data have been evaluated in the same way as in [1992ENG/ALB] -that is, the formation of ternary complexes has not been considered. The method requires a simultaneous determination of the equilibrium constants for the binary Th(IV)-OH- and Th(lV)-acetylacetonate systems as indicated in Eqs. (1) and (2) of [2000EKB/ALB]. The authors do not report the total concentration of HE (acetylacetone) used and only some of the experimental logioD -logio [H ] data are given in Table All of [2000EKB/ALB]). The equilibrium constants... 

Extensions fo Complicated Equilibria Equation 14-10 can he modified to lit many other cases. It can be extended to account for the formation of polynuclear complexes and for multiple equilibria. Ultraviolet and visible absorption spectroscopy is not. however, particularly well suited for multiple equilibria because of its lack of specificity and because each additional equilibrium adds two unknowns, a formation constant term and a molar absorptivity term. 

There is much current interest in the synthesis and reactivity of transition metal complexes containing P atoms or aggregates. In particular, (/i-t -P2) dinuclear and cyclo (t/ -Ps) mononuclear complexes have received special attention, on account of their ability to act as complex ligands in the formation of polynuclear complexes. Whereas reported syntheses of these... 

Several of the anionic versions of these complexes (E ) are excellent candidates to form bridging interactions between metal centers directed at the formation of polynuclear complexes. 

A study was also made of the system mercury (II) iodide-cadmium (II) iodide in the above solvents, and it was found that, in contrast to the results of spectrophotometric and conductivity measurements [Ga 67], the formation of polynuclear complexes could not be detected. 

Acychc and cycHc compartmental Hgands in the formation of polynuclear complexes (2006—2010) 12CCR953. 

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