Pyrophosphates metal complexes

The hydrolysis of sodium pyrophosphate was effected by using some metal complexes of poly(methacrylacetone) as catalysts138. The catalytic activity of the polymer complexes declined in the following order Zr(IV)0 > U(VI)02 >... 

In general, the electron transfer reaction (Reaction 2) controls the over-all rate of reaction, and the nature of the catalyst has a profound effect on the kinetics of oxidation (23). Thus Wallace et al. (23) have compared the catalytic efficiencies of metal phthalocyanines with metal pyrophosphates, phosphates, phosphomolybdates, and phosphotungstates. The activity of metal pyrophosphates was ascribed to the ease of electron transfer through the metal coordination shell, the reaction being suggested to occur at the solid pyrophosphate-liquid interface. On the other hand, the catalytic effectiveness of a series of metals, added to solution as simple salts, has been explained in terms of their ability to form soluble complexes containing thiols (13). It was not clear whether the high rates of oxidation were caused by the solubility of metal complexes or by the peculiar nature of the thiol complexes. 

Further evidence has been obtained to support the contention that the active catalysts are metal complexes dissolved in solution. With experiments reported in Table II, the kinetics of oxidation under standard conditions in the presence of various metal salts are compared with the rates of reaction when solid residues have been filtered from solution. The agreement between the rates in Cases 1 and 3 of Table II (where the amount of metal available is dictated by the solubility of metal complexes) shows that solid precipitates play little or no part in catalysis in all the systems studied. The amount of metal in solution has been measured in Cases 2 and 3 metal hydroxide complexes (Case 2) are not as soluble as metal-thiol complexes, and neither is as soluble as metal phthalocyanines (19). The results of experiments involving metal pyrophosphates are particularly interesting, in that it has previously been suggested that cobalt pyrophosphates act as heterogeneous catalysts. The result s in Table II show that this is not true in the present system. 

Both ADP and ATP as well as inorganic pyrophosphate form complexes with metal ions. Since the magnesium complexes are often the predominant forms of ADP and ATP under physiological conditions, we must consider the following Gibbs energy changes. These are apparent values for p = 0.2 at 25°C. 

Most of this work raises the unresolved question of the relative importance of homogeneous and heterogeneous catalysis. At least part of the catalytic activity of solid metal pyrophosphates has been shown to be due to traces of soluble complexes [140] and similar effects may well be important for other heterogeneous catalysts. The catalytic activity of traces of soluble metal complexes is so high that spurious heterogeneous catalytic effects may well be observed. 

In the case of many divalent metals the ATP complex is more stable than the pyrophosphate (ADP) complex which in turn is more stable than the monophosphate (AMP) complex. The stabilities of the divalent metal ion complexes of ATP he in the order Cu > Ni > Zn > Mn > Co > Mg > Ca > Sr > Ba. 

The simultaneous determination of Co and Ni is also made at pH 8 in the presence of pyrophosphate. The EDTA is added to the mixture of coloured complexes of these metals to bind the Cu and Zn admixtures into the inactive complexes. The optical density of the solution is measured at 530, 555 and 580 nm. The solution is heated to the boiling point to destmct the complex formed by Ni with PAR, and then is cooled. Again the measurements of optical density ai e performed at the same wavelengths. The Ni concentration is calculated from the variation in the optical density, and the Co concentration is calculated from the final values of optical density. The detection limits for these metals are 4 and 2 p.g/dm, respectively. 

Metal salts in alkaline solution Cuprammonium complex Nickel and cobalt ammonia complex Cyanides (q.v.) Copper pyrophosphates Plumbites Zincates... 

Various polyphosphates are effective sequestering agents under appropriate conditions. The best known of these is sodium hexametaphosphate (10.14), the cyclic hexamer of sodium orthophosphate. Further examples are the cyclic trimer sodium trimetaphosphate (10.15), as well as the dimeric pyrophosphate (10.16), the trimeric tripolyphosphate (10.17) and other linear polyphosphates (10.18). All of these polyanions function by withdrawing the troublesome metal cation into an innocuous and water-soluble complex anion by a process of ion exchange as shown in Scheme 10.7 for sodium hexametaphosphate. Hence these compounds are sometimes referred to as ion-exchange agents. 

Solution pH is an important variable, as it controls solubility, the deposition potential, and precursor speciation [140], As noted, one of the advantages of using an ALE process is that very different solutions can be used for each step in the cycle. For instance, pH 4 Cd solutions have been used with pH 10 Te solutions to form CdTe. Some care must be taken rinsing between solutions, however. On the other hand, Foresti et al. have used the same pH for both the metal and chalcogenide in the formation of II-VI compounds, such as CdS and CdSe on Ag single crystals [115, 116, 123, 143], To keep the Cd from precipitating in the basic solution, they complexed it with pyrophosphate. 

A H and P n.m.r. study of thiamine pyrophosphate in the presence of cobalt(ii) or nickel(ii) indicate that the TPP is bonded to the metal via pyrophosphate and the pyrimidine group, in a similar manner to ATP-M complexes. 

The structure of the active component, manganese pyrophosphate, has been reported in the literature (24). It is layer like with planes of octahedrally coordinated Hn ions being separated by planes of pyrophosphate anions (P20y ). Examination of models of this compound gave calculated Hn-Hn thru space distances of 3.26 and 3.45 angstroms, a metal-metal distance close to that found for binuclear dibridged peroxo- and superoxo- complexes of cobalt ( ). 

Returning to the ATP cleavage reaction, it may be supposed that some metal ions, probably the weakest complexers, form a monocyclic chelate by attaching to the terminal pyrophosphate only (C), whereas others, probably the strongest complexers, form a bicyclic chelate by binding the a-phosphate as well(D). By... 

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