Cobalt complexes pyrophosphates

Triphosphates and pyrophosphates can be estimated in the presence of each other and ortho and cyclic metaphosphates, with tris(ethylenediamine) cobalt chloride. At pH = 3.5, this latter reagent will form an insoluble triphosphate complex but not a pyrophosphate complex. Conversely at pH = 6.5, a pyrophosphate but not a triphosphate complex can be precipitated. Under either of these conditions ortho and cyclic metaphosphates will not give precipitates. 

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

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 ( ). 

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. 

The rate of hydrolysis of bidentate triphosphate in [Co(NH3)4HnP3Oi0] has been studied by phosphomolybdate analysis and 31P NMR.297,298 Both the X-ray crystal structure299 and the 31P NMR spectrum of [Co(NH3)4H2P3O 0] are consistent with structure (91) in which one terminal phosphate residue is not bonded to the metal centre. Kinetic studies establish that hydrolysis of the chelated ligand occurs at some two thirds of the rate for the free ligand. Hubner and Milbum300 have noted large rate increases of ca. 10s for cobalt(III) complexes with a 3 1 metal pyrophosphate stoichiometry. 

The high reactivity of the 2 1 complex can be attributed to the availability on the second cobalt of a coordinated hydroxide which can attack a phosphorus center (preferably Pg). The other cobalt, in the six-membered ring, can assist the process by functioning as an electron withdrawing center. The acceleration for the hydrolysis of ADP is marked ( 105) and comparable to the effect we have previously seen for pyrophosphate and ATP. 

Attention has also been focused on the oxidation of thiols in the presence of solid catalysts. One of the more comprehensive investigations into systems of this type has been made by Wallace et al. [133,145, 146] with a view to the possible use of phthalocyanine type complexes as commercial sweetening catalysts. Comparisons were drawn with metal pyrophosphates, phosphomolybdates, phosphotungstates, and phosphates. Pyrophosphates were found to be effective catalysts, possible due to the existence of six-membered rings involving the cobalt cation [147], which enhances the ability of the cation to donate an electron to oxygen and stabilises each oxidation state of the cation. For a series of pyrophosphates, the order of activity was Co > Cu > Ni > Fe, an activity pattern which was explained in terms of the stability of the 3d electron shells. 

Interest continues in the effects of micellar agents on rates of inorganic reactions. Aquation of oxalato-cobalt(m) and -chromium(iii) complexes is mentioned in a review of reversed micellar systems predicted and actual rates have been compared for a variety of reactions, including the mercury(n)-catalysed aquation of [CoX(NH3)g] + cations. Micellar effects on uncatalysed aquation of cis- and of tra j -[CoCl2(en)2]+ are small (as expected) but real. Other reactions for which the kinetic consequences of micelle or polyelectrolyte addition have been d cribed include complex formation from nickel(ii), the conversion of ammonium cyanate into urea, and the hydrolysis of pyrophosphate. ... 

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