Polynuclear Anions

Polynuclear anions are formed by comersharing only of boron-oxygen triangles and tetrahedra in such a manner that a compact insular group results. 

Metal carbonyls are subject to autocomplex formation in the presence of strong donor molecules 94 98>. Besides the cation which is coordinated by donor molecules, polynuclear anions are formed the latter can be degradated at higher temperatures. It may be noted that in this process of autocomplex formation changes in the oxidation numbers and thus redox reactions are involved ... 

Polynuclear anion-hridged complexes. The structure of [(NH3)3(H20)Co-(p-OH)2Co(H20XNH3)3](N03), 2H20 has been reported, although it was not possible to distinguish between the ammine and aquo ligands. When [Co(acac)2] in methanol, containing KOAc, is treated with H2O at 25°C for 1 h, [(acac)2Co(p-OH)2Co(acac)2] is formed. The structure of rac-... 

This method has the advantage of generating the desired anions quantitatively and avoids the formidable task of separating mixtures of polynuclear anions into their analytically pure components. The trinuclear anions may be isolated as alkali metal salts and used in situ or may be quantitatively metathesized to form alkyl ammonium, arsonium, or phosphonium salts. Therefore, we have found these alkali metal dianions to be a very useful, general starting material for a variety of reactions. 

Using the technique of Behrens (VII, 43), liquid ammonia was also found to be a suitable solvent. Our further experiments showed that, depending on the redox potential, it was possible to isolate polynuclear anion complexes of various types. Thus the alkali and alkaline earth metal reduction of... 

The compound Na2 [Rh12(CO)30] can be prepared by reaction of Rh2(CO)4-Cl2 with sodium acetate in methanol under an atmosphere of carbon monoxide.1 It contains one of the fust polynuclear anions to be formed when the rhodium carbonyls or carbonyl halides are reduced by the action of alkaline reagents in alcohols or by alkali metals in tetrahydrofuran (THF). It provides a unique example of a double octahedral cluster carbonyl anion in which the noble gas rule is not obeyed,1 2 and it is a starting material for the preparation of other polynuclear rhodium carbonyl anions.1 3"5 The synthesis reported here is a modification of the original method. The starting material is Rh4(CO)i2, now easily prepared at atmospheric pressure.6"8 The reaction is fast, and the overall procedure requires about 6-7 hours with 80-85% yields. 

Copper(I) Sulfido Anions. There is a remarkable array of polynuclear anions containing sulfur rings or chains that can be isolated as salts of large cations. They are formed by dissolution of copper sulfides in polysulfide solutions or by the action of sulfur and H2S on ethanolic solutions of Cu11 acetate. Examples are ions such as [Cu2S20]4-, [Cu3Si2]3, and [Cu4Si8]4-. Many of the structures are quite complex, for example, (17-H-VII) and (17-H-VIII). 

When alkali metal bases are used to raise the solution pH to moderate levels, the uranium will precipitate from the solution in the form of hydrous uranyl hydroxides or uranates, for example, Na2U207. However, through judicious choice of a base, for example, tetramethylammonium hydroxide, (TMA)OH, or tetramethylaimnoirium trifluoromethansulfonate, the study of the amphoteric behavior of uranyl hydroxides can be undertaken. Polynuclear anions of the form (U02)3(0H)7, (U02)3(0H)g, and (U02)3(OH)io are examples of soluble species in solutions where the pH < 14. When the concentration of the (TMA)OH is increased (>0.6 M OH ), highly soluble ( 0.1M) monomers ofthe form U02(0H) "(n = 3, 4,5) have been reported. These three species are in equilibrium with each other however, in solutions where the [OH ] is greater that 1M, the pentahydroxo complex predominates the speciation. 

A polynuclear anion [Nig(CO),2] reacts with a deficit of GeCl4 in acetonitrile to form [GeNi,2(CO)22] xs tetrachloride yields [GeNi,o(CO)2o] - These compounds represent the first examples of encapsulated Ge, which is contained in a pentagonal antiprism in the Ni species, and this is bicapped in the Ni,2 species. 

The polynuclear anions are usually chlorocuprate(I) complexes, but compound K[Cu2(CN)a] HgO has been identified in the phase diagram of the CuCN-KCN-HaO system 334), The anion s structure is made up of [CuaiCNlg] sheets with the lattice water clathrated in (OuCN)e rings 93). 

A polynuclear anion of a different kind was prepared in connection with earlier systematic investigations on adducts of SeOCb with other halides (see, e.g., 197) from 1 5 mixtures of [(CH3)4N]C1 and SeOCl2 (198). It contains two Cl ions which are solvated by 10 SeOCb mole-... 

Fig. 24. Arrangement of the SeOCfj molecules around the solvated chlorides in the polynuclear anion of [(CH3)4N ]2[(Cn2(SeOCl2)io]. The Se -Cl distances are given...

Polynuclear anions are generated readily, and some polynuclear hydrides that can be made by this method "- are listed " in Table 1. Although these anions are isolated and characterized, polynuclear hydrides frequently are made from anions prepared in situ. As with the cationic polynuclear hydrides discussed previously, the location of the H atom may be unknown, even though the H NMR specifies that it is attached to the metal core. 

A large kinetic-isotope effect on protonation rates of polynuclear anions indicate... 

Table 1. Formation of Polynuclear Hydrides by Protonation of Polynuclear Anions ...

Some polynuclear anions change the structure of their metal cores on protonation. For example, the dianion [OSj(CO),J " has O, symmetry the first proton bridges an O, face, and the second causes the metal framework to rearrange to a monocapped square pyramid in which the exact location of the hydrogen ligands is not clear ... 

The same factors (structural rearrangement on protonation and delocalization of negative charge) that produce the low proton basicity of polynuclear anions are probably also responsible for the slow rate at which protonation equilibria are established with polynuclear systems (see Table 2). If [HOs(CO) Os(CO)J is compared with the mononuclear Os anions in the previous two subsections, it is clear that the second 05(00)4 unit has decreased the rate of protonation as well as the proton basicity. 

A. P. Humphries, H. D. Kaesz, Prog. Inorg. Chem., 25, 145 (1979) a comprehensive review of polynuclear hydrides, including a section on their formation by protonation of polynuclear anions. 

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