Anions catenated

A salt with yet another conposition is obtained when [N2 2 2 2] is used as the counter-cation. [N3 3 3 3][Cu Br2] and [N3 3 3 3][Cu Cl2] [460] both melt at 99-100°C with very similar values of L/l, 413 and 416 kJ mor respectively. The X-Cu-X angles in the two salts are also the same 178.4° (X = Br) and 178.5° (X = Cl). Comparisons with salts obtained with other cations suggest [460] that the coordination number of copper and anion catenation both... 

NaAs03 has an infinite polymeric chain anion similar to that in diopside (pp. 349, 529) but with a trimeric repeat unit LiAs03 is similar but with a dimeric repeat unit whereas /6-KASO3 appears to have a cyclic trimeric anion As309 which resembles the cyc/o-trimetaphosphates (p. 530). There is thus a certain structural similarity between arsenates and phosphates, though arsenic acid and the arsenates show less tendency to catenation (p. 526). The tetrahedral As 04) group also resembles PO4) in forming the central unit in several heteropoly acid anions (p. 1014). 

The propensity for iodine to catenate is well illustrated by the numerous polyiodides which crystallize from solutions containing iodide ions and iodine. The symmetrical and unsymmetrical 13 ions (Table 17.15) have already been mentioned as have the I5- and anions and the extended networks of stoichiometry (Fig. 17.12). The stoichiometry of the crystals and the detailed geometry of the polyhalide depend sensitively on the relative concentrations of the components and the nature of the cation. For example, the linear ion may have the following dimensions ... 

Sulfur has a striking ability to catenate, or form chains of atoms. Oxygen s ability to form chains is very limited, with H202, 03, and the anions 02, 022-, and OG the only examples. Sulfur s ability is much more pronounced. It appears, for instance, in the existence of Ss rings, their fragments, and the long strands of plastic sulfur that form when sulfur is heated to about 200°C and suddenly... 

In these, As04 tetrahedra are built into the polyphosphate chains (309). Since the P—0—As bond has much the same sensitivity to hydrolysis as the As—0—As bond, they are rapidly hydrolyzed in aqueous solution to monoarsenate and mixtures of polyphosphates, the mean chain length of which depends on the As P ratio in the starting material (74)- Contrary to an idea based originally on gravimetric analysis (309), the As atoms are not distributed regularly in the chain, but statistically (308). The observation that, after careful hydrolysis, the phosphate content is exclusively in the form of polyphosphates provides chemical proof of catenation in the poly-arsenatophosphate anions. When arsenatophosphates with more than five P atoms per atom of As are hydrolyzed by hot water trimetaphosphates are formed, just as they are formed for all other high-molecular polyphosphates in solution (316). 

The aqueous chemistry of aluminum(III) above pH 6 differs from that of silicates in that the only important species, other than solid Al(OH)3 at pH 5-8, is Al(OH)4, which, although isoelectronic with Si(OH)4, shows no tendency to catenate. On the other hand, below pH 5 Alm, unlike the poorly soluble Si(OH)4, is freely soluble as Al3+(aq) [actually Al(OH2)63+, Section 13.2], while at intermediate pH hydrolytic A1 species, including the ion Ali304(0H)24(0H2)127+ referred to above, predominate in solution. However, Al(OH)4 units can readily insert themselves into silicate anion species in solution. The result is usually the prompt precipitation of an aluminosilicate gel (a typical zeolite precursor), although over some limited Al, Si, and OH- concentration ranges quite high concentrations of dissolved aluminosilicates can be maintained over many months.9... 

There are no well-characterized polymeric bismuth catenates, although a black ether-insoluble substance, described as (PhBi)x, results from the reduction of phenylbismuth halides with LiAlH4.200 A polymeric bismuthine is thought to result from the decomposition of l,T-bibismolane.201 To date, bismuth catenation is limited to Bi2 in the case of free molecules and to some complex anionic and cationic clusters. 

By comparison with the mercury(I) and mercury(II) ions (Chapter 56.1), the coordination chemistry of (10) and (11) has received little attention. Both ions disproportionate to Hg° and Hg2+ /Hg2+ in media more basic than those from which they can be prepared. However, the existence of (11) (which can be regarded as a complex of (10) with Hg°) and the cation-anion coordination found in solid salts of (10)38 suggest that this ion, at least, might form stable complexes with suitable weak donors. In addition, the formation of as yet incompletely characterized Hg2BrCI04 2SnBr2, which may contain both Hg—Hg and Hg—Sn bonds,42 and the isolation and characterization of [ (np3)Co HgHg Co(np3) ] (1 see Section 11.2) may presage a wider occurrence of catenated heterometallic polymercury species. Slow disproportionation of (11) into (10) and Hg3-X(MF6) occurs even in liquid S02.39 As discussed below, there is evidence for mercury atom transfer between (10) and (11) in liquid S02. 

The homopolyhalogen anions are formed mainly by iodine, which exhibits the highest tendency to form stable catenated anionic species. Numerous examples of small polyiodides, such as 1, I% and IJ, and extended discrete oligomeric anionic polyiodides, such as I7, Ig-, Ij7, Ijg, I4g, I42 and I29, and polymeric (17 ) networks have been reported. These polyiodides are all formed by the relatively loose association of several I2 molecules with several I- and/or 1 ... 

The study of catenated nitrogen ligands has been extended to N5 systems by the synthesis of transition metal complexes containing coordinated 1,5-diarylpentaza-l,4-diene-3-ide anions (6). Although substituted pentaza-1,4-... 

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