Borate anions tetrahedral

Most of the isolated borate anions shown above may polymerize in a variety of ways to form extended chains, sheets, and networks. Polymerization occurs by either sharing an exocyclic oxygen atom or by sharing an intracyclic tetrahedral boron atom. These are exemplified by the structures of the important industrial minerals colemanite [29], Ca[6304(0H)3] H2O... 

Similar to their spontaneous 1,4-hydration in water, heterocycles 49a, 50a-e, and 50g all react simply upon dissolution in warm methanol to form his-addition products. The X-ray crystal structure of the bis-methanol adduct (51) derived from the diazaborine 50b clearly showed it to be a zwitterion comprised of tetrahedral borate anion and formamidinium cation fragments. Once again, 50f proved to be the only exception A weakly chelated dimethylborate ester (not shown) was found to be its structure in MeOH by B NMR. 

Does the diol react with trigonal boric acid or with the tetrahedral borate anion ... 

The pK value for this dissociation is about 9.14. Reports of lower values (15) may result from the formation of polyborate ions or from the presence of chloride ions. If a knowledge of Ka is important in a particular study, the possible effect of chloride ion should be investigated before it is introduced into the system. Dilute boric acid solutions probably contain only two boroxy species, trigonal boric acid and tetrahedral borate anion. 

Isotope labeling experiments indicate that the B—O bond is broken and not the C—O bond in the formation of the diol-boric acid complexes (18). This indicates that the initial step in the mechanism may be an attack on the boron atom by an oxygen of the diol, followed by the release of water. This could occur without developing any charge separation. If such a mechanism were correct, it would seem that an attack on the boron atom would be easier for trigonal boric acid than for the tetrahedral borate anion (Figure 1). 

A successful structural model for this process has been developed, which involves the tripodal tris (pyrazolyl) borate anion, resulting in the distorted tetrahedral species 12.20. An analogue of the key hydrogen carbonate catalytic intermediate in which the Zn2+ centre is five-coordinate is also known in the form of a bidentate nitrate analogue [Zn 73 HB(3 Phpz)3 (772 NO,)]. 

Boron, having the electronic configuration s22s22p, has 3 valence electrons, and forms planar, tricovalent derivatives that are electron deficient, and which, as Lewis acids, accept two electrons from bases to complete the boron outer-shell octet and give tetrahedral adducts. Boric acid exemplifies this behavior by ionizing, in aqueous solution, not by direct deprotonation, but by hydration and subsequent ionization, to give the symmetrical borate anion ... 

Boron oxides and their derivatives are technologically important and are relatively inexpensive to produce since, as noted earlier, essentially all boron in Nature is in oxygenated form. Boron-oxygen compounds contain predominantly trigonal planar B03, and to a lesser degree tetrahedral B04 units, as in the borate anions in Fig. 5-25. 

This relationship held quite well for a range of neutral trigonal planar boranes BXYZ and analogous carbocations (CX YZ), though it proved less precise for systems in which there was conjugation between the substituent (X, Y, or Z) and three-coordinate boron. When data for tetrahedral borate anions BXt and neutral carbon species CX4 were considered, a similar relationship [of slightly different slope, Eq. (5.36)] was found ... 

Fig. 12.17 The structures of selected borate anions trigonal planar and tetrahedral B atoms are present, and each tetrahedral B carries a negative charge. The [B405(011)4] anion occurs in the minerals borax and kernite. In the pyroborate ion, [B205]", the B—O—B bond angle depends on the cation present, e.g. ZB—O—B = 153° in C02B2O5, and 131.5° in Mg2B205.

At this relatively high ligand concentrations predominantly one-to-two complexes are formed. This ist not surprising since the tetrahedral borate anion should allow the attachment of even large polyol molecules... 

Boron. There is relatively little information on the coordination and speciation of B on mineral surfaces. Palmer et al. (77) using B isotopic fractionation in the pH range of 6.65 to 8.45, suggested that B is tetrahedrally coordinated with clay surfaces. Su and Suarez (18) examined coordination of B adsorbed on Al and Fe oxides, allophane, and kaolinite using ATR-FTIR. The dominant B species in solution are boric acid, B(OH)3 and borate anion, B(OH)4, with a pK. of 9.23. These two species can be readily distinguished since the B atom in boric acid is trigonally coordinated while B in the borate anion is tetrahedrally coordinated. 

Polymerization is not observed above pH 11 because borate anions repel each other and because tetrahedrally coordinated boron without any available d orbitals is coordinatively saturated and unable to participate in the sp d intermediate required for condensation via an Sjv mechanism [181], Below pH 4 polymerization is inhibited because the OH ligands on boric acid are not sufficiently strong nucleophiles to have an affinity for other boric add molecules [181]. 

Based on these rules, the structures of the stable dimer, trimer, and tetramer are shown in Fig. 32. It is interesting to note that similar structures composed of three-mem bered rings (Fig. 33) were proposed by Krogh-Moe [183] to exist in alkali borate glasses where alkali ions (rather than protons) compensate the charges on the tetrahedral borate anions. 

Two triborate moieties may link together by sharing a common tetrahedral boron atom to form the pentaborate anions shown in Fig. 5. The pentaborate monoanion shown in Fig. 5a is observed in solution (see Fig. 2) and occurs in synthetic and mineral borates. Examples are the synthetic commercial ammonium pentaborate, NH4[Bs06(0H)4] 2 H2O [ = (NH4)20 5 B2O3 ... 

At pH 10, diphenylborinic acid gives a tetrahedral anion that complexes with various diol systems, and thus it can be used in electrophoresis like borate.109 In a more detailed study of such complexing,110 diols were examined by 13C-n.m.r. spectroscopy, before and after addition of sodium diphenylborinate, and complexes were detected, and their spectra observed, for a variety of carbohydrate derivatives. 1,2-Diol groupings in acyclic and cis-cyclic compounds, 1,3-related diols at C-4,C-6 of hexopyranosides, the 3,5-diols of glucofuranoses, and 2,4-diols of the anomeric methyl 3,6-anhydro-D-glucopyranosides were all found to react. No interaction occurred with l,6-anhydro-/3-D-glucopyranose (compare Section V,2). 

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