Metaborate ring

Fig. 8-1.—A portion of the infinite metaborate chain (BO ) in CaBt04 (left), and the metaborate ring [B 0 ] in K BtO . Small circles represent boron atoms, large circles oxygen atoms.

Figure 8 Packing of the cyclic metaborate rings in the structure of /3-BaB204...

The oxygen to boron ratio of the metaborates (2 1) requires that at least one of the oxygen atoms must be shared between two boron atoms to obtain the customary boron coordination number. Metaborates with the triangular BO3 group crystallize in (a) a trimeric ring structure, or (h) a chain structure. Sodium and potassium metaborates and orthorhombic... 

Ur any 1, Sodium and Calcium Metaborate. The spectra of uranyl, sodium, and calcium metaborates are illustrated in Figure 1, and Table II lists the absorption maxima for three compounds. The bands in the 92% B compounds are also located. Data on the sodium and calcium salts, as representative of the ring and chain metaborate structures, are in agreement with results reported by Goubeau and Hummel (6). The spectra given by Wier and Schroeder (20) are more complex and may not represent pure phases. 

The three metaborate spectra each have two strong bands in the asymmetric B—O stretching region. — 1450 and — 1220 cm." ). The higher frequency band is identified with the side-chain or branch oxygen atom, while the lower frequency band is identified with a ring or chain... 

Of the borates much the most common are the metaborates these have chain and ring forms. Sodium metaborate, correctly formulated NagBgOg, contains the six-membered ring ions ... 

As already noted, the structures of hydroxy-acids, which include B(0H)3 and Te(OH)g, are described in Chapter 14 with hydroxides, to which they are more closely related stmcturally, the molecules being linked by long 0-H—0 bonds (two from each OH) as in many organic hydroxy-compounds. One form of metaboric acid, consisting of cyclic 6303(011)3 molecules, is also of this type, for the three 0 atoms in the ring are not involved in hydrogen bonding. 

Compounds RB(0H)2 polymerize to cyclic trimers only, in contrast to R2Si(OH)2 which give polymeric linear siloxanes compare the formation of only one cyclic metaborate ion, 8306 . The formation of 6-membered rings is a prominent feature of boron chemistry, as in boroxine (p. 862) and the numerous substituted... 

Alkyl metaborate esters in combination with organic hydroperoxides are reported to be active for epoxidation [182]. The borate esters are cyclic and have general formula (ROBO)3, where R is an alkyl group like cyclohexyl (CeHn). They are six-membered ring compoimds formed from O-B units with a pendant RO, and are 1 1 dehydro adducts of alcohol and boric acid. 

Other discrete and chain-polymer anions can be formed by linking of two or more rings by shared tetrahedral boron atoms, in some cases with dehydration (cf. metaborate below). 

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