Tetrahydroborate anion

The preparation of oximes from olefins is a valuable approach for the synthesis of nitrogen-containing compounds such as amino acids and heterocycles. Okamoto and colleagues have reported that a catalytic reduction-nitrosation of styrenes 31 with ethyl nitrite and tetrahydroborate anion by the use of bis(dimethylglyoximato)cobalt(II) complex afford the corresponding acetophenone oximes 32 (Scheme 23). 

Hydride abstraction from tetrahydroborate anions is the most convenient route for the preparation of boranes - . In these processes an unstable intermediate transfers a BH3 moiety and in so doing forms new BHB bridge bonds. The simplest of these reactions involves the formation of B H from NaBH, and BF3 at 25°C. In addition to forming new B—H—B bonds in BjH, 1 mol of [HBF3] is formed per mol of [BH,]- ... 

Insertion of boron atoms from larger boranes into the B—B bonds of higher tetrahydroborate anions proceeds with the formation of new B—H bonds and represents a rational approach to the synthesis of higher boranes, e.g., B-H., combines ... 

The simplest binary borane species is the tetrahydroborate anion, BH4", which is isoelectronic with CH4 and NH4" . Many salts of this anion, such as LiBH4 and NaBH4, are essentially ionic and have been used for more than 50 years as versatile reducing agents (7, 27). However, BH4" can also react by ligand displacement to form covalently bonded complexes in which it acts as a monohapto, dihapto, or trihapto... 

Earlier kinetic studies on the acid hydrolysis of the tetrahydroborate ion (BH4 ) in moist acetonitrile were interpreted in terms of the accumulation of an intermediate complex of acetic acid and the tetrahydroborate anion. However, nmr measurements and kinetic isotope effects now show that while this intermediate is formed, and allows isotope exchange between BH4 and CH3COOD, it does not accumulate. The rate-determining step involves a second intermediate CHaCOOBHa", so that the earlier study on the hydrolysis of tetrahydroborate actually related to the hydrolysis of this species. It is suggested that this occurs... 

In the chromium triad, the only claimed tetranuclear compound is the anion [Mo4(CO)u ]2 which was mentioned together with [Mo3(CO)14]2, in a study of the reaction of sodium tetrahydroborate and hexacarbonyl-molybdenum in liquid ammonia14). Later work failed to confirm the existence of the trinuclear anions, [M3(CO)i4]2 (M = Cr, Mo, W)109 1S0,169 and, therefore, the real existence of the tetranuclear anion also seems dubious. 

The physical and chemical properties of the tetrahydroborates show more contrasts than the salts of nearly any other anion. The alkali metal salts are the most stable. In dry air, NaBH4 is stable at 300°C and in vacuo to 400°C with only partial decomposition. In contrast, several tetrahydroborates, including the titanium, thallium, gallium, copper, and silver salts, are unstable at or slightly above ambient temperatures. The chemical and physical properties of the tetrahydroborates are closely related to molecular structure. Sodium tetrahydroborate, which is typical of the alkali metal tetrahydroborates except for the lithium salt, has a face-centered cubic (fee) crystal lattice which is essentially ionic and contains the tetrahedral [BHJ- anion. The tetrahydroborates of the polyvalent metals are in many cases the most volatile derivatives of these metals known. Aluminum tris(tetrahydroborate)... 

Arsenic species were preconcentrated on Zipax, a pellicular anion exchange material and separated on a column packed with high performance liquid chromatography grade strong anion exchange resin, then continuously reduced with sodium tetrahydroborate and detected by atomic absorption spectrometiy. Detection limits were 2ng for arsenite, arsenate and monomethylarsinate and lng for dimethylarsonate. 

Tetrahydroborate or other hydridic anions produce [RhH(PPh3)3], widely used by Robinson s group as the starting point for the preparation of carboxylato and other complexes (equation 18). The IR spectra of [Rh(02CR)(PPh3)3] show that the carboxylate is monodentate and therefore Rh is four-coordinate this has been confirmed by X-ray crystallography for the benzoate. 

With this repertoire of bonding possibilities at our disposal, we car construct the molecular structures of various boron-hydrogen compounds, both neutral species and anions. The simplest is the tetrahydroborate 26 or borohydride ion, BH. Although borane is unstable with respect to dimerization, the addition of a Lewis base, H . satisfies the fourth valency of boron and provides a stable entity. Other Lewis bases can coordinate as well. 

The [M2(CO),o(p.-H)] species (M = Cr, Mo, or W) were first isolated as tetramethylammonium salts by water treatment of the [M2(CO),0]2 anions, produced by the reaction of M(CO)6 with NaBH4 in liquid ammonia. Later, they were obtained as tetraethylammonium salts by the reaction of M(CO)6 with tetraethylammonium tetrahydroborate(l -) in diglyme, 1,l -oxybis(2-methoxy-ethane),2 and by the more convenient reaction of M(CO)6 with NaBH, in te-trahydrofuran, followed by treatment with EttNBr.3... 

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