Tetrahydroborates stability

The stability of the tetrahydroborates correlates with the desorption temperature according to T = AH/AS for p = p°. Furthermore, the desorption temperature correlates with the Pauling electronegativity of the atom forming the cation (Figures 5.48 and 5.49). 

The dominant effect of carbonyl groups on the degradation of oxidized cellulose in an alkaline medium is evident also from the conversion of alkali-unstable oxidized celluloses into stable materials by reduction with sodium tetrahydroborate in a weakly alkaline medium, i.e., under conditions such that aldehyde groups are converted into alcoholic groups. The stability of oxidized celluloses is considerably enhanced also when carbonyl groups are oxidized to carboxy groups. 

Commercially available solutions of H3B-THF in THF (Aldrich-Boranes, Inc., 2371 N. 30th St., Milwaukee, WI 53210) which are used in this synthesis, contain approximately 5% of sodium tetrahydroboratefl —) as a stabilizer. Tri(sec-butyl)borane prepared from these solutions should be distilled. An alternative commercial (Aldrich-Boranes, Inc.) hydroborating agent free of sodium tetrahydroborate(l —) is borane-methyl sulfide. 

The ability of phosphines to act as both a metal oxidation states and as a 7r acceptor to relieve electron density from lower metal oxidation states explains, in part, the observation that diphosphines stabilize the +3, +1, and 0 oxidation states at the expense of the +2 oxidation state.870 Vanadium(II) diphosphine halides, [V(dmpe)2X2] (dmpe = dimethyl-dimethylphosphinoethane, X = Br or I), were prepared as side products in the synthesis of [XV(CO)2(dmpe)2].885 The structurally characterized diphosphino V11 tetrahydroborate complex, tra .s-[V(BH4)2(dmpe)2], (223) was synthesized via the reduction of similar vanadium(III) precursors.832 The reaction of a vanadium(II) chloride salt with dmpe afforded the tra s-[VCl2(dmpe)2] complex which can be alkylated with MeLi or MgMe2 to afford the organometallic trans-[V(Me)2(dm pcVJ compound, which in turn reacts with thiocyanate to form tnms -[V(NCS)2(dmpe)2].8 87 Acetonitrile or propionitrile readily displace the chlorides from trans- WCl2(dmpe)2] to form // tf/ ,v-[V( NCR)2(dmpc)2][BI>h4]2 (R = Me or Et) the acetonitrile derivative reacts with HCPh(S02CF3)2 in acetonitrile solvent to yield the hexaacetonitrile species [V(NCMe)6]2+.887... 

Complex 185 can be considered as the result of the stabilization of the unsaturated fragment [OsH(CO)(P Pr3)2] by 184. So, the mononuclear tetrahydroborate complex is a metalloligand that can be displaced by other... 

Preliminary studies indicate that these new dihalides can be converted to alkyl, aryl, and tetrahydroborate derivatives of varying stability (96, 99). Another useful precursor for some types of biscyclopentadienyl derivatives of uranium(IV) is the amido compound (C5H5)2U[N(C2H5)2]2 (100). 

The stability of tetrahydroborates can be considered from different viewpoints ... 

The stability as an enthalpy difference between the complex hydride and an intermediate product in the decomposition reaction M[BH4] MH + B. AH tetrahydroborates where the MH exhibits a heat of formation between the complex hydride and its elements go through the above-mentioned intermediate product in the thermal desorption of hydrogen. 

The alkali metal tetrahydroborates are ionic, white, crystalline, high-melting soUds that are sensitive to moisture but not to oxygen. Group 3 and transition metal tetrahydroborates are covalent bonded and are either Uquids or subhmable solids. The alkaline earth tetrahydroborates are intermediate between ionic and covalent The tetrahydroa-luminates are very much less stable than the tetrahydroborates and therefore considerably more reactive. The difference between the stability of the tetrahydroaluminate and the tetrahydroborates is due to the different Pauling electronegativity of B and A1 which is 2.04 and 1.61, respectively. 

The synthesis, stability, and properties of phenylaluminium tetrahydroborates have been studied, PhgAl dissolves readily in Al(BH4)s to give the solid dimeric compound PhaAKBH ). ... 

The aim of the screening study is to provide novel, mixed metal tetrahydroborate candidate materials along with their estimated stabilities as obtained from destabilizing the alkali tetrahydroborates by substituting part of the alkali metal atoms with other metals. The screened combinations are limited to ternary tetrahydroborates with a one-to-one mixture of the alkali and the other metal that are calculated in a number of prototype structures [19]. 

Sodium borohydride, or correctly sodium tetrahydroborate, is a white crystalline solid that is stable in dry air to 300°C. It is highly soluble in water and slowly decomposes until the solution becomes alkaline. Decomposition is more rapid in acidic solution, but NaBH4 is kinetically stable in alkaline solutions when the pH is above about 12.9. For this reason, 2%-3% sodium hydroxide is typically added to NaBH4 solutions to provide stability. Although kinetically stable, aqueous solutions of NaBH4 are thermodynamically unstable, and decomposition can be catalyzed by a number of transition metals. 

The IR and NMR data were consistent with the bidentate bridging mode of the HBBN ligand as confirmed by X-ray analysis. The pseudotetrahedral arrangement of the ligands around U is characterized by the average Cg-U-Cg angle of 115(1)". The U-C(cp)av is 2.76(1) A, and the U-B distance 2.78(3) A is typical for a bidentate bridge (the H were not located). The complex exhibits the characteristic properties of tetrahydroborate uranium complexes as molecularity, volatility, thermal stability and resistance to Lewis bases. 

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