Preparation and stoichiometry

Boron carbide (p. 149) is a most useful and economic source of B and will react with most metals or their oxides. It is produced in tonnage quantities by direct reduction of B2O3 with C at 1600° a C resistor is embedded in a mixture of B2O3 and C, and a heavy electric current passed. 

The various stoichiometries are not equally common, as can be seen from Fig. 6.5 the most frequently occurring are M2B, MB, MB2, MB4 and MBfi, and these five classes account for 75% of the compounds. At the other extreme RunBg is the only known example of this stoichiometry. Metal-rich borides tend to be formed by the transition elements whereas the boron-rich borides are characteristic of the more electropositive elements in Groups 1-3, the lanthanides and the actinides. Only the diborides MB2 are common to both classes. 

---

A number of basic oxalates, such as (U02)2C204(0]rn2 2H20, and salts of basic oxalato complex ions of the types M 3U02(C204)2(0H) and M 5(1102)2(0204)4(01 ) have also been reported, as well as salts of a wide range of peroxo-, halogeno-, sulfato-, selenito-, selenato-, thiocyanato-, and carbonato-oxalato complex anions. Only the preparation and stoichiometries of these complexes have been reported. 

Iwamoto, T. Miyoshi, T. Miyamoto. T. Sasaki, Y. The metal amminc cyanide aromatics clathrate. I. The preparation and stoichiometry of diammincmetal(II) tetracyano-niccolate(II) dibenzene and dianiline. Bull. Chem. Soc. Jpn. 1967,40. 1174-1178. 

YbX compounds with X = N, P, As, and Sb have a cubic rocksalt structure and a nearly integral 4-3 valence for the Yb ions. Details of their physical properties depend on sample preparation and stoichiometry (see, e.g. Donni et al. 1990a). Their main properties are ... 

Thin oxide films may be prepared by substrate oxidation or by vapour deposition onto a suitable substrate. An example of the fomrer method is the preparation of silicon oxide thin-films by oxidation of a silicon wafer. In general, however, the thickness and stoichiometry of a film prepared by this method are difficult to control. 

Metal organic decomposition (MOD) is a synthesis technique in which metal-containing organic chemicals react with water in a nonaqueous solvent to produce a metal hydroxide or hydrous oxide, or in special cases, an anhydrous metal oxide (7). MOD techniques can also be used to prepare nonoxide powders (8,9). Powders may require calcination to obtain the desired phase. A major advantage of the MOD method is the control over purity and stoichiometry that can be achieved. Two limitations are atmosphere control (if required) and expense of the chemicals. However, the cost of metal organic chemicals is decreasing with greater use of MOD techniques. 

There is an extensive literature devoted to the preparation and structure determination of coordination compounds. Thermal analysis (Chap. 2, Sect. 4) has been widely and successfully applied in determinations [1113, 1114] of the stoichiometry and thermochemistry of the rate processes which contribute to the decompositions of these compounds. These stages may overlap and may be reversible, making non-isothermal kinetic data of dubious value (Chap. 3, Sect. 6). There is, however, a comparatively small number of detailed isothermal kinetic investigations, together with supporting microscopic and other studies, of the decomposition of coordination compounds which yields valuable mechanistic information. 

With 2-formylpyridine S-methyldithiocarbazate, planar, diamagnetic nickel(II) complexes with stoichiometry [Ni(6-H)A], where A = Cl, Br, I, and NCS, have been isolated [126]. Also, paramagnetic, octahedral [Ni(6-H)2] has been prepared and spectrally characterized. These complexes have NNS coordination as does the related S-benzyldithiocarbazate, 24, in [Ni(31-H)A], A = Cl, Br [165]. 

Pyrazines and substituted pyrazines have long been known to act as exo-bidentate ligands to linearly bridge metal ions, and then have also been employed for the self-assembly of polymeric materials. With pyrazine (prz) varying the reaction conditions and stoichiometry, ID, 2D, and 3D structures containing silver(I) atoms with several coordination modes have been prepared.606-609 A 3D racemate is present in [Ag2(2,3-Me2prz)3](SbF6)2.610 A sawhorse connection has been achieved in the compound [Ag(N02)(pyz)] .611 With 2,2 -bipyrazine an infinite loop has been obtained.612... 

In 1979 the bieyclic diol exo-2,ejco-6-dihydroxy-2,6-dimethylbicyclo[3.3.1]nonane (i) was prepared and observed to co-crystallise with various solvents, including ethyl acetate, chloroform, toluene, dioxane, and acetone. A crystal structure determination of the ethyl acetate compound revealed the occurrence of a helical canal host structure, containing ethyl acetate as guest (with 3 1 diol ethyl acetate stoichiometry), and that spontaneous resolution had occurred on crystallisation of the multimolecular inclusion compound 6>. 

By using different Cp ligands (Cp, Cp, ebthi), additional ligands (THF, pyridine, acetone), and metals (Ti, Zr), a fine-tuning of the reactions of these complexes has been feasible. Additional influences are exerted by, e. g., the substituents on the substrate, the stoichiometry used, the solvents, and other reaction conditions. Complexes of this type have also been prepared and used in connection with a multitude of substrates, and in many cases the products differ markedly from those obtained with conventional metallocene sources. 

This procedure describes the preparation and use of an effective chiral catalyst for the asymmetric allylation of aldehydes. A previous synthesis of optically pure 1-(phenylmethoxy)-4-penten 2-ol requires seven steps from D-mannitol.4 This procedure has been employed successfully with other aldehydes,5 and also with methallyltributylstannane5 (see Table). Catalysts prepared from (R)- or (S)-BINOL and Ti(0-i-Pr)4 at 2 1 stoichiometry have also proven useful in these reactions.The olefinic products may be regarded as latent aldol products between aldehydes and the enolate of actetaldehyde or acetone. In all cases examined thus far, enantioselectivity... 

Precursor Techniques A homogeneous sample containing all of the non-volatile constituents is prepared first. Stoichiometry is easily maintained in the precursor and reasonable reaction times and temperatures can be employed. Generally, the precursor will be a mixture of phases, however. Then, the precursor and volatile constituents are formed into an intimate mixture and are subjected to a brief thermal treatment to form the desired phase. 

Lithium niobate is a ferroelectric material used as an optical switch. Preparation by the simple ceramic method leads to problems in obtaining the correct stoichiometry, and a mixture of phases often results. Several sol-gel preparations have been described, their advantage being the lower temperature required for the preparation and the greater homogeneity of the product. One such preparation starts with lithium ethoxide (LiOC2H5 (or LiOEt)) and niobium ethoxide Nb2(OEt)io. Each ethoxide was dissolved in absolute ethanol and the two solutions mixed. The addition of water leads to partial hydrolysis giving hydroxy-alkoxides, for example ... 

Reproducibility. Homogeneous catalysts have the advantage over heterogeneous catalysts of being totally reproducible because they have a definite stoichiometry and structure. By contrast, the structure of the surface of a heterogeneous catalyst is heavily dependent on both its method of preparation and its history subsequent to preparation. 

---