Fluorine compounds and complexes

V.4 Group 17 (halogen) compounds and complexes V.4.1 Fluorine compounds and complexes V.4.1.1 Aqueous zirconium fluorides... 

VIII. 1 Fluorine compounds and complexes VIILl.l Solid and gaseous thorium fluorides VIILl.1.1 ThF(g), ThF Cg), ThFjCg)... 

The result is that the first element of the main groups (lithium, beryllium as well as boron, carbon, nitrogen, oxygen, and fluorine described in Chapter 12) are different from the rest of their group members. Usually, the first element is much smaller than the rest of the elements in that group, and it tends to form covalent compounds and complexes. This tendency is called Fajans rule after Kazimierz Fajans, a physical chemist who in 1923 postulated it. 

Specific hiocatalytic reaction classes have heen chosen to target amide and chiral amine synthesis, stereo- and regio-specific hydrojgrlation of complex molecules as well as other redox reactions, the use of enzymes for stereo-controlled synthesis of fluorinated compounds, and production of homo-chiral quaternary centres. 

The vast range in elemental properties, from those of metallic elanents such as thallium and lead that have very low electronegativities of 1.8 and 1.9 (respectively) to those of the nonmetalUc elements such as oxygen and fluorine that have the highest electronegativities of 3.5 and 4.0 (respectively), results in the great chemical diversity of the elements in the p block. These elements include metals, alloys, simple covalent compounds, enormous covalent network compounds, simple binary ionic compounds, and complex chain and layered ionic compounds. 

The fluorination reaction is best described as a radical-chain process involving fluorine atoms (19) and hydrogen abstraction as the initiation step. If the molecule contains unsaturation, addition of fluorine also takes place (17). Gomplete fluorination of complex molecules can be conducted using this method (see Fluorine compounds, organic-direct fluorination). 

The second method of tantalum and niobium production is related historically to Marignac s process of tantalum and niobium separation, in the form of complex fluoride compounds, and is based on the fluorination of raw material. The modem production process consists of slightly different steps, as described below. 

The structure of KNbF6 consists of potassium ions and isolated NbF6 complex ions that were shown by Bode and Dohren to occur in the lattice in a configuration similar to that of a-CsCl [165]. The complex anion Nb(Ta)F6 has a configuration of a distorted bi-pyramid (four fluorine atoms are shifted in pairs from their positions in the basic plane, towards the vertexes). The structure of KNb(Ta)F6 compounds and of the Nb(Ta)F6 polyhedron are shown in Fig. 26. Nb/Ta-F distances are equal to 2.13 and 2.15 A, respectively, and F-F distances are 2.61, 3.03, 3.22 and 3.55 A. Each potassium atom is surrounded by 12 fluorine atoms that are at unequal distances from each other 8 of them are 2.50 A apart and four others are 2.94 A apart. 

In both cases, the fluorination of the complex oxides of tantalum and niobium leads to the formation of the water-soluble compounds (NH4)2TaF7 and (NH4)3NbOF6, the insoluble lithium fluoride and die gaseous components H20, NH3 and HF. 

Ammonium hydrofluoride is relatively stable, even in the molten state. In addition to being in contact with tantalum or niobium oxide, the compound will initiate the fluorination process yielding complex tantalum or niobium fluoride compounds. There is no doubt that thermal treatment of the hydroxides at high temperatures and/or at a high temperature rate leads to the enhancement of the defluorination processes, which in turn results in an increase in fluorine content of the final oxides. 

A comprehensive review of the preparation, reactions, and n.m.r. spectra of phosphorus-fluorine compounds has appeared. This year s literature has been notable for the first detailed applications of ab initio SCF-MO calculations to the problems of bonding in halogenophosphines and their derivatives. - Comparison of the results of such theoretical calculations with experimental data obtained from photoelectron spectra shows a good correlation in the case of phosphorus trichloride and phosphoryl chloride, and of phosphorus trifluoride and its borane complex. ... 

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