1,1,1-Trihalides 1.1.1- trifluorides

The anhydrous trihalides are ionic, high melting, crystalline substances which, apart from the trifluorides are extremely deliquescent. As can be seen from Table 30.4, the coordination number of the Ln changes with the radii of the ions, from 9 for the trifluorides of the large lanthanides to 6 for the triiodides of the smaller lanthanides. Their chief importance has been as materials from which the pure metals can be prepared. 

Molybdenum hexafluoride. 3,1412 Molybdenum-iron-sulfur complexes, 4,241 Molybdenum oxide amino acid formation prebiotic systems, 6, 872 Molybdenum storage protein microorganisms, 6, 681 Molybdenum telluride, 3, 1431 Molybdenum tetraalkoxides physical properties, 2, 347 Molybdenum tribromide, 3,1330 Molybdenum trichloride, 3,1330 Molybdenum trifluoride, 3, 1330 Molybdenum trihalides, 3, 1330 bond lengths, 3, 1330 magnetic moments, 3,1330 preparation, 3,1330 properties, 3, 1330 structure, 3,1330 Molybdenum triiodide, 3,1330 Molybdenum trioxide complexes, 3, 1379 Molybdenum triselenide, 3, 143)... 

Potassium reacts explosively with all antimony trihalides except trifluoride, as with tin halides (p.223). 

Exceptions to the rule are observed for compounds with low polarity, i.e. when covalent bonds predominate. Fluorides and oxides (including silicates) usually fulfill the rule, whereas it is inapplicable to chlorides, bromides, iodides, and sulfides. For instance, in metal trifluorides like FeF3 octahedra sharing vertices are present, while in most other trihalides octahedra usually share edges or even faces. 

Bismuth salts, 4 25 Bismuth sesquisulfide, 4 24 Bismuth subcarbonate, 4 36 Bismuth subgallate, 4 36 Bismuth subhalides, 4 19 Bismuth subnitrate, 4 36 Bismuth subsalicylate, 4 1, 36 medical applications of, 22 11-12 Bismuth(III) sulfate, 4 25 Bismuth(III) sulfide, 4 24 Bismuth sulfides, 4 24-25 Bismuth thiolates, 4 25 Bismuth-tin alloy waterfowl shot, 4 15 Bismuth triacetate, 4 25 Bismuth tribromide, 4 21 physical properties of, 4 20t Bismuth trichloride, 4 19-20 physical properties of, 4 20t Bismuth trifluoride, 4 19 physical properties of, 4 20t Bismuth trihalides, 4 19 Bismuth triiodide, 4 21-22 physical properties of, 4 20t Bismuth trinitrate pentahydrate, 4 25 Bismuth trioxide, 4 23-24 physical properties of, 4 20t Bismuth triperchlorate pentahydrate, 4 25... 

The preparations of rare-earth trihalides can be found in various books (2-8) and in Taylor s review (2 ). This review, however, did not include the preparation of scandium and yttrium trihalides, and only covered the preparation of the trifluorides very briefly. We have reviewed the preparation of all the trihalides (including scandium and yttrium) from Taylor s review up to June 1979 and have also included some methods and references missed by Taylor. Although we have mentioned all the methods available for the preparation of the trihalides, emphasis has been placed on the methods used since Taylor s review, and these have been referenced fully, whereas for the other methods, Taylor s review is recommended as a source of references. 

Taylor s treatment of lanthanide trifluorides was considerably more cursory than that of the other three trihalides, but nonetheless still provides a starting point (2). More recently, the extraction, preparation, and treatment of the trifluorides have been dealt with in Gmelin (4a). 

Reactions with halogens are slow at room temperature but progress rapidly above 200°C forming ytterbium trihalides. All the trihalides namely, the YbCls, YbBrs, and Ybis with the exception of trifluoride, YbFs, are hygroscopic and soluble in water. 

Pyridine readily forms stable coordination compounds. Thus, boron, aluminum and gallium trihalides react at 0°C in an inert solvent to give 1 1 adducts (cf. 85). Steric factors are important, and a-substituents decrease the ease of reaction. This is illustrated by the heats of reaction of pyridine, 2-methylpyridine and 2,6-dimethylpyridine with boron trifluoride which are 101.3, 94.1 and 73.2 kJ mol-1, respectively. The marked decrease in exothermicity here should be contrasted with the small steric requirement of the proton as shown by the pA., values of substituted pyridines (see Section 3.2.1.3.4). 

All four trihalides of scandium are known. The trifluoride is very slightly soluble in H2Q. and is precipitated front scandium nitrate, Sc(NC>3)3,... 

The structure of the complex (96) between benzaldehyde (95) and boron trifluoride (equation 15) was investigated by X-ray crystallography169. In 96, BF3 is in the anti position to the phenyl ring and this geometry remains also in solutions, as tested by the 19F-NMR spectrum in CD2CI2. An ab-initio study170 on interactions between formaldehyde and boron trihalides showed that these complexes (mainly donor-acceptor complexes) affect spectroscopic properties and the reactivity of the carbonyl group the polarization of the C=0 bond favours the attack of nucleophiles. 

The strength or coordinating power of different Lewis acids can vary widely against different Lewis bases. Thus, for example, in the case of boron trihalides, boron trifluoride coordinates best with fluorides, but not with chlorides, bromides, or iodides. In coordination with Lewis bases such as amines and phosphines, BF3 shows preference to the former (as determined by equilibrium constant measurements).66 The same set of bases behaves differently with the Ag+ ion. The Ag+ ion complexes phosphines much more strongly than amines. In the case of halides (F, CP, Br, and P), fluoride is the most effective base in protic acid solution. However, the order... 

Organo tellurium trifluorides, trichlorides, tribromides, and triiodides are known. Aryl tellurium trihalides are generally more stable than alkyl tellurium trihalides. The sensitivity toward atmospheric agents decreases from the trichlorides to the triiodides. Very few organo tellurium trifluorides have been prepared and little is known about their reactivity. The general reactivity of aryl tellurium trihalides decreases from the chlorides to the iodides. 2-Chloroalkyl tellurium trichlorides, products of the addition of tellurium... 

Table 13.2 shows some of the properties of the trihalides of the Group VA elements. Several trends in the data shown in Table 13.2 are of interest. For example, the trihalides of phosphorus and arsenic can be considered as covalent molecules. As a result, the intermolecular forces are dipole-dipole and London forces that are weak. Therefore, the melting and boiling points increase with molecular weight as expected. The trifluorides of antimony and bismuth are essentially ionic compounds and the melting points are much higher than those of the halogen derivatives that are more covalent. 

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