Group trihalides

Figure 7.7 Trends in the standard enthalpies of formation AH] for Groups 3 and 13 trihalides as illustrated by data for MF3 and MBrj.

The thermochemistiy of the Group 13 trihalide complexes has been extensively studied and several stability sequences have been... 

A coordination compound, or complex, is formed when a Lewis base (ligand) is attached to a Lewis acid (acceptor) by means of a lone-pair of electrons. Where the ligand is composed of a number of atoms, the one which is directly attached to the acceptor is called the donor atom . This type of bonding has already been discussed (p. 198) and is exemplified by the addition compounds formed by the trihalides of the elements of Group 13 (p. 237) it is also the basis of much of the chemistry of the... 

The preparation of a-iodocarboxylic acids is of particular interest, since iodide is a better leaving group as is chloride or bromide. A similar a-iodination with a phosphorus trihalide as catalyst is not known. However the iodination can be achieved in the presence of chlorosulfonic acid mechanistically the intermediate formation of a ketene 10 by dehydration of the carboxylic acid is assumed ... 

Wells et al. characterized group 13-stibine adducts by single crystal X-ray structure analyses first in 1997 [35]. The solid state structures of three borane-stibine adducts of the type X3B—Sb(Tms)3 (X = Cl 6, Br 7, I 8), obtained by reaction of boron trihalides BX3 and Sb(Tms)3 in n-pentane, were determined. 

Only some Lewis acid-base adducts of group 13 trialkyls R3M or trihalides CI3M and transition metal complexes of the type LnFe—E=CR2, (E = P, As) Weber L, Scheffer MH, Stammler HG, Stammler A (1999) Eur J Inorg Chem 1607 (LnW=P) Scheer M, Muller J, Baum G, Haser M (1998) J Chem Soc Chem Commun 1051, have been synthesized and structurally characterized... 

The complexes Fe(CNR)4(CN)2 (R = H, CH3, C2H5) are reported to form 1 2 complexes with boron trihalides (65). In these complexes the BX3 group coordinates to the cyanide nitrogen, giving the ligand group [CNBXj] . A mention of a similar complex was made earlier 161). 

Boron trihalides are strong Lewis acids that react with a wide collection of Lewis bases. Many adducts form with donor atoms from Group 15 (N, P, As) or Group 16 (O, S). Metal fluorides transfer F ion to BF3 to give tetrafluoroborate salts LiF + BF3 LiBF4 Tetrafluoroborate anion is an important derivative of BF3 because it is nonreactive. With four <7 bonds, [BF4 ] anion has no tendency to coordinate further ligands. Tetrafluoroborate salts are used in synthesis when a bulky inert anion is necessary. 

Although 1,3,2-diazaphospholenium cations are usually prepared from neutral NHPs or 1,3,2-diazaphospholes via Lewis-acid induced substituent abstraction or A-alkylation, respectively (cf. Sect. 3.1.2), the group of Cowley was the first to describe a direct conversion of a-diimines into cationic heterocycles by means of a reaction that can be described as capture of a P(I) cation by diazabutadiene via [4+1] cycloaddition [31] (Scheme 4). The P(I) moiety is either generated by reduction of phosphorus trihalides with tin dichloride in the presence of the diimine [31] or, even more simply, by spontaneous disproportionation of phosphorus triiodide in the presence of the diimine [32], The reaction is of particular value as it provides a straightforward access to annulated heterocyclic ring systems. Thus, the tricyclic structure of 11 is readily assembled by addition of a P(I) moiety to an acenaphthene-diimine [31], and the pyrido-annulated cationic NHP 12 is generated by action of appropriate... 

Intramolecular adducts have also been used as gallium and indium precursor complexes they are synthesized by the alkylation of the group 13 metal trihalide with an appropriate Grignard reagent,89 as shown Figure 20. 

Electric discharge in a mixture of PC13 and H2 produces P2C14, and white phosphorus dissolved in carbon disulfide reacts with I2 to produce P2I4. All of the trihalides of the group VA elements are known, and they can be prepared by reaction of the elements, although there are other preparative methods. The fluorides are prepared as follows ... 

Table 14.6 shows some of the properties of the trihalides of the group VA elements. 

Because of the reactive covalent bonds to halogen atoms, all of the trihalides of the group VA elements hydrolyze in water. It is found that the rates decrease in the order P > As > Sb > Bi, which agrees with the decrease in covalent bond character that results from the increase in metallic character of the central atoms. Not all of the trihalides react in the same way. The phosphorus trihalides react according to the equation... 

Table 14.6 Physical Properties of the Trihalides of Group VA Elements. ...

The determination of many thermodynamic solution properties of these trihalides, especially enthalpies of solution, requires the use of anhydrous materials. These are by no means always readily available for this group of compounds. Therefore we precede our discussion of thermochemical results with a section on preparative methods, in which we deal summarily with many of the relevant papers published since Taylor s extensively referenced review, published in 1962 (2). 

With a functional substituent in the alkyl group, the self-association may be intramolecular. Thus, in the tu-hydroxyalkyltin trihalides, HO(CI I2) SnGI3, when n 3 or 4, the molecules are intramolecularly coordinated, whereas when n = 5, they form a linear polymer.336 Similarly, MeC02(CH2) SnCl3 forms a cyclic monomer when n = 3, but a cyclic dimer when 11 = 2, and an oligomer when n = 4.337... 

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