Chloride ion donor

A table of relative chloride ion donor and acceptor abilities can be establched17 from equilibrium and displacement reactions (Table 10.5). As expected, good donors are generally poor acceptors and vice versa with but few exceptions (e.g., HgCl2). 

Relative chloride ion donor Chloride km donors Chloride ton acceptors... 

Extensive studies of reactions between chloride ion donors (bases) and chloride ion acceptors (acids) have been conducted by Cutmann,7 who interpreted them in terms of the above equilibria. An example is the reaction between tetramethylammonhim chloride and iron(lil) chloride, which may be carried out as a titration and followed conductometrically ... 

In acidic melts, the A12C17 ions are predominant. In basic melts, per is constant, in the presence of solid NaCl [401], and it is equal to 1.1 at 175°C. Likewise, in the molten chloroaluminate systems the terms acid and base denote a chloride ion acceptor and a chloride ion donor, respectively. The pCL may be measured with either an aluminum or a chlorine electrode immersed in the melt. 

Antimony pentachloride, SbCh, is a liquid (mp 4°C, bp 140 °C dec ), which can be obtained by reaction of SbCls with elemental chlorine. Sohd SbCls exists in two modifications above 54.1 °C, it is trigonal bipyramidal, below this temperature, it changes reversibly into a double chlorine-bridged dimer. SbCls decomposes at 140 °C with formation of SbCls and CI2. It is a Lewis acid with a strong tendency to interact with a ligand (L) to give octahedral complexes LSbCls. Reactions with chloride ion donors lead to ionic compounds with SbCle" anions and unusual cations. Examples are shown in equations (10) and (11). 

In the melts, chloride-ion donors were bases and substances increasing AICI3 concentration were acids. The pK values decrease with the temperature (from 7.1 to 5.0 at temperatures 175 and 400 C, respectively, Na-based melF) elevation and from Cs to Li (at 400 C pK were 3.8,5.0,5.8,7.4 for Li-, Na-, K- and Cs-based melts, respectively ). The latter effect may be explained from the point of view of hard and soft acids and bases" - Li-Cl complexes should be more stable than Cs-Cl ones as formed by hard base (Cl ) and more hard acid (LF). 

Phosphorus(V) fluoride reacts in liquid hydrogen chloride with chloride ion donors to give hexafluorophosphate ... 

It may be noted that the free halogens will not react with the respective liquid hydrogen halides. Not even iodine reacts with hydrogen iodide, where the formation oi a polyiodide anion might be expected. On the other hand iodine monochloride will react with chloride ion donors in hydrogen chloride just as well as in the absence of a solvent32... 

Conductometric titrations show that the formation of hexachlorostannates, hexa-chlorotellurates and of hexachlorotitanates occurs in two steps in molten iodine monochlorideii and in liquid arsenic(III) chloride i. At a 1 1-molar ratio of the basic chloride and the acceptor chloride the solution reaches a maximum in conductivity and with further addition of the chloride ion-donor insoluble hexa-chlorometallate is formed, allowing the conductivity to pass through a minimum. The formation of an acidic-salt has been postulated, but conclusive evidence is not available. 

Thionyli i and sulphuryl chlorides 20 have exceedingly low donor properties, but strong chloride ion-donor properties and these are useful media for the formation of chloro-complexes. Liquid nitrosyl chloride does not show any 0-donor properties, but it is easily ionized i 22 ... 

The chlorides of the alkali- and alkaline earth metals are practically insoluble in the pure liquid solvents, because the low donor numbers do not allow extensive coordination of the cations to take place. Tetraalkylammonium chlorides and phosphorus(V) chloride act as chloride ion donors. Triethylamine, pyridine, quinoline, other N-bases and ketones give conducting solutions which have been interpreted as due to the formation of chloride ions ... 

The same conclusions were drawn from infrared and Raman work ". Data on the chloride ion donor strength of the oxyhalide solvents is, however, not available. One of the main difficulties is the removal of the last traces of water and its elimination during the reactions. Although experimental methods have been considerably improved, it must be born in mind that apparently nobody has ever been successful in working in the complete absence of moisture or of hydrolysis products. Traces of water appear to remain even in reactive liquids, such as the oxychlorides under consideration. It seems that in phosphorus oxychloride small amounts of water form HaO+Cl" which is dissociated in the solution and contrasts with the behaviour of anhydrous hydrogen chloride when dissolved in the same solvent. Thus purified phosphorus oxychloride contains approximately moles of water per liter and this must be taken into consideration, when the pure solvent or when dilute solutions are considered. The assumption of the seK-ion-ization equilibria in the liquid sol vents, 79... 

Boron(III) chloride gives moderately conducting solutions in phosphorus oxychloride and phenylphosphonic dichloride . Conductivity data and the absence of a reaction with a strong chloride ion donor in phosphorus oxychloride exclude the presence of [Cl3POBCl2]" Cl in this solvent, but in C6H5POCI2 a reaction with ferric chloride is indicated ... 

Boron(Ill) chloride reacts with chloride ion donors in phosphorus oxychloride but no exchange of labelled chlorine is observed between boron(III) chloride and phosphorus oxychloride o unless the latter is present in large excess. 

Tungsten hexachloride, the only hexachloride investigated, accepts one chloride ion from chloride ion donors in phosphorus oxychloride . The complex ion seems to have a coordination number higher than seven due to additional solvent coordination, possibly [Cl7WOPCl3] but is decomposed by removal of the solvent ... 

Hexachloro- and tetrachlorometallate ions are unsolvated and not associated in solutions. The reluctance of di- and tetrachlorides to accept a second chloride ion from triphenylchloromethane is remarkable, showing that the chloride ion-donor properties of triphenylchloromethane are weak. In solvents of lower donor number such as benzoyl chloride two chloride ions can be accepted from triphenylchloromethane. In the stronger donor solvents, phosphorus oxychloride and phenylphosphonic dichloride, stronger chloride ion donors, such as tetraethyl-ammonium chloride, are neccessary to obtain the fully chloride-coordinated anionic species. 

High formation constants of the [(C6H5)3C][MCln+i] complexes correspond approximately to high —JH values for the formation of the chloro-complexes in a particular solvent using tetraethylammonium chloride as a chloride ion donor. 

The extent of chloride ion coordination to an acceptor chloride is influenced by the nature of the chloride ion donor, since ionization of triphenylchloro-methane does not occur in a donor solvent, but occurs in the presence of an acceptor. 

The acceptor strength of strong electron pair acceptors is independent of the properties of the chloride ion donors, but weak electron pair acceptors, such as titanium(IV) chloride or zinc chloride, interact stronger with covalent chloride ion donors than with ionic chlorides. At the same time they accept only one chloride ion from covalent triphenylchloromethane, but two from ionic tetraalkylammo-nium chloride. 

Potentiometric and spectrophotometric methods have been used to obtain information about the extent of chloride ion transfer with the formation of chloronium ions and thus about the chloride ion donor strength of metal chlorides in different oxyhalides solvents. The chloride ion donor properties are increased by a solvent of high donor number and decreased by a weak donor solvent, if the chloronium ions are solvated. Thus the chloride ion donor strength of a particular chloride is higher in phenylphosphonic dichloride than in phosphorus oxychloride and is usually non-apparent in benzoyl chloride ... 

Table 59. Relative Orders of Chloride Ion-Donor Strength (towards FeClaJ and Chloride Ion-Acceptor Strength (towards (CeHsjaCClj in Phosphorus Oxychloride and Phenylphosphonic...

The relative order of chloride ion donor strength (or of the relative extent of ionization) is similar in phosphorus oxychloride and in phenylphosphonic dichloride, the only exception being the dichlorides of zinc and mercury. The... 

The order of zinc chloride and titanium(IV) chloride found in phosphorus oxychloride is a reversal of that in phenylphosphonic dichloride, but is reciprocal in the respective solvents. The differences can be accounted for by assuming a solvent contribution, which has opposite effects toward donor and acceptor functions Zinc chloride has a higher chloride ion donor function in phenylphosphonic dichloride than in phosphorus oxychloride owing to differences in dielectric constant and in solvation by which at the same time its chloride ion acceptor function in phenylphosphonic dichloride is suppressed. 

The main reason for the reciprocity of chloride ion donor and chloride ion acceptor functions is associated with the properties of the chlorides. In each pair in Table 60 the stronger chloride ion donor is at the same time the weaker chloride ion-acceptor and vice versa irrespective of the solvent. The only exception is zinc chloride, which is stronger than mercuric chloride both as chloride ion donor and as chloride ion acceptor ... 

Table 60. Relations of Chloride Ion Donor and Chloride Ion-Acceptor Properties of Some Pairs of Chlorides in CeHsPOCh and POCI3...

It is to be expected that such compounds can easily be converted into the corresponding chloro-complexes by addition of chloride ion-donors, but such experiments have not been carried out yet in acetic anhydride. 

It has been shown that in phosphorus oxychloride, which is somewhat smaller in donor number than acetonitrile, certain chlorides are able to act as chloride ion donors to give chloronium ions. Thus phosphorus(V) chloride gives PCI4+ ions ... 

In acetonitrile vanadium(III) chloride gives VCI3 (AN)3, titanium(III) chloride o gives TiCl3(AN)3 and chromium(III) chloride CrCl3(AN)3. These complexes in acetonitrile solution were all found to act as chloride ion donors in the presence of strong chloride ion acceptors i ... 

An interesting example of the influence of the donor number on the mode of ionization of a dissolved compound is the behaviour of boron(III) chloride. It reacts both in diethyl ether and in tetrahydrofurane as a chloride ion donor towards ferric chloridei24 ... 

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