Autocomplex formation

Boron trichloride undergoes autocomplex formation by strong donors (see Sect. 4.4), but the iodide is easily ionized by interaction with pyridine 35>... 

The ionization of trimethyltin iodide by neutral donors is an example of a heterolytic fission of a covalent bond. The ionization process is more complicated if the substrate contains more than one ionizable bond, in particular, if the anions formed are capable of competing successfully with the donor molecules for coordination at the substrate. If they are successful both complex cations and complex anions are formed and this process is known as autocomplex formation or ligand disproportionation ... 

Autocomplex formation is favored when the substrate is difficult to ionize. 

In a strong donor solvent the substrate tends toward autocomplex formation if it is not easily ionized. On the other hand, a substrate which is easily ionized will not tend to give autocomplex ions in a strong donor solvent. 

Frequently, adduct formation, ionization, and autocomplex formation, may occur simultaneously and to an extent which is governed by the stabilities of the resulting species. This process is also influenced by the molar ratios of donor and substrate, a factor which has frequently been ignored. 

NMR measurements, spectrophotometric, kinetic, potentiometric, polaro-graphic, and conductometric investigations, are helpful in elucidating the various types of coordination in solution. Conductometric titration in a coordinating inert medium of reasonable dielectric constant has proved to be very useful for obtaining indications about the superposition of autocomplex formation, adduct formation and ionization. 

Autocomplex formation of tin (IV) iodide is indicated by the conductometric and spectrophotometric measurements 88h Tin (IV) iodide gives a yellow nonconducting solution in nitrobenzene. The solution turns red upon addition of a donor with strong donors, such as TBP, DMF, DMSO, or HMPA, the color change occurs on addition of the first drop with weak donors considerable amounts may be necessary. The spectra show the presence of hexaiodostannate. At the same time the solution becomes conducting. The comparison of the conductivities at a molar ratio D Snl4 v = 3 shows a relationship to the donicities of the neutral donors in the order... 

The extent of autocomplex formation depends on both, the nature and the quantity of the donor added. For a given molar ratio the autocomplex formation is decreased by. an increase in donicity, since the iodide ions are less successful in competing with an excess of donor molecules. At the same time the donor molecules replace iodide coordination in hexaiodostannate ... 

This mode of autocomplex formation is observed to a slight extent in a solution of iron (III) chloride in nitrobenzene. The low conductivity of this solution is increased according to the amount and donicity of the donor added (Fig. 12). 

HMPA gives, however, poorly conducting solutions 89 In the course of the conductometric titration of FeCl3 with HMPA in nitrobenzene a conductivity maximum is observed at a molar ratio HMPA FeCl3 = 1 2 and [FeCl4 ] ions are present at this composition of the solution. It is likely that the complex cation which is simultaneously produced by autocomplex formation may contain coordinated nitrobenzene molecules ... 

The conductivity curve is analogous to that of the system HMPA-FeCl3 and the conductivities are lower than those in the presence of PhjPOCl, which is a weaker donor than Ph3PO (Fig. 13). This observation is interpreted by analogy to the HMPA-FeCl3 system. The autocomplex formation... 

With the exception of HMPA strong neutral donors give non-conducting solutions up to a molar ratio D SbCls = 1 1 further addition of the donor increases the conductivity apparently due to autocomplex formation ... 

The final conductivity of the Ph3 PO system is lower than that of the Ph2POCl system, although the former is known to have a higher donicity than the latter. Cryoscopic measurements in the Ph2POCl system reveal that the number of particles is increased by the ionization process and thus autocomplex formation can be excluded. The following mode of ion formation has been suggested ... 

In the HMPA-SbCls system the conductivity curve is similar to that in the HMPA-SnI4 system. Since SbCl5 is much more difficult to ionize, autocomplex formation is more likely than simple ionization ... 

This observation suggests that the ionization process is endothermic. Conductometric titrations of the trihalides with HMPA in 1,2-dichloroethane suggest that at low D MX3 ratios some autocomplex formation may occur. At a molar ratio of 1 1 inflections are found indicating that the mode of the ionic equilibrium is essentially changed, apparently to that of simple ionization (Fig. 15)... 

Metal carbonyls are subject to autocomplex formation in the presence of strong donor molecules 94 98>. Besides the cation which is coordinated by donor molecules, polynuclear anions are formed the latter can be degradated at higher temperatures. It may be noted that in this process of autocomplex formation changes in the oxidation numbers and thus redox reactions are involved ... 

For example, complexes with very strong EPD ligands, such as Ng ", NCS ", CN, or F may exist even in solvents of high DN such as HMPA or DMSO. In solvents of weak or medium EPD properties, complex formation is essentially quantitative. On the other hand, bromo and iodo complexes usually exist only in weak EPD solvents, such as NM, PDC, or AN, and are completely ionized in solvents such as DMF, DMSO, or HMPA. The stabilities of chloro complexes are somewhat higher in the respective solvents. According to Table VII the chloride ion has an EPD strength similar to that of DMF or DMSO. Consequently chloro complexes in these solvents (compare Table IV) are ionized to some extent, sometimes with autocomplex formation. 

A third possibility is that [1] is followed simultaneously by both [2] and [4] when reaction [4] may provide X units, which may be consumed by reaction [2]. This is likely to occur when the donor properties of the donor solvent and competitive ligand are not vastly different. This type of ionization is termed autocomplex-formation and is known to occur in many systems. 

While the formation of an anionic complex is supported by a low donor number of the solvent and by high donor properties of X, the formation of solvated cations (ionization) is favoured by a high donor number of the solvent. Autocomplex formation will be expected to occur, when the donor properties of solvent and anionic ligand are similar. 

Examples of autocomplex formation can be seen from Table 10, where A (= anionic complex) denotes the ready formation of complex anions in the presence of the competitive anionic ligand X. ... 

Similar results are found for iron (III) compounds. While FeCls undergoes autocomplex formation in triethylphosphate (DNsbCh = 23) it is simply ionized in a solvent of higher DNsbCh, such as dimethyl sulphoxide 60. Ferric chloride prefers to form tetrachloroferrate in a solvent of much lower donor number, for example phosphorus oxychloride (DNsbCU = H)- With a stronger competitive ligand, such as azide, autocomplex formation is found in dimethyl sulphoxide, where with the stronger donating cyanide ion anionic cyanocomplexes are easily formed . ... 

The data are interpreted by normal mobilities of the cations and thus do not support the conclusions drawn from transport number measurements82j for which autocomplex formation in the system AICI3—POCI3 is more likely to be responsible. 

The same conclusions are reached from electrolysis experiments in concentrated solutions . Meek and Drago regard the presence of [FeCU]- ions in dilute solutions as due to the occurrance of autocomplex formation as has been found to occur in solutions of ferric chloride in trimethylphosphate 4 e.g. ... 

A further contribution in assisting autocomplex formation is provided by the high dielectric constant of the solvent. 

In the vanadyl systems the species existing in PDC-solutions appear to have distorted octahedral structures . The highest chloride-coordinated ion is [VOCU] -, which may be present to some extent in the absence of excess chloride ions, due to some degree of autocomplex formation by vanadium oxydichloride . Likewise the ultimate azidocomplex seems to have the composition [VO(N3)4]2-. It has been concluded from the spectra that all vanadyl species in propanediol-1,2-carbonate are of lower symmetries than the corresponding species in acetonitrile or in dimethyl sulphoxide, and this has been ascribed to stronger 7r-bonding contributions to the V = 0 bond in propanediol-1,2-carbonate . 

Spectrophotometric studies have been made on the ligand exchange between the hydrated nickel(II) ion and bromide ions . In those systems which were, however, not anhydrous the species [NiBr]+, NiBr2, [NiBra] and [NiBr4] were detected nickel(II) bromide was observed to have a low stability, thus indicating considerable autocomplex formation . ... 

SnCU, TiCU, SO3 and SbCls were found to give conducting solutions possibly due to autocomplex formation equilibria, in ethyl acetate (D), such as 2D. SbCls [D2SbCl4]+ + [SbCle]"... 

In the respective chloro-systems tetrahedral MCl2(TMP)2, [MCl3(TMP)] and [MCU]"" units were detected for cobalt(II) and nickel(II) and it appears that autocomplex formation occurs to a certain degree, when the dichlorides are dissolved in trimethyl phosphate . It may be noted that [CoCl(TMP)5]+ has not been detected in TMP, while both [CoBr(TMP)5]+ and [CoN3(TMP)5]+ are readily formed. Higher azide-coordinated species of cobalt(II) are tetrahedral. [Co(N3)4] is practically undissociated in TMP while in acetonitrile dissociation takes place s . In the nickel(II) azidosystem octahedral hexaazidonickelate [Ni(N3)6] appears to be present according to spectral evidence but potentiometric and conductometric results indicate the formation of Ni(N3)2 and [Ni(N3)4]2- which also seem to be hexacoordinated . 

The vanadyl ion gives with chloride and azide ions both neutral compounds and anionic species , the latter being presumably [VOCl4] and [VO(N3)5] "" respectively. Autocomplex formation seems to occur in these systems . 

In the ferric chloride systems the tetrahedral species [FeCl2(TMP)2]+, FeCl3(TMP) and [FeCU]" are present and it appears that FeCls dissolved in trimethyl phosphate undergoes autocomplex formation" 127 is known to occur in triethyl phosphatei s. With azide ions as competitive ligands octahedral units are produced" . 

The number of coordination forms found in trimethyl phosphate within a particular system is smaller than in acetonitrile or propanediol-1,2-carbonate. Some of the lower X -coordinated species show high kinetic stabilities. The tendency to form solvate bonds is reflected in autocomplex formation for many systems and in the ionization of manganese(II) bromide and nickel(II) iodide. 

Ferric chloride which is known to undergo autocomplex formation in trimethyl phosphate, but although being extensively ionized in TBP, the halide remains undissociated owing to the low dielectric constant of this solvent ... 

Autocomplex formation is even more evident in the nickel(II) and copper(II) chloro-systems, where only [MC1]+ and [MCl3] were detected O ... 

Since iV, iV -dimethylacetamide has more favourable steric properties than tributyl phosphate, ferric chloride is converted into tetrachloroferrate(III) in the presence of chloride ions in A7, iV -dimethylacetamide ferric chloride also undergoes considerable autocomplex formation in the pure solvent i ... 

In the chloro-system both tetrahedral C0CI2 and [C0CI4]— have been found and the occurrence of autocomplex formation is indicated. These findings in solution are also in accordance with the formulation of the solid solvate as [Co(DMSO)6][CoCl4]. The analogous coordination forms are found also in the azide systems. The... 

Even nickel(II) chloride undergoes ionization, 68 nickel(II) azide undergoes autocomplex formation o Although four azide-units are coordinated at nickel(II) in the anionic azido complex the spectra suggest a hexacoordinated species apparently arising from additional solvent coordination at the apices of the octahedron. 

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