EPD/EPA complexes

When tetrachloromethane solutions of yellow chloranil and colourless hexamethyl-benzene are mixed, an intensely red solution is formed (Imax = 517 nm [50]). This is due to the formation of a complex between the two components, and is only one example of a large number of so-called electron-pair donorjelectron-pair acceptor complexes (EPDjEPA complexes) It is generally accepted that the characteristic long-wavelength absorptions of these EPD/EPA complexes are associated with an electron transfer from the donor to the acceptor molecule. Mulliken termed these absorptions charge-transfer (CT) absorptions [51]. 

No general agreement exists as to the relative importance of the different inter-molecular forces in making up the EPD/EPA complexes. According to Mulliken s VB description of weak EPD/EPA complexes, the electronic ground state can be considered as a hybrid of two limiting structures a) and b) in Fig. 2-5. 

Fig. 2-5. Formation and optical excitation of an EPD/EPA complex between donor D and acceptor A (the predominating mesomeric structure in the ground and excited states is underlined).

Sulfur dioxide is a r-electron-pair acceptor. The standard explanation for the strong ionizing power of SO2 is the formation of an EPD—EPA complex between the halide anion and the sulfur dioxide molecules [148], Table 2-11 summarizes some of the available data concerning the comparative efficiencies of various solvents in promoting the ionization of chloro-triphenylmethane [150],... 

Formally analogous to Eq. (4-29), Eq. (4-30) describes in a simplified manner the reaction between a Lewis acid A and a Lewis base B , via tight ion pairs (which can sometimes be considered as EPD/EPA complexes), to give the covalent ionogen A—B [cf. also Eq. (2-13) in Section 2.6],... 

The solvent-induced change in rate is, however, much larger than expected from the relatively small difference in polarity between nitromethane and hexamethylphos-phoric triamide. This, together with the correlation between rate decrease and increase in the solvent donor number DN cf. Table 2-3 in Section 2.2.6), suggests that specific solvation and stabilization of the diazonium ion by EPD solvents play a dominant role in the reaction (5-27). Very likely, formation of an EPD/EPA complex between the reactants in a rapid preequilibrium step precedes the rate-controlHng first step [504, 792],... 

A strongly solvent-dependent electrophilic reaction is the addition of halogens to alkenes [79-81] and alkynes [81a]. In a rapid equilibrium, a loose transitory EPD/EPA complex (1 1) between halogen and alkene is formed [512]. This is followed by the ratedetermining step, which involves an SNl-hke unimolecular ionization to form a halo-nium intermediate which can be either symmetrical or unsymmetrical. This then reacts with a nucleophile Nu to give the products cf. Eq. (5-29). 

A second limitation of the Hughes-Ingold theory concerns the fact that the solvent is treated as dielectric continuum, characterized by one of the following its relative permittivity, e, the dipole moment, fi, or by its electrostatic factor, EF, defined as the product of and [27]. The term solvent polarity refers then to the ability of a solvent to interact electrostatically with solute molecules. It should be remembered, however, that solvents can also interact with solute molecules through specific inter-molecular forces like hydrogen bonding or EPD/EPA complexation cf. Section 2.2). For example, specific solvation of anionic solutes by pro tic solvents may reduce their nucleophilic reactivity, whereas in dipolar aprotic solvents solvation of anions is less,... 

Not only intramolecularly ionic compounds such as dipolar meropolymethine dyes, but also EPD/EPA complexes [cf. Section 2.2.6) with an intermolecular charge-transfer (CT) absorption can exhibit a pronounced solvatoehromism. The CT transition also involves ground and excited states with very different dipole moments. This suggests that the CT absorption band should exhibit marked solvent polarity effects [17, 63, 64]. 

Less pronounced, but nevertheless significant solvent shifts of the CT band are observed for EPD/EPA complexes, if the ground state is not ionic and the excited state is ionic An example is the CT absorption band of the acenaphthene/3,5-... 

Charge transfer forces involve the movement of electrons or protons from one molecule to another. Electron pair donor - electron pair acceptor complexes (EPD-EPA) result from the donation of a pair of electrons giving rise to electrostatic attraction between two charged species. The difference between this type of bond and a normal chemical bond is that both bonding electrons are derived from the same molecule (the EPD), the role of the EPA being to provide an empty orbital. It is important not to confuse the EPD-EPA complex with ion pair formation resulting from proton transfer [34],... 

More recent interesting examples of r-EPD/ r-EPA complexes can be found in references [335, 336] and of r-EPD/t)-EPA complexes (i.e. r/cation interactions) in references [337, 338]. For the synthesis of the first free, non-coordinated silyl cation in solution [i.e. trimesitylsilylium tetrakis(pentafluorophenyl)borate], the careful selection of a non-coordinating solvent, which nevertheless dissolves educts and product, was of crucial importance. Only with arenes as weak EPD solvents, bulky substituents around the silicon atom, and a weak nucleophiUc anion, was the synthesis of (Mes)3Si" (FsC6)4B in solution possible [338]. 

Another description of EPD/EPA interactions, particularly useful for strong complexes, is based on the coordinative interaction between Lewis bases or nucleophiles (as EPD) and Lewis acids or electrophiles (as EPA) [53, 58], The intermolecular bonding is seen not as a hybrid of electrostatic and charge-transfer forces, but as one of electrostatic and covalent ones. The interaction of the acceptor A with the electron pair of the donor D is a result of an overlap of the orbitals of the two molecules consequently, a finite electron density is created between the two partners according to Eq. (2-9). 

The HSAB behaviour is dependent on the medium in which EPD/EPA reactions are carried out. For example, the order of stability of complexes of metal ions with halide ions in the gas phase is F > Cl > Br > I , which makes all metal ions appear hard in the gas phase. However, in aqueous solution, the stability order is reversed to F < Cl < Br < 1 for those metal ions classified as soft [69],... 

In other words, whether or not an Sn2 reaction has a tight or loose activated complex will not only depend upon the nature of the reactants Y and R-X, in solution it will also be affected by the nature of the solvent. Better solvation of the activated complex of a type II Sn2 reaction by solvents with improved EPD/EPA properties will lead to a loosening of the activated complex. Transferring this activated complex from solution to the gas phase, with subsequent loss of the charge-separation stabilizing solvation, will therefore increase its tightness cf. also [499]. 

Nevertheless, there are some examples known with larger, although still moderate, solvent effects [124, 125, 129, 538-540]. Over a range of solvents from o-xylene to trichloromethane, the reaction rates for the addition of tetracyanoethene to anthracene have been found to increase by a factor of 70 [125]. In the case of the reaction between cyclopentadiene and acrolein, changing the solvent from ethyl acetate to acetic acid causes a 35-fold acceleration in rate [129]. A strongly dipolar activated complex is unlikely, as reflected by this small sensitivity to solvent. These data are more consistent with the following mechanism first the diene and dienophile form an EPD/EPA com-... 

Deviations from the E1/2-DN curves may occur with EPD molecules which can coordinate in different ways. For instance, nitrobenzene coordinates towards SbCls via an oxygen atom of the nitro group, while towards a soft EPA cation coordination via the aromatic ring appears possible. In the latter case E1/2 values would be determined not by the donicity (97) (measured towards SbCls) but by the r-donor properties of nitrobenzene. Studies of the reduction of Ag+ at the rotating platinum electrode in various EPD solvents indicate that nitrobenzene behaves as a stronger EPD than expected on basis of its donicity 39). In fact, Ag+ forms complexes with various aromatic compounds 46). Specific EPD—EPA interactions were observed in the reduction of Cu+, Ag+... 

The second stabilizing rule suggests that the anion, or more generally speaking, the EPD unit, may be the coordination center in a complex compound2). The EPA unit is consequently considered the ligand. 

This ion is readily hydrated since, in agreement with the functional principle, the oxygen atoms are capable of developing the EPD function to form outer-sphere complexes with water functioning as EPA ... 

In pentacyano complexes of cobalt(III) the net charge at the coordination center is considerably decreased by the strong coordinate bonds of the five cyano groups. Thus the EPA properties of cobalt(III) are considerably lower in this complex unit, but further stabilization may still be effected by a sixth EPD ligand of high donicity like ammonia. By the reaction... 

Reducing this complex to [Co(CN)5]3 decreases the EPA properties of cobalt so that coordination of the sixth EPD ligand is not favored. The net charge at the cobalt (I I) in this complex ion is so low that cobalt(II) hardly acts as EPA at all and prefers to coordinate a radical ... 

Further reduction to cobalt (I) further increases the electron population of the coordination center and the radical-bonding properties of cobalt are no longer favored. Instead, the EPD properties that prevail at the coordination center allow coordination by EPA units according to the second stabilizing rule the complex ion is stabilized ) as a hydrido complex ... 

The situation becomes more complicated if a given ionization reaction is studied in solvents that differ both in their EPD and EPA properties. This may be illustrated for the complex formation between Co and Cl ions. Qualitatively, stabilities of cobalt-chloro complexes usually decrease with increasing EPD strength of the solvent (25, 26). Quantitative measurements reveal, however, a number of irregularities which cannot be understood by considering the differences in solvent donicities. Accurate thermodynamic data have recently been determined for the reaction... 

In this reaction an anionic leaving group is replaced by a neutral donor molecule. Consequently, if relative EPD strengths of both neutral and anionic EPD units are known, it should be possible to make at least qualitative predictions about the ionizability of substrates A—Bj, A—Bg,. .. in various EPD solvents. Approximate values of relative EPD strength of halide, pseudohalide, and neutral EPD units have been determined using vanadyl(IV)acetylacetonate, VO(acac)2, as reference EPA (46) and are listed in Table VII. These values have proved useful for the interpretation of numerous reactions involving complex formation between transition metal ions and halide or pseudohalide ions... 

---