18 2 Solute-Solvent Interactions The approximate quantum mechanical description of proton states by linear combination of these protomeric structures has been called protomerism(symbol p)[323 324.It seems to be applicable to hydrogen bond systems in which a proton transfer may occur between two potential minima of equal depth [323.3241. Solvents containing proton-donor groups are designated protic solvents 1361 or HBD solvents [196]:solvents containing proton-acceptor groups are called HBA sol- vents [1961.The abbreviations HBD(hydrogen-bond donor)and HB4(hydrogen-bond acceptor)refer to donation and acceptance of the proton,and not to the electron pair ogen honding Solvents without proton-donor groups have been designated aprotic solvents [36 However.this term is rather misleading sir ce for example solvents commonly refe to as dipolar otic (e.g.CH3SOCH. CH:CN.CHiNO)are in fac the pd by or better still by ents 197 Typical p or HRD solve ats are wa e onia.alcohols.carboxvlic acids Typical HBA as HBD sly (eg water atohon 24 A hydr the solute acts as a HBA-bas and the solvent as a HBD. the le are reversed 19 esp ependent self-and heter amphiprot vent lcohols,amide Th ture of binary HBD/HBA solvent mixtures i largely deter mined by intermole cular hydrogen b ing between the two compone leads to pronou ced deviations from ideal solution 327小.Repre sentati example s are t ne/acetone 326 and trich orom nane/dimethy sulfoxide mixtures [327],which readily form hydrogen-bonded 1:1 and 2:1 complexes, respectively,with distinct changes in their physical properties as a consequence. Hydrogen bonding plays a particularly important role in the interactions between anions and HBD solvents.Hence,HBD solvents are good anion solvators.Due to the small size of the hydrogen atom, small anions like Fe,Cl or HO are more effec- tively solvated by such solvents than the larger onesSCNo th picrate ion 36 This is also one of the reasons why the Gibs eno hydration,AG of the halide ions decreases in the series F>Cle>Br Hydrogen bonding is of paramount importance for the stabilization and the shape of large biological molecules in living organisms(e.g.cellulose,proteins,nucleic acids) For instance,the anaesthetic properties of some halogen-containing solvents such as chloroform,halothane(CF3-CHCIBr),and methoxyflurane (CH3O-CF2-CHCl2) have been connected with their ability to hinder the formation of biologically important hydrogen bonds.This is shown in the following equilibrium 300: ClaC-H. N-H---0=C Cl3C-H-…0=C +N-H
The approximate quantum mechanical description of proton states by linear combination of these protomeric structures has been called protomerism (symbol p) [323, 324]. It seems to be applicable to hydrogen bond systems in which a proton transfer may occur between two potential minima of equal depth [323, 324]. Solvents containing proton-donor groups are designated protic solvents [36] or HBD solvents [196]; solvents containing proton-acceptor groups are called HBA solvents [196]. The abbreviations HBD (hydrogen-bond donor) and HBA (hydrogen-bond acceptor) refer to donation and acceptance of the proton, and not to the electron pair involved in hydrogen bonding. Solvents without proton-donor groups have been designated aprotic solvents [36]. However, this term is rather misleading, since, for example, solvents commonly referred to as dipolar aprotic (e.g. CH3SOCH3, CH3CN, CH3NO2) are in fact not aprotic. In reactions where strong bases are employed, their protic character can be recognized. Therefore, the term aprotic solvents should be replaced by nonhydroxylic or better still by non-HBD solvents [197]. Typical protic or HBD solvents are water, ammonia, alcohols, carboxylic acids, and primary amides. Typical HBA solvents are amines, ethers, ketones, and sulfoxides. Amphiprotic solvents can act both as HBD and as HBA solvents simultaneously (e.g. water, alcohols, amides; cf. Fig. 2-4). In type-A hydrogen bonding, the solute acts as a HBA-base and the solvent as a HBD-acid; in type-B hydrogen bonding, the roles are reversed [196]. Hydrogen bonding is responsible for the strong, temperature-dependent self- and hetero-association of amphiprotic solvents (e.g. water, alcohols, amides). The molecular structure of binary HBD/HBA solvent mixtures is largely determined by intermolecular hydrogen bonding between the two components, which usually leads to pronounced deviations from ideal solution behaviour [306, 325–327]. Representative examples are trichloromethane/acetone [326] and trichloromethane/dimethyl sulfoxide mixtures [327], which readily form hydrogen-bonded 1:1 and 2:1 complexes, respectively, with distinct changes in their physical properties as a consequence. Hydrogen bonding plays a particularly important role in the interactions between anions and HBD solvents. Hence, HBD solvents are good anion solvators. Due to the small size of the hydrogen atom, small anions like Fm, Clm, or HOm are more e¤ectively solvated by such solvents than the larger ones, e.g. I m 3 , Im, SCNm, or the picrate ion [36]. This is also one of the reasons why the Gibbs energy of hydration, DGsolv, of the halide ions decreases in the series Fm > Clm > Brm > I m [49]. Hydrogen bonding is of paramount importance for the stabilization and the shape of large biological molecules in living organisms (e.g. cellulose, proteins, nucleic acids). For instance, the anaesthetic properties of some halogen-containing solvents such as chloroform, halothane (CF3 aaCHClBr), and methoxyflurane (CH3OaaCF2 aaCHCl2) have been connected with their ability to hinder the formation of biologically important hydrogen bonds. This is shown in the following equilibrium [300]: 18 2 Solute-Solvent Interactions
2.2 Intermolecular Forces 19 Halohydrocarbon solvents containing an acidic C -H bond shift this equilibrium in favour of free or less associated species,thus perturbing the ion channels which deter mine the permeability of neuron membranes to K/Na ions in the nervous system Hydrogen bonds play a decisive role in determining the structure and dimension of these ion channels,on which this permeability depends (300]. Hydrogen-bonding also seems to be the molecular basis of sweetness.All sweet compounds seemingly have a H-bond donor and a H-bond acceptor ca.250...400 pm apart,which can form hydrogen bonds with a complementary pair on the sweet receptor in the tastebuds of the tongue [328]. The effectiveness of solvents (and solutes)as hydrogen-bond donors and/or acceptors has been studied experimentally using suitable reference compounds,com- prising representative HBDs or HBAs,in order to construct quantitative scales of sol- vent (and solute)hydrogen-bond acidity and hydrogen-bond basicity,respectively.For reviews on their construction and application to physicochemical and biochemical pro- cesses,see references [329-334]as well as Chapter 7.Scales of hydrogen-bond acidity and basicity have mostly been set up using complex formation constants,as determined in inert solvents [329-3321.For example,the strength of HBAs has been measured from the Gibbs ene y change AGug for the formation of 1:1 hydrogen-bonded complexes between all kinds of HBAs (ases)and the reference HBD 4-fuorophenol in tetra- chloromethane at 25C [331,332].Other attempts to construct scales of HBD and HBA trengths.ea.the and scale of Taft and Kamlet 1333.3341.are described in Chapter 7 Not une the pu scales derived in this way do not correspond to the common pk,and pk scales,i.e.to the normal acidity or basicity constants. 2.2.6 Electron-Pair Donor/Electron-Pair Acceptor Interactions(EPD/EPA Interactions) 150-59.59a.59h When tetrachloromethane solutions of yellow chloranil and colourless hexamethyl- n inte ation of the s s formed 517 nm (50).This is d nts and r a la led electro (EPDIEPA accepte that wav ons c ese EPD/EPA n electro to th or molecule Mulliken terme these absorptions charge-transfer(CT)abs A necessary condition or the matio of an additional bondin interactio between two valency-saturated molecules is the presence of an occupied molecular EPD/EPA comnlex ar or (EDA pler [50 ecular com- Lewis ba s)and clec air ac rs (lewis acids) of the stabilitie olexes or the charges of the components
Halohydrocarbon solvents containing an acidic CaaH bond shift this equilibrium in favour of free or less associated species, thus perturbing the ion channels which determine the permeability of neuron membranes to Kl/Nal ions in the nervous system. Hydrogen bonds play a decisive role in determining the structure and dimension of these ion channels, on which this permeability depends [300]. Hydrogen-bonding also seems to be the molecular basis of sweetness. All sweet compounds seemingly have a H-bond donor and a H-bond acceptor ca. 250 . . . 400 pm apart, which can form hydrogen bonds with a complementary pair on the sweet receptor in the tastebuds of the tongue [328]. The e¤ectiveness of solvents (and solutes) as hydrogen-bond donors and/or acceptors has been studied experimentally using suitable reference compounds, comprising representative HBDs or HBAs, in order to construct quantitative scales of solvent (and solute) hydrogen-bond acidity and hydrogen-bond basicity, respectively. For reviews on their construction and application to physicochemical and biochemical processes, see references [329–334] as well as Chapter 7. Scales of hydrogen-bond acidity and basicity have mostly been set up using complex formation constants, as determined in inert solvents [329–332]. For example, the strength of HBAs has been measured from the Gibbs energy change DGHB for the formation of 1:1 hydrogen-bonded complexes between all kinds of HBAs (bases) and the reference HBD 4-fluorophenol in tetrachloromethane at 25 C [331, 332]. Other attempts to construct scales of HBD and HBA strengths, e.g. the a and b scale of Taft and Kamlet [333, 334], are described in Chapter 7. Not unexpectedly, the pKHB scales derived in this way do not correspond to the common pKa and pKb scales, i.e. to the normal acidity or basicity constants. 2.2.6 Electron-Pair Donor/Electron-Pair Acceptor Interactions (EPD/EPA Interactions) [50–59, 59a, 59b] When tetrachloromethane solutions of yellow chloranil and colourless hexamethylbenzene are mixed, an intensely red solution is formed (lmax ¼ 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 donor/electron-pair acceptor complexes (EPD/EPA complexes)*). It is generally accepted that the characteristic longwavelength 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]. A necessary condition for the formation of an additional bonding interaction between two valency-saturated molecules is the presence of an occupied molecular * Synonyms for EPD/EPA complex are electron donor acceptor (EDA) complex [50], molecular complex [57, 58], and charge-transfer (CT) complex [51]. Since normally the term molecular complex is only used for weak complexes between neutral molecules, and the appearance of a chargetransfer absorption band does not necessarily prove the existence of a stable complex, the more general expression EPD/EPA complex, proposed by Gutmann [53], will be used here. This will comprise all complexes whose formation is due to an interaction between electron-pair donors (Lewis bases) and electron-pair acceptors (Lewis acids), irrespective of the stabilities of the complexes or the charges of the components. 2.2 Intermolecular Forces 19
20 2 Solute-Solvent Interactions orbital of sufficiently high energy in the EPD molecule,and the presence of a sufficiently low unoccupied orbital in the EPA molecule".Based on the type of orbitals involved in bonding interactions,all EPD molecules can be divided into three groups [51,53:n- -and -EPD.In the first group,the energetically highest orbital is that of the lone pair of n-electrons on the heteroatoms (R2O.RiN.R-SO).in the second it is that of the electron pair of a a-hond (R-Hal evelonronane)and in the third it is that of the pair of electrons of unsaturated and aromatic compounds (alkenes,alkylbenzenes,poly- 53, ,andπ-EPA.The lov est orbital in the first g is a vacant valenc orbital of a metal atom(Ag，certain organometallic co ounds).in the second it is a non bonding g-orbital (Br2.ICD).and in the third it is a system of -bonds (aromatic aturated c nds with electro withdra substituents such as matic is able to for nine diff nt EPD/EPA C s.The lar st number of ir ns have with lexes of -EPD/-EPA the ntion he with) nd元-EPD/a-EPA(Gf aromat arbons and referen s335 ofπ-EPD/-EPA mplexe can be found in xes(e 1337338 the ons)in ref. n- i.e is nyl]borat car selecuo olve which an produ ly with aren as w solvents ulky su on atom,and a nucleophilic anion,was the synthesis of (Mes)3Si (FsC6)4B AH,for the formation of strong EPD/EPA often used as a measure of the ond energies,lie between -42 and -188 kJ/mol (-10 to -45 kcal/mol)[59].n-EPD/t-EPA comp exes are partic lar members of this group (e.g BF3,△H= -50 kJ/mol or -11.9 kcal/mol 60).For weak complexes, usually larger than the dispersion energies but smaller than about 42 kJ/mol (10 kcal mol)[59] -EPD/-EPA complexes between neutral molecules are examples( 0.21 kJ/mol or 0...5 kcal/mol)e.a.benzene/1.3.5-trinitrobenzene (A=-8kJ/mo or -1.9 kcal/mol [57)). 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. The non-ionic structure (a)represents a state without any donor-acceptor inter- actions,in which only non-specific intermolecular forces hold D and A together.The mesomeric structure(b)characterizes a state in which an ionic bond has been formed by The fundamental difference be this epd/EPa bondine interaction and a n nal chemical eptor) vides the
orbital of su‰ciently high energy in the EPD molecule, and the presence of a su‰ciently low unoccupied orbital in the EPA molecule*). Based on the type of orbitals involved in bonding interactions, all EPD molecules can be divided into three groups [51, 53]: n-, s-, and p-EPD. In the first group, the energetically highest orbital is that of the lone pair of n-electrons on the heteroatoms (R2O, R3N, R2SO), in the second it is that of the electron pair of a s-bond (RaaHal, cyclopropane), and in the third it is that of the pair of p electrons of unsaturated and aromatic compounds (alkenes, alkylbenzenes, polycyclic aromatics). Similarly, EPA molecules can also be divided into three groups [51, 53]: v-, s-, and p-EPA. The lowest orbital in the first group is a vacant valency-orbital of a metal atom (Agl, certain organometallic compounds), in the second it is a nonbonding s-orbital (I2, Br2, ICl), and in the third it is a system of p-bonds (aromatic and unsaturated compounds with electron-withdrawing substituents such as aromatic polynitro compounds, halobenzoquinones, tetracyanoethene). Because, in principle, any donor is able to form a complex with any acceptor, there exist nine di¤erent types of EPD/EPA complexes. The largest number of investigations have been concerned with complexes of the type p-EPD/p-EPA (cf. the above-mentioned hexamethylbenzene/ chloranil complex) and p-EPD/s-EPA (cf. complexes of aromatic hydrocarbons and alkenes with halogens and interhalogens). More recent interesting examples of p-EPD/p-EPA complexes can be found in references [335, 336] and of p-EPD/v-EPA complexes (i.e. p/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 nucleophilic anion, was the synthesis of (Mes)3Siþ (F5C6)4B in solution possible [338]. The reaction enthalpies, DH, for the formation of strong EPD/EPA complexes, often used as a measure of the bond energies, lie between 42 and 188 kJ/mol (10 to 45 kcal/mol) [59]. n-EPD/v-EPA complexes are particular members of this group (e.g. Et2OaaBF3, DH ¼ 50 kJ/mol or 11.9 kcal/mol [60]). For weak complexes, DH is usually larger than the dispersion energies but smaller than about 42 kJ/mol (10 kcal/ mol) [59]. p-EPD/p-EPA complexes between neutral molecules are examples (DH ¼ 0 ... 21 kJ/mol or 0 ... 5 kcal/mol), e.g. benzene/1,3,5-trinitrobenzene (DH ¼ 8 kJ/mol or 1.9 kcal/mol [57]). No general agreement exists as to the relative importance of the di¤erent intermolecular 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. The non-ionic structure (a) represents a state without any donor-acceptor interactions, in which only non-specific intermolecular forces hold D and A together. The mesomeric structure (b) characterizes a state in which an ionic bond has been formed by * The fundamental di¤erence between this EPD/EPA bonding interaction and a normal chemical bond is that in an ordinary chemical bond each atom supplies one electron to the bond, whereas in EPD/EPA bonding one molecule (the donor) supplies the pair of electrons, while the second molecule (the acceptor) provides the vacant molecular orbital. 20 2 Solute-Solvent Interactions
2.2 Intermolecular Forces 21 D·A=DA一0PA91hLD-A+9A⊙ to) (bj h me即ha cceptor A (the predominating mesomene structure I d states is transfer of an electron from D to A.This electron transfer will be easier the lower the ionization potential of the donor [61,63],and the higher the electron affinity of the acceptor [62,63].The ionic limiting structure(b)is relatively energy-rich and contributes only slightly to the ground state.Nevertheless,this small contribution is sufficient in establishing an extra bonding interaction in addition to the non-specific van der Waals forces.However,subsequent investigations showed that these charge-transfer forces are weaker than was previously believed,and that the classical van der Waals forces (including electrostatic forces)suffice in explaining the stabilities of EPD/EPA com- plexes [59,64,198].The relative importance of contributions from the electrostatic and charge-transfer forces in the ground state of EPD/EPA complexes has been studied by many authors.For a review,see reference [183;Vol.1,p.6ff.].It seems that both elec trostatic and charge-transfer interactions are important in the ground state of EPD/EPA complexes.Their relative contribution,however,varies widely in different EPD/EPA complexes 183]. 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 electro tatic and covalent ones.The interaction of the tor A with the electron pair of the donor Disa 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). D:·AD-A (2-9) Hence,the structure D isa A can b cribed asa covalent one and the EPD/EPA interactior acid/ 65 and r-EPD);alco hols,ethe ar onors (-EPD) s,car and Mors ide nors(a-EPD and anti PA as are h logens and mixe gens -EPA),anc sulfur dio le(-EPA).In principl all solvents are amphoter in this respect,i onor(nucleophile)and an accepto r (electrophile)s Itaneously.Fo water ca act as a donor (by means of the oxygen atom as well asas ar acceptor(by forming hydrogen bonds).This is one of the reasons for the exceptional importance of water as a solvent. n-Donor s are particularly important for the solvation of cations.Exam ples are hexamethylphosphoric triamide,pyridine,dimethyl sulfoxide,N,N-dimethyl formamide,acetone,methanol,and water.Their specific EPD properties make them excellent cation solvators,and they are,therefore,good solvents for salts.They are
transfer of an electron from D to A. This electron transfer will be easier the lower the ionization potential of the donor [61, 63], and the higher the electron a‰nity of the acceptor [62, 63]. The ionic limiting structure (b) is relatively energy-rich and contributes only slightly to the ground state. Nevertheless, this small contribution is su‰cient in establishing an extra bonding interaction in addition to the non-specific van der Waals forces. However, subsequent investigations showed that these charge-transfer forces are weaker than was previously believed, and that the classical van der Waals forces (including electrostatic forces) su‰ce in explaining the stabilities of EPD/EPA complexes [59, 64, 198]. The relative importance of contributions from the electrostatic and charge-transfer forces in the ground state of EPD/EPA complexes has been studied by many authors. For a review, see reference [183; Vol. 1, p. 6¤.]. It seems that both electrostatic and charge-transfer interactions are important in the ground state of EPD/EPA complexes. Their relative contribution, however, varies widely in di¤erent EPD/EPA complexes [183]. 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). ð2-9Þ Hence, the structure DlaaAm is a covalent one and the EPD/EPA interaction between D and A can be described as a Lewis acid/base interaction [65]. Of the solvents, aromatic and olefinic hydrocarbons are p-donors (p-EPD); alcohols, ethers, amines, carboxamides, nitriles, ketones, sulfoxides and N- and P-oxides are n-donors (n-EPD), and haloalkanes are s-donors (s-EPD). Boron and antimony trihalides are acceptor solvents (v-EPA), as are halogens and mixed halogens (s-EPA), and liquid sulfur dioxide (p-EPA). In principle, all solvents are amphoteric in this respect, i.e. they may act as a donor (nucleophile) and an acceptor (electrophile) simultaneously. For example, water can act as a donor (by means of the oxygen atom) as well as as an acceptor (by forming hydrogen bonds). This is one of the reasons for the exceptional importance of water as a solvent. n-Donor solvents are particularly important for the solvation of cations. Examples are hexamethylphosphoric triamide, pyridine, dimethyl sulfoxide, N,N-dimethylformamide, acetone, methanol, and water. Their specific EPD properties make them excellent cation solvators, and they are, therefore, good solvents for salts. They are 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). 2.2 Intermolecular Forces 21
22 2 Solute-Solvent Interactions also known as coordinating solvents66.The majority of inorganic reactions are carried out in coordinating solvents. An empirical semiquantitative measure of the nucleophilic properties of EPD sol vents is provided by the so-called donor number DN (or donicity)of Gutmann [53.67 (cf.also Section 7.2).This donor number has been defined as the negative AH values for 1.1 adduct formation between antimony pentachloride and electron pair donor solvents (D)in dilute solution in the non-coordinating solvent 1,2-dichloroethane,according to Eq.(2-10) (2-10) Solvent Donor Number DN =-AHp-stci/(kcal.mol-1) The linear relationship between and the logarithm of the corre- sponding equilibrium constant (Ig Kp-sbci,)shows that the entropy contributions are equal for all the studied acceptor/donor solvent reactions.Therefore,one is justified in considering the donor numbers as semiquantitative expressions for the degree of coor dination interaction between EPD solvents and antimony pentachloride.Antimony pentachloride is regarded as an acceptor on the borderline between hard and soft Lewis acids.A list of organic solvents ordered according to increasing donicity is given in Table 2-3.From this it is seen that,for example,nitromethane and acetonitrile are weak donor solvents,whereas dimethyl sulfoxide and triethylamine are very strong donors The higher the donor number,the stronger the interaction between solvent and acceptor Unfortunately,donor numbers have been defined in the non-SI unit kcal.mol- Marcus has presented a scale of dimensionless,normalized donor numbers DNN,which are defined according to DNN=DN/(388 kcal.mol-l)12001.The non-donor solven 1,2-dichloroethane (DN DNN =0.0)and the strong donor solvent hexamethyl phosphoric triamide(HMPT:DN =38.8 kcal.mol-;DNN =1.0)have been used to define the scale.Although solvents with higher donicity than HMPT are known(ef. Table 2-3),it is expedient to choose the solvent with the highest directly (ie.calori metrically)determined DN value so far as the second reference solvent [200**).The DNN values are included in Table 2-3. A visual estimate of the diferent donicities of EPD solvents can easily he made using the colour reaction with copper(),nickel(II),or vanadyl(IV)complexes as acceptor solutes 2041. The donor number has proven very useful in coordination chemistry.since it can be correlated with other physical obser vables for such reactions,e.g.thermodynamic ard accepor (r SC)whi thevomd The dorme given by Gutmann [67].It should be was subs measuring th
also known as coordinating solvents [66]. The majority of inorganic reactions are carried out in coordinating solvents. An empirical semiquantitative measure of the nucleophilic properties of EPD solvents is provided by the so-called donor number DN (or donicity) of Gutmann [53, 67] (cf. also Section 7.2). This donor number has been defined as the negative DH values for 1:1 adduct formation between antimony pentachloride and electron-pair donor solvents (D) in dilute solution in the non-coordinating solvent 1,2-dichloroethane, according to Eq. (2-10)*). ð2-10Þ Solvent Donor Number DN ¼ DHDaSbCl5 /(kcal mol1) The linear relationship between DHDaaSbCl5 and the logarithm of the corresponding equilibrium constant (lg KDaaSbCl5 ) shows that the entropy contributions are equal for all the studied acceptor/donor solvent reactions. Therefore, one is justified in considering the donor numbers as semiquantitative expressions for the degree of coordination interaction between EPD solvents and antimony pentachloride. Antimony pentachloride is regarded as an acceptor on the borderline between hard and soft Lewis acids. A list of organic solvents ordered according to increasing donicity is given in Table 2-3. From this it is seen that, for example, nitromethane and acetonitrile are weak donor solvents, whereas dimethyl sulfoxide and triethylamine are very strong donors. The higher the donor number, the stronger the interaction between solvent and acceptor. Unfortunately, donor numbers have been defined in the non-SI unit kcal mol1. Marcus has presented a scale of dimensionless, normalized donor numbers DN N, which are defined according to DN N ¼ DN/(38.8 kcal mol1) [200]. The non-donor solvent 1,2-dichloroethane (DN ¼ DN N ¼ 0:0) and the strong donor solvent hexamethylphosphoric triamide (HMPT: DN ¼ 38:8 kcal mol1; DN N ¼ 1:0) have been used to define the scale. Although solvents with higher donicity than HMPT are known (cf. Table 2-3), it is expedient to choose the solvent with the highest directly (i.e. calorimetrically) determined DN value so far as the second reference solvent [200]**). The DN N values are included in Table 2-3. A visual estimate of the di¤erent donicities of EPD solvents can easily be made using the colour reaction with copper(II ), nickel(II ), or vanadyl(IV ) complexes as acceptor solutes [204]. The donor number has proven very useful in coordination chemistry, since it can be correlated with other physical observables for such reactions, e.g. thermodynamic * An analogous approach was first used by Lindqvist and Zackrisson [67a]. The authors established a series of EPD solvents calorimetrically, based on their increasing donor capacities relative to a standard acceptor (SbCl5 or SnCl4) with which the given donor was combined in 1,2- dichloroethane. ** The donor number of 38.8 kcal mol1 for HMPT was given by Gutmann [67]. It should be mentioned, however, that a much higher DN value of 50.3 kcal mol1 was subsequently measured for this solvent by Bollinger et al. [214]. This shows that serious problems arise in measuring the Lewis basicity of this EPD solvent towards SbCl5. 22 2 Solute-Solvent Interactions