摘要:
1979 1143Thermochemistry of Alkali-metal Hexachloro- and Hexabromo-tung-states(iv) and -rhenates(iv)By John Burgess, S. James Cartwright, Ian Haigh, Raymond D. Peacock, and Paul Taylor, DepartmentH. Donald B. Jenkins and Kenneth F. Pratt, Department of Molecular Sciences, University of Warwick,Enthalpies of hydrolysis or of oxidative hydrolysis are reported for the hexahalogenometallates(lv) K,[WC16],K,[WBr6], Rb,[WBr,], Cs,[WBr,], K,[ReCI,], and K2[ReBr6], and for ReCI, and ReBr,. Enthalpies of formationof these compounds have been calculated from these hydrolysis results. Halide-ion affinities of WCI,, WBr,,ReCI,, and ReBr,, and two-electron affinities of wcl6 and WBr,, have been estimated from these and other thermo-chemical results. These estimates have been made using the new direct-minimisation method of Jenkins andPratt based on the Huggins and Mayer potential to assign the lattice energies of the hexahalogenometallate saltsinvolved.of Chemistry, University of Leicester, Leicester LE1 7RHCoventry CV4 7ALENTHALPIES of reaction of tetrahalides with halide ions[equation (l)] can be estimated from the enthalpies offormation of the reactants and products.It is oftenpossible to obtain enthalpies of formation of tetrahalidesand of halogeno-anions from calorimetric determinationsof the enthalpies of hydrolysis of appropriate compounds ;enthalpies of formation of halide ions are well estab-lished. In the present paper we report enthalpies of(oxidative) hydrolysis of several salts of hexahalogeno-metallates(Iv), specifically of K,[WCl,], K,[WBr,],Rb,[WBr,], Cs,[WBr6], K,[ReCl,], and K,[ReBr,], andof rhenium tetrachloride and rhenium tetrabromide.From these results and published thermochemical datawe have estimated halide-ion affinities of the respectivetetrahalides, AH2= [equations (2) and (3), X = C1 or Br1.tWhen the enthalpies of formation both of a hexa-halogenometallate and of the parent hexahalide areknown, then the n-electron affinity, AH,,, of the hexa-halide can be deduced [equation (4)] .t Since the enthal-MX, $- He- [MX6]%-; AHne (4)pies of formation of the hexachloride and hexabromide oftungsten are known, we have been able to estimate two-electron affinities of these compounds [equation (4) ;M = W, X = C1 or Br, n == 21 from our thermochemicalresults.For rhenium hexachloride, whose existencerequires confirmation, we derive an estimate for thetwo-electron affinity based on an assumed value for theenthalpy of formation of this compound.t Strictly these quantities should refer to gas-phase species.However, it is not always possible, due to lack of knowledge onenthalpies of sublimation or of vaporisation, to calculate halide-ion affinities and electron affinities for the gas phase. In suchcases these affinities have to be referred to the solid or liquidstate. Indeed, for the practising chemist, the halide-ion affinityof a solid or liquid halide may be a more useful guide than thethermodynamically ideal gas-phase value. We shall indicate inthe text the phases of the species involved in the halide-ion andelectron affinities we quote.EXPERIMENTALPreparations.-The compounds K2[WC16],2 K,[WBI-,],~Rb,[WBr,] ,2 Cs,[WBr,] ,, K,[ReC1,],3 and K,[ReBr,] wereprepared by published methods.Their purity was estab-lished by standard analytical procedures (see below), andtheir lattice constants determined or checked from theirX-ray diffraction patterns (Debye-Scherrer) .Samples of rhenium tetrachloride were prepared by thereaction of rhenium metal with antimony pentachlorideor by the reaction of rhenium pentachloride, itself preparedfrom rhenium metal and ~hlorine,~ with antimony tri-chloride., Rhenium tetrabromide was prepared by thedissolution of rhenium dioxide in concentrated hydrobromicacid, followed by evaporation to small volume.The result-ing concentrated solution was dried in a desiccator overpotassium hydroxide and phosphorus pentaoxide.The compositions of the hexahalogenometallates and ofthe tetrahalides were confirmed by the following analyticalmethods. Tungsten was determined by precipitation ofthe cinchonine-tannin complex and ignition of this t otungsten trioxide.' Rhenium was estimated gravimetric-ally as tetraplienylarsonium tetraoxorhenate(v1x) .8 Chlor-ide and bromide were determined by precipitation of theirsilver ~ a l t s . ~Calorimetry.-Two calorimeters were employed, the firsta modified version of a calorimeter described by Myers andBrady,lo the second a standard LKB model 8700 precisioncalorimeter.Either calorimeter could be incorporated intoa Wheatstone bridge circuit, constructed in these labora-tories, employing a Kipp-Zonen BD5 recorder as monitor.Both calorimeters were operated a t 298.2 K. Both calori-metric assemblies were checked periodically against thewell established enthalpies of solution of potassium chloridein water l1 or of neutralisation of tris(hydroxymethy1)-methylamine .I2 The number of independently preparedsamples of each compound, and the hydrolysis conditions inthe calorimeters, are indicated in Table 1. Sample weightsof 0.1-0.2 g were hydrolysed in 100 or 150 cm3 of solution.It was impossible to obtain satisfactory Calorimetricresults for the rubidium and caesium salts of the hexa-halogenorhenate(xv) anions, since these salts were tooinsoluble in the reaction mixture for the hydrolysis reactionto proceed to completion within an acceptable time.Results obtained for the oxidative hydrolysis of K,[ReC16]and of K,[ReBr,] to tetraoxorhenate(vI1) were much lesssatisfactory than those obtained for hydrolysis to rheniumdioxide ; the former have therefore been discarded1144 J.C.S.DaltonTABLE 1No. ofsamples23333232Thermochemical results for hydrolysesNo. ofhydro1 yses6999916913[NaOH]mol dm-30.860.860.860.862222Mean AHh,a- 738-778- 735-714- 264- 287- 400- 275o akJ mol-179675101517AHf-1 380’- 1 066-1 107-1 142- 1 3356- 1 038- 360- 303a Standard deviations, from which standard errors of the means and their confidence limits can be calculated by standardprocedures (see, for example, E.S. Swinbourne, ‘Analysis of Kinetic Data,’ Nelson, London, 1971, pp. 7-9). Containing 1.5%sodium hypochlorite. cf. AHf(K,WCl,,c) = - 1 359 kJ mol-l by bomb calorimetry (D. V. Korol’kov and G. N. Kudryashova,Russ. J . Inorg. Chem., 1970, 15, 1759). Corrected for reaction of the bromide ion released with hypochlorite, for which the stan-dard enthalpy change is -32.6 k 0.4 kJ mol-l (our measurements, cf. -33.1 kJ mol-l quoted in J. P. King and J. W. Cobble, J .Amer. Chem. Soc., 1960, 82, 2111). cf. AHt(K,ReCl,, c) = -1 331 kJ mo1-I (R. H. Busey, K. H. Gayer, R. A. Gilbert, and R. B.Bevan, J . Phys. Chem., 1966,70, 2609).RESULTS AND DISCUSSIONEnthalpies of Formation.-Our calorimetric results aresummarised in Table 1.The hydrolyses studied pro-ceeded quantitatively in alkaline or in alkaline hypo-chlorite solution, as shown in equations (5) for the hexa-halogenotungstates(Iv), (6) for the hexahalogenorhen-ates(Iv), and (7) for the rhenium tetrahalides. TheA,[WX,I(c) + [C10l-(aq) + G[OHI-(aq) - 2A+(aq) + [W0412-(aq) + 6X-(aq) +Cl-(aq) + 3H20(1) (5)K,CReX,I(c) + 4[0Hl-(aq) -Rex&) + 4[0Hl-(aq) +2K+(aq) + Re0,*2H20(c) + 6X-(aq) +2H,O(l) (6)ReO2*2H,O(c) + 4X-(aq) + 2H20(1) (7)enthalpy of formation, AHf,$ of each compound wascalculated from its measured enthalpy of hydrolysis andthe appropriate ancillary thermochemical data (Table 2).For oxidative hydrolysis of hexabromo-salts, it isnecessary to make allowance for the bromide-hypo-chlorite reaction (see footnote to Table 1).The calorimetric results for ReC1, given in Table 1were all obtained from samples prepared from thereaction of rhenium metal with SbCl,, using the frangiblebulb apparatus.Results from these same samplesobtained using the LKB calorimeter were more widelyscattered, due to technical difficulties in handling thissensitive compound. These difficulties are associatedwith the filling and weighing of the fragile sampleampoules. Nonetheless the mean enthalpy of hydro-lysis, -398 kJ mol-1, agrees well with that reported inTable 1. Samples of ReC1, prepared from ReCl, andSbC1, were impossible to purify, completely, but theirmean enthalpy of hydrolysis, -382 kJ mol-l in 2 moldm-, Na[OH], supports the Table 1 value.Enthalpies of formation of chlorides and bromides oftungsten and of rhenium are listed in Table 3.Theenthalpy of formation of ReC1, is close to that of ReC1,.It is not possible to obtain standard enthalpies of formation,AHp, from our results. However, it seems likely that our AHfvalues are very close to AHp, probably within our stated un-certainties.This indicates that the stability of ReCl,(c) is marginalwith respect to ReCl,(c). However, reasonable extra-polation of the available enthalpies of sublimationshows that ReCl,(g) is likely to be appreciably morestable than ReCl,(g). Our data are in accord with theTABLE 2Ancillary thermochemical data and sources used for thecalculation of AHf (Table 1) from calorimetric measure-ments of hydrolysis, and for subsequent estimates ofhalide and electron affinitiesa H.Ion AHt/k J mol-1 Ref.[W04l2- ( a 4 -1 073.2 a[Reo,I- (aq) -791.6 Cc1- (4 -167.1 bBr- (aq) - 121.5 b[ClOI- (as) - 107.1 d- 230.0 b- 252.2 b-251.1 b- 258.0 b- 285.8 bReO,*SH,O(c) CC.KO. T. Matsui. and L. G. Heder. Thermochim. Acta.[OH]- (aq)K+ (aq)Rb+ ( a 4cs+ (4H20(1)-1 011.71974, 10, 211‘; I. Dellien, F. M. Hall, 2nd L. G. Hepler, Chem:Rev., 1976, 76, 283. J . Chem. Thermodynamics, 1975, 7, 1 ;1976, 8, 603. R. H. Busey, K. H. Gayer, R. A. Gilbert, andR. B. Bevan, J . Phys. Chem., 1966, 70, 2609. J. E. Mc-Donald, J.P. King, and J. W. Cobble, J . Phys. Chem., 1960,64,1345; J. D. Cox, J. B. Pedley, A. Kirk, S. Seilman, and L. G.Heath, CATCH Tables, Halogen Compounds, University ofSussex, 1972.TABLE 3Enthalpies of formation of (solid) chlorides and bromidesof tungsten a and of rheniumCompound AHf/k J mol-l Compound AHf/k J rno1-IWCl, - 255 ReC1, -264WCl, ca. -450 ReC1, -361 wc1, - 510 ReC1, -372WCl, - 594ReBr, -167 ’WBr, - 146 ReBr, -303 CWBr, -315or -300WBr, - 348a Values for tungsten compounds taken from I. Dellien, F. M.Hall, and L. G. Hepler, Chem. Rev., 1976, 76, 283. J. P.King and J, W. Cobble, J . Amer. Chem. Soc., 1950, 82, 2111;NBS Technical Note 270/4, 1969. This work. J. Burgess,C. J. W. Fraser, I. Haigh, and R. D. Peacock, J.C.S.Dalton,1973, 501. This value was reported in NBS Circular 600,1952, but has been omitted from later compilations (cf. text)1979 1145known properties of these two chlorides, especially sinceReC1, is prepared in the presence of an excess of chlorine.It is possible to hazard a guess at the enthalpy of form-ation of ReCl, (see above) from the values given inTable 3. We shall use an estimated value of -350 k Jmol-l for AHf(ReCl,,C) later in this paper.ations of thermodynamic data. Comparison with datafor tetrabromides and tetrachlorides of other transitionelements (Table 4) suggests that AHf(WBr,,c) may beca.-300 kJ mol-l.Average bond-dissociation energies of transition-metaltetrachlorides are collected in Table 5, which shows theWe shall use the latter estimate.TABLE 4Enthalpies of formation (k J mol-I) of tetrachlorides and of tetrabromides of d- and f-block elements(in their standard states)TetrachloridesTiCl, -804" VCl, -569" CrCI, - 102ZrCl, -981 " NbC1, -695" MoC1, -4480" RuC1, -52ThCl, - 1 192" PaCl, -1 045f UC1, -1 0199 NpCI, -992 "HfCl, -990" TaC1, -702 * WC1, ca.-450' ReC1, -361 OsC1, -255 PtC1, - 326 bTetrabromidesTiBr, -617 5 VRr, -337 5ZrBr, -761 MoBr, -295ThBr, -950" UBr, -823 NpBr, -5502"WBr, -300h ReBr, -303 PtBr, -1598c I. Dellien, F. M. Hall, and L. G. Hepler, Chem. Rev.,1976, 76, 283. This work. NBS Circular 500, 1952. f D. Brown, personal communication. E. H. P. Cordfunke, W. Ouwelt-jes, and G. Prins, J . Chem. Thermodynamics, 1976, 8, 241.NBS Technical Note 270/5, 1971.NBS Technical Note 270/4, 1969.Estimated value (see text).TABLE 5Mean bond-dissociation energies a (k J mol-l) for tetrachlorides of transition elements (in the gas phase) bTiCl, 430 VCI, 382 CrC1, 338ZrC1, 491 NbC1, 443 MoCl, 380 RuC1, 295HfC1, 498 TaC1, 457 WCl, 410" ReC1, 362C*d osc1, 339 PtC1, 277d" Mean bond-dissociation energies (b.d.e.) have been calculated from: b.d.e. = t[- AHf(MCl,, g) + AH,,b(M) + 2AHdi8,(C1,, g)].The required thermodynamic data have been taken from NBS Technical Notes 270/4, 1969, or 270/5, 1971, unless stated other-d Enthalpies of sublimation of 170 and 180 kJ mol-1 wise.have been estimated by interpolation for the tetrachlorides of rhenium and platinum.Enthalpies of formation determined in the present investigation.TABLE 6Enthalpies of formation (k J mol-I) of alkali-metal salts of transition-metal [MC1,I2- and [MBr,]2- anionsM = Ti -1 747 5 -1 767" -1 797" -1 493" - 1 517," -1 553,"K2Wl6I R~,[Mc~,I Cs2[MC1,1 K,[MBr,l Rb,[MBr,l WMB~,I-1 612 - 1 641Zr -1 932 " -1 992"Hf - 1 957 5Nb -1 594 5 -1 619 " - 1 663 "Ta -1 648," -1 669," -1 711,"-1 707" - 1 736' -1 774=Mo -1 46gd - 1 495d -1 527W -1 359," - 1 429" -1 446 "-1 380fRe -1 335,f-1 331 g-1 065f -1 106f - 1 133f-1 036f0s -1 171"I r -1 197"Pd -1 187"Pt -1 040"5 Ref.15. S. A. Shchukarev, D. V. Korol'kova, and I. V. Vasil'kova, Russ. J . Inorg. Chem., 1964, 9, 980. V. M. TsintsiusA. I.D. V. Korol'kova and G. M. Kudryashova, Russ. J . Inorg.g R.H. Busey, K. H. Gayer, R. A. Gilbert, and R. B. Bevan, J . P h y s . Chem., 1977, 70, 2609.and E. K. Smirnova, Russ. J . Inorg. Chem., 1969, 14, 1729; E. K. Smirnova and I. V. Vasil'kova, ibid., 1967, 12, 292.Efimov and L. P. Belorukova, Russ. J . Inorg. Chem., 1967, 12, 792.Chem., 1970, 15, 1759. f This work.Rhenium tetrabromide has still not been fullycharacterised; our enthalpy value relates to the materialof the correct composition prepared as described in theExperimental section. This value for AHf(ReBr,,c) ismore exothermic than expected by comparison withother data (Tables 3 and 4), but some of these are alsosubject to some uncertainty. The situation with respectto WBr, is even less satisfactory. A value of -146 kJmol-l for AHf(WBr,,c) was given in NBS Circular 500,published in 1952, but does not appear in later compil-expected trends of values both along rows and downcolumns of the Periodic Table.Our determined enthalpies of formation of hexa-chloro- and hexabromo-tungstates( ~ v ) and -rhenates( 1v)are compared with data for other transition-metalA,[MX,] salts in Table 6.This Table shows howAHf(A,MX,) varies along rows and down columns of thePeriodic Table, as well as the effects of changing A+(A = K, Rb, or Cs) and X- (X = C1 or Br). The agree-ment of our results with those from earlier workers fo1146 J.C.S. Daltonthe standard enthalpies of formation of K,[WCl,] andof K,[ReCl,] is also apparent from the appropriateentries in this Table.Lattice Energies.-For the purposes of calculating thelattice energies required in this study we employ therecently developed direct minimisation of the Hugginsand Mayer l3 potential by Jenkins and Pratt,l4?l5 whichhas been tested for cyanides.16J7 The method isdescribed in full in ref. 15; its outline is presentedbriefly here.We write the total lattice-potential energy, UPOT, asthe sum of the electrostatic, U E L E C , dipole-dipoledispersion, Udd, dipole-quadrupole dispersion, Uqd, andUPOT = UELEC + u d d + Uqd - UIL (8)( aupoT~Mx6) ) a=a, = o (9)repulsion, UR, terms [equation (S)].Applying thecondition [equation (9); a, is the equilibrium value ofTABLE 7Coefficients for the total lattice potential, equation (1 2)UpoT(A2MX,)/k J mol-1aolA9.87510.0010.2710.5010.7010.3859.840rA ,157015611 53715761 5031 4931511A 1'2 80266238280262243275which is determined in the study, has a close parametricrelationship with p.The total lattice-potential energies of the salts con-sidered in the present work approximate closely to theparametric form of equation (12) in the region of QXvalues of chemical interest.Values of the coefficientsA , and A, of equation (12) are listed in Table 7.UPOT = A , + AiqX (12)TABLE 8Data for the estimation of halide-ion affinities of tetrahalides of tungsten and of rhenium (all values in kJ mol-1)K2[WC1,] Rb2[WC1,] cs2[wc&] K2[ReC1,] Rb,[WBr,] Cs,[WBr,] K2[ReBr,]514.5 a 494.9 b 460.0 a 514.5 494.9 a 460.0 514.5AHf(A&X,, C) -1 380" -1 42gd -1 446d -1 333" -1 107" -1 142' -1 038"AHr"(A+, 8)- 838 - 858 - 829 - 785 - 594 - 569 - 555AHf(MX62-, 8) Bl 280 266 238 280 262 243 275total -1 012 -1 023 - 977 - 942 - 738 - 703 - 693-447 -447 -447 -360 ca.-300f ca. -300f - 303-341 ' -341 " -341- 246 6 - 246 ' -246 -246 -234 ' - 234 ' -234 'ro AHf (MX,) { 13101 81 110 69 20 45 2162 80 266 238 280 262 243AHf(XtiC, AH2x ' Co(g) -5 - 25 19 275a Ref. 22. NBS Circular 500, 1952. c This work, see Table 1. D. V. Korol'kov and G. N. Kudryashova, Russ. J . Inorg.Chem., 1970, 15, 1759. ' CATCH Tables, University of Sussex, 1974. f Estimated value, see text.the cell length a ] , true for all crystal lattices, to equation(8) leads to (lo), and thence to (11).In equation (ll),We have also estimated lattice energies by approxi-mate empirical methods, either by using repulsions inalkali-metal halides as a basis, or by interpolating appro-priate values of ro in the Born-Mayer equation from theA-M distance for qx = 0 to the A-X distance for qx =a UELEC a u d d a U q d -1. Such values are close to those derived from thedirect-minimisation calculations detailed above.affinities for the tetrachlorides and tetrabromides of=( ~ ) , = , + ( ~),=,, + (a,>,=,, (lo)(11)3 Halide-ion Afinities.-In order to calculate halide-ion7(MX,2-) = pin[+, + ( 2 +&7xcj-1))*]j=l(A2MX6) * 2A+ (g) + [MX6l2-(g)QX is the charge on the halogen atoms of the complex ion[MX6I2-, p is the repulsion exponent, 4, to $3 are cal- A,[ MX61 (c)culable,14 and 7(MX,2-) is the Huggins basic radius for athe [MX,I2- anion.This radius is used within theHuggins and Mayer formalism to calculate the repulsion AHf(A2MXg.C)energy, UR, and hence the total lattice potential energy,U~OT, via equation (8). In the present work, admittinga variable p parameter would introduce complicationsand in the absence of compressibility data make theevaluation of UR difficult. The assumption that p isconstant (taking the value to be 0.345 A) within theframework of the present method is made prior to thesolution of equation (ll), and hence errors introducedby such an assumption will be minimised since 7(MX6,-),2A(c) + M(c) + 3Xz(ss) 'SCHEME 1tungsten and of rhenium [equations (2) and (3)] werequire values for AHf(MX,,-,g), AH,(X-,g), and AH,-(MX,,g). Equations (13) and (14) are generated fromthe Born-Fajans-Haber cycle (Scheme 1 ; ss = standardstate) for salts with A = K, Rb, or Cs.Equation (141979 1147UPoT(A2MX6) =2AHf(A+,g) AH~(MXG~-,~) - AHf(A&'Kpc) (13)AHf(MX62-,g) =UPoT(A2MX6) - 2AHf(Af,g) + AHf(A2MX6,c) (14)AHf(MX62-,g) = BO + B1qx (15)can be expressed in the form of (15); values of coeffi-cients B, and B, are given in Table 8. Equations (16)and (17) are now used to obtain halide-ion affinities * forMX, ; coefficients Co and C, [equation (17)] are also givenin Table 8. We now need to know values of qx, theAH2x =AHf(MX62-,g) - 2AHf(X-,g) - AHf(MX4) (16)(17)effective charge on the halogen atoms, for each of ourhexahalogenometallate(1v) complex anions.There is noself-consistent set of values for these. Kubo andNakamura18 estimated qx values for a range of hexa-halogenometallate(1v) anions from n.q.r. measure-ments. Subsequently Brown et aE.19 estimated qx for asimilar range of complexes, again from n.q.r. measure-ments, and obtained values 0.05 (or more) more negativethan Kubo and Nakamura in the cases where data wereavailable from both sources. Molecular-orbital (m.0.)calculations 2o tend to support the values of Brown et al.We therefore use their values of qcl = -0.62 for[wC16l2- and qcl = -0.56 for [ReCl6l2-, and, on thebasis of n.q.r. and m.0. evidence, estimate a value ofFrom Table 8 and qcl = -0.62,19 we find the valuesfor the two-chloride ion affinity, AHzl, for WCl, givenin equations (18)-(20).The two-chloride ion affinityfor ReC1, is given by equation (21), using a value ofqcl = -0.56.19 The values for the two-bromide ionAH2c1 (WQ, C) =qBr = -0.55 for [WBr6l2- and -0.50 for [&!Br6l2-.101 + 28Oqcl = -73 kJ mol-l (K+ salt)81 + 266qc1 = -84 kJ mol-l (Rb+ salt)110 + 238qcl = -38 kJ mol-l (Cs+ salt)68 + 280 qcl = -89 kJ mol-l (K+ salt)(18)(19)(20)(21)AH2c1 (WC4, C) =AHzc1(WC1,,c) =AHZCI (ReCl, , c) =affinity for WBr, depend strongly on the value taken forits standard enthalpy of formation. From our estimateof AHf(WBr,,c) = -300 kJ mol-l (see above), weobtain the estimates for the two-bromide ion affinityshown in equations (22) and (23).In these equationsAHzB~(WB~~,C) = 175 + 262q~, =31 kJ mol-1 (Rb+ salt)75 k J mol-l (Cs+ salt)(22)(23)AHzBr(WBr,,c) = 209 + 243q~r =* See footnote on p. 1143.we use an estimated value (see above) of -0.55 forqBr. The two-bromide ion affinity of ReBr, is given byequation (24), in which we use an estimate for q R r of-0.50 (see above).AHzBr(ReBr4,c) = 218 + 275 qBr =81 kJ mol-l (K+ salt) (24)There are two unsatisfactory features in the abovederivations of two-halide ion affinities. The first is thelack of a reliable value for the standard enthalpy offormation of WBr,. For the present we feel that it isbest to use our estimate of -300 kJ mol-l. The otherthermochemical value which causes some concern is thatfor AHfe(Cs+,g).There are two significantly differentvalues published for this in standard thermochemicalcompilations.21, 22 Neither of these are quite as expectedby extrapolation from the other alkali metals. Indeed,a value estimated from the trend for the other alkalimetals would make our values for halide-ion affinitiesfrom caesium salts [equations (20) and (23)] closer tothose from the analogous potassium and rubidium salts[equations (18), (19), and (22)]. We have accordinglychosen to give a lower weighting to the caesium-derivedresults in our final recommendations for two-halide ionaffinities of these tetrahalides of tungsten and of rhen-ium (Table 9). It is impossible to give statisticallyTABLE 9Recommended (rounded) values for two-halide-ion affinitiesof tungsten and rhenium tetrahalidesCompound k J mol-l Compound k J mo1-IAHZCll AHZBr/5080based uncertainty limits for these Table 9 values.Oursubjective judgement suggests 90% confidence limits ofthe order of A40 kJ mol-l, taking into account the diffi-culties mentioned in this paragraph and the uncertaintiesattendant on the choice of the values of qx used in theassignment of the lattice energies. The two-chlorideion affinities of tungsten and rhenium tetrachlorides arecompared with those for other tetrachlorides, both ofsp- and of d-block elements, in Table 10.Electron Afinities.-Two-electron affinities [equation(4); n = 21 can be estimated for WC1, and WBr,, andalso for ReC1, if one assumes a reasonable value for theenthalpy of formation of this elusive compound.Anappropriate thermochemical cycle is Scheme 2. Fromthe relevant data given in Tables 8 and 11, the two-electron affinities shown in Table 11 have been cal-culated.Despite the uncertainties in the estimates of electronaffinity, it is clear that the magnitude of that for WC16 isgreater than that for WBr,, and that the tentative valuefor ReCl, is more negative than that for Wcl,. Thesetwo-electron affinities are less than those for two atomsof the respective halogens; the electron affinities forchlorine and bromine atoms in the gas phase are -696and -648 kJ per two gram atoms respectively.%- 70 WBr,(c)- 90 ReBr,(c)WCl,(C)ReCl, (cI I48 J.C.S. DaltonTABLE 10Two-chloride-ion affinities (k J mol-l) of tetrahalides aGeC1,SnC1,PbC1,TiC1,ZrCl,HfC1,- 36- 76- 54- 164- 158- 259NbC1,TaCl,TeC1,MoCl,wc1, ReC1, { (4 - 90b 0sc1, { [; -6; PtC1, { (c) - 37(I Taken from ref.15 unless where otherwise stated. This work.TABLE 11are in kJ mol-l and refer to 298 KEstimation of two-electron affinities, AH2e [equation (4) ; n = 21 for hexahalides of tungsten and rhenium. All valuesSalt UPOT(&MX& C) AHt(MX6, C) AH2e(MXfj, C) AHt(MX6, g) AH2~(MX6, g)1396 - 594 -411 - 494 -5111396 - 594 - 422 - 494 - 5221389 - 594 - 376 - 494 - 461- 348 - 383 -257 - 474 1 3591359 - 348 - 348 -257 - 439K2[ReCl,I 1419 - 350 - 585 -255 - 680K2[WCl6IRb2[WC161Cs2[WC1,1Rb2 [WBr,lCs2[WBr61Using the yx values given in the text and A , and A , values from Table 7. An estimated value of 91 kJ mol-l has been usedThis is an extrapolated estimate for the enthalpy of formation of this elusive com- for the enthalpy of sublimation of WBr,.pound.An estimated value of 95 k J mol-l has been used for the enthalpy of sublimation of ReC1,.Interestingly, the electron affinity for WF, (g) is -447kJ m ~ l - l , ~ ~ in comparison with an electron affinity of-333 kJ per gram atom for fluorine (gas phase).23Single-ion Hydration Entha1pies.-The calculation oflattice energies is also important in the estimation ofsingle-ion hydration enthalpies from measurements ofenthalpies of solution of appropriate salts. Recently, weestimated single-ion hydration enthalpies for the hexa-chloro- and hexabromo-rhenate(1v) anions from enthal-pies of solution of the respective caesium25 and potas-sium 26 salts.The simple Born-Mayer equation wasused to calculate the lattice energies. We do not havesufficient data to apply the direct-minimisat ion approachto the determination of lattice energies of these caesiumsalts, but we cite above (Tables 7 and 10) values forK2[ReCl$ . Using this direct-minimisation lattice energywe derive a value of -740 kJ mol-l for the single-ionhydration energy of the hexachlororhenate(1v) anion ;the Born-Mayer lattice energy leads to a value of -827kJ mol-l. This difference arises from the differencesobtained for the lattice energies from the two approaches.We thank P. D. Robinson for some of the thermochemicalresults on K,[WCl,], Thorn Lighting Ltd.for helpfuldiscussions and co-operation, and the S.R.C. both for a grantto purchase the LKB calorimeter and for the award ofstudentships (to S. J. C., I. H., K. F. P., and P. T.).[7/1216 Received, 11th JuZy, 19771REFERENCESJ . Chem. Soc., 1964, 714.R. Colton, Nature, 1962,194, 374; D. Brown and R. Colton,C. D. Kennedy and R. D. Peacock, J . Chem. Soc., 1963,3392.G. W. Watt and R. J. 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ISSN:1477-9226
DOI:10.1039/DT9790001143
出版商:RSC
年代:1979
数据来源: RSC