J. CHEM. SOC. FARADAY TRANS., 1994, 90(21), 3293-3299 3293 Effect of Solvent on the Reaction of Coordination Complexes Part 19.T-Base Hydrolysisof (ap)-(0-Methoxy benzoato)(tetraethylenepentamine)-cobalt(iii) in Aquo-organic Solvent Media Achyuta N. Acharya and Anadi C. Dash* Department of Chemistry, Utkal University, Bhubanes war-751 004, India The kinetics of base hydrolysis of (afl)-(o-methoxy benzoato)(tetren)cobalt(itt) have been investigated in aquo- organic solvent media [O-70% (v/v) cosolvents] at 10 d t/"C d 40 (I = 0.01 mol dm-3) using methanol, ethanol, propan-1-01, propan-2-01, butan-1-01, tert-butyl alcohol, ethylene glycol, 2-methoxyethanol, acetone, acetonitrile, 1,4-dioxane and dimethyl sulfoxide as cosolvents. The second-order base hydrolysis rate constant increased non-linearly with increasing mole fraction (XOrs)of all cosolvents, except for the ethylene glycol-water system ; ethylene glycol had a rate-retarding effect.The transfer Gibbs energy of the transition state (TS) relative to that of the initial state (IS),for transfer of species from water to mixed solvent varied non-linearly with 0;'andX,,, , reflecting the individuality of the cosolvents and thereby suggesting that the relative stabilities of the transition state and the initial state were governed by the preferential solvation effect. The solvent stabilisation of the initial state and the transition state has been assessed for the methanol-water and ethanol-water systems by combining the solubility data of the dithionate salt of the complex with the transfer Gibbs energy data for S,0,2-.The thermodynamic parameters (AH* and ASt)were sensitive to the structural changes in the bulk soIven t phase. The importance of hydrophobic effects,'V2 preferential Experimental s~lvation~-~and solvent structural effects7-' on the kinetics (agS)-(o-Methoxybenzoato)(tetren)cobalt(rn) diperchlorateand energetics of the ligand-substitution reaction of was prepared as described earlier.17 The analytical data for transition-metal complexes has been emphasized in several the complex (C, H, N and Co) agreed with those predicted by recent studies.' I-' Recently, we reported the base hydrolysis its molecular formula. (aBS)-(0-MethoxybenzoatoXtetren)inof (a~S)-(salicylato~tetraethylenepentamine)cobalt(~~~) cobalt(II1) dithionate was prepared by repeated rec-mixed-solvent media4 using methanol and dimethyl sulfoxide rystallization of the perchlorate complex salt with sodium as cosolvents.It was evident that solvation of the initial state di thiona te, washed successively with water, absolute alcohol, and the transition state was strongly influenced by these diethyl ether and stored over fused calcium chloride. Calc. for cosolvents. However, (salicylato)(tetren)cobalt(IIr) (where [Co(tetren)0,CC,H,(OCH3)]S206 :Co, 10.6. Found : Co,tetren = tetraethylenepentamine) was capable of generating a 10.4. The dithionate complex exhibits AJnm (&/dm3 mol- ' reactive amid-imido conjugate base via intramolecular cm-')at 490 (199) in 0.1 mol dm-3 HClO, which closely acid-base equilibrium, -0--* -NH-7HO.* .N--.16 agreed with the data of the corresponding perchlorateSuch an effect will be absent in the corresponding o-complex.' methoxybenzoato complex. Hence the solvent perturbation Organic solvents? (analytical grade) were distilled after effects of the tetren ligand and the aromatic moiety on drying over 4A molecular sieve. 1,4-Dioxane and 2-the base hydrolysis of (apS)-(o-methoxybenzoato)(tetren)-methoxyethanol were made peroxide-free by repeatedlycobalt(II1) (I) can be examined much more clearly. With this refluxing them over sodium hydroxide and metallic sodium, aim we made a thorough study of this reaction in aquo- respectively, and distilling before use. Density and boiling organic solvent media which differed in their structural, elec- points were taken as a check of the purity of the solvents.trostatic and hydrophobic interaction propensities. Fresh solvent mixtures were prepared by mixing a known volume of organic solvent with water before recording any kinetic run. Doubly distilled water was used; the second distillation was made from alkaline KMnO, using an all- glass distillation still. All other chemicals used were of ana- lytical grade. Solubilities of the dithionate complex in MeOH-H,O and EtOH-H,O were determined by equilibrating an excess of the solid complex in the acidified mixed solvent ([HClOJ x lo-, mol dm-3) for 3-4 h in stoppered flasks, thermostatted at 25.0 +_ 0.2"C. Samples of the solutions were withdrawn b""' and the concentrations were determined spectrophotometri- cally at 490 nm. All the spectral measurements were made I using a Jasco 7800 UV-VIS recording spectrophotometer.(cr/?S)-(o-methoxybenzoato)(tetren)cobalt(ltl) ion ~~ ~ ~ Part 18: A. C. Dash and G. C. Pradham, Znd.J. Chem., Sect. A, 7 Abbreviations : methanol, MeOH ;ethanol, EtOH ; propan-1-01, 1994, 33, 661. This work was partly presented at the 23rd IUPAC PrOH ; propan-2-01, Pr'OH ; butan-1-01, BuOH ; tert-butyl alcohol, Conference on Solution Chemistry, held at the University of Leices- Bu'OH ; ethylene glycol, EG; 2-methoxyethanol, ME; acetone, AC; ter, UK, 16-23 August, 1993. acetonitrile, AN; dimethyl sulfoxide, DMSO; 1,4-dioxane, D. Kinetics The base hydrolysis of the title complex (perchlorate form) was studied under pseudo-first-order conditions ([OH-]~/[complex]~2 25) at 320 nm using an automated Hi-Tech (UK) SF 51 stopped-flow spectrophotometer con- trolled by an Apple IIGS PC.The decay of the complex was found to be single-exponential for any run under the experi- mental conditions, even after extending the reaction over an expanded timescale (0.01-50 s). The rate measurements were made at [NaOH], = 0.01 mol dm-3 and 10.0 < t/"C < 40.0, with At/"C 2 20 (at least four temperatures) for any solvent composition. The other experimental details were the same as described earlier.' The pseudo-first-order rate constant and its standard deviation for any run was calculated from at least seven replicate measurements. Results and Discussion The rate data (kobs) in the fully aqueous medium strictly obey a second-order relationship, kobs= ko,[OH -]T [see eqn.(I)] (tetren)CO02CC6H,(o-oMe2+ + OH-kon-(tetren)CoOH'+ + -O,CC,H,(o-OMe) (I) in the range 0.010 < [OH-],/mol dm-3 < 0.10 at 20.0 Q t/ "C < 40.0 (I= 0.10 mol dm-3) (see Table 1). The activation parameters (AHS and ASS)agree well with those reported by us earlier' '(see Table 1) at I = 1.0 mol dm -'. The large posi- tive value of AS: is consistent with the S,1 conjugate-base (CB) mechanism '' involving dissociatively activated conju- gate base of the complex generated by the deprotonation of the N-H group of the coordinated tetren. Variation of Rate Constant with Solvent Composition The second-order base hydrolysis rate constants, koH ,in dif- ferent aquo-organic solvent media at I = 0.01 mol dm-' are collected in Table 2.All aquo-organic solvent systems except EG-H,O exerted a marked accelerating effect over the com- position range studied. Since ko, is subject to ionic strength effects, its correction to I = 0 (k&) was made by using the relationship in eqn. (1) log (koH)= log kgH + 22, 2B AI1',/( 1 + (1) where Z, and zB denote the charges of the reacting species, A = 1.8246 x 106/(DsT)3/2,B = (50.29 x 108)/(D,T)lI2,D,is the bulk relative permitti~ity'~ and Q was chosen to be 5 A. log kgH at 25 "C increased non-linearly (except for MeOH and DMSO) with increasing mole fraction of the organic solvent, Xorg (see Fig.1); such plots are linear for MeOH-H,O and DMSO-H,O media. In contrast, non-J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 4.6 4.0 3.4 I 00 Y 0-2.8 2.2 Y Y n -I I I I I1.6 0.10 0.20 0.30 0.40 0.50 0. 0 XO,, Fig. 1 log k&., us. X, for different aquo-organic solvent media at 25 "C: (a) EG, (b)MeOH), (c) DMSO, (6)EtOH), (e) AN, (f)PrOH, (9)Pr'OH, (h) AC, (i) ME, (j) Bu'OH, (k)D the EG-H,O system (see Fig. 1). The rate effect is modestly sensitive to the nature of the cosolvents, being maximum for 1,4-dioxane. The variation of log kgH with 0,' is generally non-linear ; linearity is observed for DMSO-H,O. Plots of log kg, us. E; and Grunwald Winstein's solvent ionising parameter, Y for which E; (MeOH-H,O, Pr'OH-H,O, AC-H,O, DMSO-H,O, AN-H,O),' and Y (MeOH, Pr'OH, Bu'OH and AC)2' data are available, are not linear.These facts reflect the inadequacy of the structureless dielectric con- tinuum model, which depicts the solvent and solute as uni- formly charged rigid spheres. The importance of hydrogen bonding and other non-electrostatic medium effects in con- trolling the reactivity of the substrate is apparent. The observed solvent effect may be interpreted in terms of the conjugate base mechanism: kca (CB+)' -H2O Products (11) linear rate retardation with increasing X,, was observed for fast Table 1 Rate data for base hydrolysis of I in aqueous medium" [OH-]/10-2 mol dmT3 20.0 * 0.01"C 25.0 f0.1 "C 35.0 f0.1 "C 40.0_+ 0.1 "C 1.o 0.22 f0.01 0.44 f0.02 1.54 f0.09 2.89 f 0.09 2.0 0.46 f0.01 0.86 f0.02 2.95 f 0.07 5.38 0.27 5.0 1.20 * 0.04 2.28 f0.01 7.85 k0.36 13.2 & 0.7 7.50 1.90 k 0.08 3.60 f0.14 11.9 f0.2 23.3 f1.2 10.0 2.54 f0.10 4.99 0.14 17.4 f1.2 30.6 f2.1 k,,/dm3 mol s-l 23.4 f0.6 44.9 k1.3 155 f3 284 f7 AH:fiJ mol -' 92.5 f 0.3(93 f2)bAS/J K-' mol-' 97 f l(90 f5)b [complex], = (3-4) x I = 0.1 mol dm-3, 1 = 320 nm.I = 1.0 mol dm-3.'7 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 3295 Table 2 Rate data for base hydrolysis of I in mixed-solvent media with various amounts of organic solvent (in vol.%) at different temperatures k,,"/dm mol -s -t/T 5.2 vol.% 10 vol.% 20 vol.% 30 vol.% 40 vol.% 50 VO~.%O 60 vol.% 70 VO~.% MeOH 20.0 56f 1 60f 1 67 f2 75 f2 87 f1 105 f2 136 k 3 188 f7 25.0 108 f3 117 f 4 134 f3 159 f2 188 & 7 230 k 2 301 f13 386 f13 35.0 393 f9 425 f8 489 f4 632 k 2 814 f8 lo00 f20 1295 f34 1733 f98 40.0 676 f24 749 f20 958 f 15 1177 k 32 1495 f 44 1903 k 76 2769 k 87 3618 f162 (0.024)b (0.047) (0.099) (0.159) (0.228) (0.306) (0.398) (0.507) EtOH 20.0 60f 1 73 f 1 102 f2 149 f3 217 f9 318 k 12 448f 15 680 f16 25.0 114 f4 137 f3 197 f2 309f7 440f9 605 f14 950 f11 1412 f30 35.0 387 f7 475 f4 740f 17 1106 f27 1629 f50 2127 k 80 3357 f 53 5100 f 118 40.0 724f 11 900f9 1291 f 22 1996 f44 3132 f68 4180 90 6043 f210 9060 f300 (0.017)b (0.033) (0.07 1) (0.116) (0.170) (0.235) (0.3 15) (0.416) PrOH 20.0 54f 1 67 f2 95 f1 136 f7 202 f 5 265 k6 362 f 15 514 f5 25.0 102 _+ 1 125 k 8 190 f3 285 _+ 4 404f6 541 f16 754 f 14 985 f20 35.0 356 f 8 446f 10 659 f 13 1029 f15 1494 f34 1916 f40 2549 f 86 4085 k 100 40.0 639 f18 836 f6 1261 f18 1918 _+ 56 2732 f 39 3684 f81 5005 f 180 6700 f320 (0.01 3)b (0.026) (0.057) (0.093) (0.127) (0.179) (0.246) (0.336) Pr'OH 20.0 62 f2 82 f2 128 f3 218 _+ 3 356 f9 483 f 11 900f 11 1533 f 60 25.0 120 f3 152 f6 244f5 426 f 10 701 & 10 1111 f50 1818 f50 3079 f 174 35.0 396 f5 524 k 7 891 f5 1467 k31 2401 f39 3658 f60 5795 f56 9504 f294 40.0 733 f17 1OOOf 18 1580 f19 2417 f52 3965 f 103 6296 f318 loo00 f500 16500 f900 (0.01 3)b (0.025) (0.055) (0.091) (0.135) (0.190) (0.260) (0.3 5 2) Bu'OH' 20.0 62f 1 82 f1 134 f3 178 f 3 344f7 529 f11 858 f5 1429 f18 25.0 121 f 3 154 f3 251 f2 343 f5 631 f7 1014 f 21 1646 f35 2878 _+ 76 35.0 408f8 536 f5 883 f4 1197 f13 2322 f112 3346 f88 4970 i-39 8660 f300 40.0 759 f14 990f 15 1635 f31 2101 f51 4004 f32 4963 f220 9591 f476 15330 f1032 (0.010)b (0.021) (0.045) (0.075) (0.1 12) (0.159) (0.221) (0.306) ME 20.0 56 & 1 63 f2 73 f1 97 f2 130 f3 196 f1 258 f6 427 f12 25.0 109f6 123 f3 151 f4 206f9 283 f5 395 f9 568 f6 904 k 24 35.0 346 f16 418 f9 544 f22 703 f23 1064 f28 1493 f31 2141 k 83 3444 f102 40.0 664 f 16 794 f21 1047 f28 1442 f 26 2104 f41 2895 f106 4498 f69 7259 f198 (0.012)b (0.025) (0.054) (0.089) (0.132) (0.185) (0.254) (0.346) EG 20.0 39 f1 34 f1 24 f1 19 f1 16f 1 12 k 1 10.3 f0.3 9.9 f0.4 25.0 76f 1 66 f2 49 f 1 38 f 1 32f 1 28 f1 26 & 2 24 f 1 35.0 267 f1 246f6 189 f5 157 f2 136 k 3 125 f2 118 * 2 115 f6 40.0 499 f8 466 f 11 377 f3 330 f8 291 i-8 265 f5 257 f5 248 f4 (0.017)b (0.035) (0.075) (0.121) (0.177) (0.243) (0.3 25) (0.428) AC 20.0 64f2 77 f1 129 f3 231 f 4 446 f10 855 i-11 1702 f42 3232 f142 25.0 123 f3 159 f3 252 f 10 402 f10 820 f20 1538 f93 3273 f84 6200 f200 35.0 413 f8 531 f 18 937 f24 1657 f29 3024 f122 5608 f335 10800 k 300 19300 f1300 -40.0 761 f29 976 f 25 1685 f37 2980 f67 5597 f192 9988 k 583 19200 f1400 (0.01 3)b (0.026) (0.057) (0.094) (0.139) (0.195) (0.267) (0.361) AN 20.0 56+ 1 65 f1 97 f2 156 f2 262 f6 448 5 819 f6 1657 f39 25.0 104f2 128 f 2 189 f4 316 8 537 f5 926 f13 1720 44 3264 160 35.0 374 f 8 442 f 14 717 f13 1136 f38 2045 f27 3527 f95 6275 f155 12400 f600 40.0 659 f11 807 f 16 1312 & 14 2185 f89 3838 f118 6510 _+ 158 12300 k 700 25800 f1100 (0.01 8)b (0.036) (0.079) (0.127) (0.185) (0.254) (0.3 38) (0.442) DMSO 10.0 16f 1 17 f1 18 f1 27 _+ 1 33 f1 47 * 3 20.0 58 f1 65 f1 83 f2 119 f4 159 f5 261 f7 25.0 118 f5 137 f2 165 f5 214 k9 320 & 9 463 f23 35.0 387 k8 422 f 14 547 f4 712 f31 1221 f31 2116 f 74 40.0 765 + 18 860f 15 1066 f29 1421 f69 2287 f79 3754 f141 (0.014)b (0.027) (0.059) (0.68) (0.124) (0.201) 3296 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Continued t/OC 5.2 VO~.% 10 VO~.% 20 vol.O/o 30 VO~.% 40 vol.O/o 50 vol.% 60 vol.% 70 vol.% D 15.0 33 f1 43 f1 74 f2 146 f4 278 f8 420 f3 810 f10 1491 f32 20.0 62 f2 84 + 2 151 f3 304f3 527 f12 lo00 f50[235 f7Id [397 f31' 1878 f43 [872 f131" 3346 f122 30.0 35.0 25.0 228 + 6 419 f5 123 f2 300 f5 556 + 11 163 +3 513 f 11 1027 f25 285f2 1066 f 22 2022 k107 581 f6 2084 f49 3835 f92 1050 f27 3272 f35 5900 f265 1900 f58 6849 250 10300 f400 3735 f59 6100 f140 12100 f 400 16700 f1200 (0.01 l)b (0.023) (0.050) (0.083) (0.123) [11400 f5001' (0.173) C16600 f14001' (0.239) [32700 f 12001' (0.328) a k;,/dm3 mol-' s-l = 52.4 f 1.2, 98.2 f 1.7, 338 f 7 and 603 f15 at 20.0, 25.0, 35.0 and 4O.O"C, respectively, I = 0.01 mol dm-3, 1 = 320 nm.Values in parentheses denote mole fraction of the organic component. 'For BuOH, kOH/dm3 mol-' s-' (vol.% BuOH): 54 1 (2.0), 59 f 1 (4.0), 63 f1 (6.0), 67 f 2 (8.0) at 20.0"C; 101 f1 (2.0), 109 f3 (4.0), 118 + 1 (6.0), 131 f4 (8.0) at 250°C; 340 f12 (2.0),377 f10 (4.0), 409 f8 (6.0), 438 f11 (8.0) at 35.0"C;659 f9 (2.0), 704 f 14 (4.0), 778 f16 (6.0), 844 f 8 (8.0) at 40°C.'At 10°C. At 40°C. Table 3 Calculated values of [A,G"(CB+)* -A, G"(C2+)],,,JkJ mol-for different aquo-organic solvent systems at 25 "C" [A, G"(CB+): -Al G"(C2')]~,+w,/kJmol-' organicsolvent 10 VOl.Yob 20 vol.% 30 vol.% 40 vol.% 50 ~01.96 60 vol.% 70 vol.% ~ MeOH' -0.50 & 0.10 -0.92 f0.08 -1.57 f0.07 -1.98 & 0.10 -2.15 f0.06 -1.94 f0.12 -1.42 f0.10 -EtOH -0.32 f0.08 -0.44 f0.06 -0.60 f0.08 0.05 f 0.07 1.32 0.08 2.90 f0.06 Pr'OH 0.88 f0.11 0.76 f0.07 0.60 f0.08 0.79 -t 0.08 1.40 f0.12 2.06 f0.08 -Bu'OH -0.33 f0.07 0.16 f0.05 1.61 f0.06 2.11 f0.06 2.68 0.07 --EG -0.07 f 0.09 0.03 f0.09 -0.06 f0.06 -0.19 f0.11 -0.48 f 0.09 -0.18 f0.17 ME 0.17 f0.08 1.27 f0.08 2.13 f0.12 2.55 & 0.07 2.66 f0.08 2.54 f0.06 0.61 f0.08 -D 1.56 f0.07 3.65 f0.05 5.05 f0.06 4.80 f0.08 4.22 f0.09 --AC 2.02 & 0.07 3.96 f0.1 1 6.65 f0.08 9.40 0.08 12.48 f0.16 -DMSO 1.26 f0.07 3.49 f0.09 6.44 f0.12 10.02 f0.09 15.46 f0.13 Based on the TATB assumption.Amount of organic solvent (vol.%) 'Based on the TPTB assumption. a where R = (tetren)cobalt(w) and X = (o-OCH,)C,H,CO, -. where GS(CB+)$ denotes the transition-state Gibbs energy of ko, takes the form : the monopositive conjugate base [(CB'): = (tetran-H) Co2'. * * -O,CC,H,(o-OCH,XTs)], and G,"(C2+) and kOH = kCB KCB (2) G,"(OH-) the standard Gibbs energies of the dipositive sub- where KcB and kc, are as defined in eqn. (11).The activation strate (c2') (Is) and OH-, respectively, in the medium s. Gibbs energy for a given solvent composition s (see Fig 2) Eqn. (4) yields the transfer Gibbs energy of the transition can be given by state (CB+): relative to that of the initial state (C2') of the -complex when transfer of species occurs from water (w) to AGK = G,(CB+)' -G:(C2+) -G,"(OH-) (3) mixed solvent (s). -[A, G"(OH-)I,+w, (4) The relative transfer Gibbs energy term, A[At G*llstw), at zero ionic strength, could be calculated from the rate data using the relationship: A[A, Gx](stwl= RT In (k&/kgH). The differences between the transfer Gibbs energies of the tran- sition state and the initial state of the complex ion, [A, G"(CB+): -A, Go(C2+)](s+w),were calculated [see eqn.(4)] using available data for [At G(OH -)Istw,.22tThese values (see Table 3), based on the TATB-TPTB scale, reflect the effects of differential solvation of the transition and initial states. Note that with increasing hydrophobicity of the alco- hols or the aprotic cosolvents, [A, G"(CB+)t],,tw, tends to be more positive than [At Go(C2+)](stw) except for EG-H,O. Plots of [A, G"(CB+): -At Go(C2')](s+w)vs. 102/D, are gener- ally non-linear and also solvent specific (see Fig. 3); linearity is, however, maintained for DMSO-H,O and AC-H,O products r,.FR~.1 Gibbs energy profile of the reaction, RX2+f= t [A,G"(OH-)],,+,JkJ mol-' for MeOH-H,O is based on the ji:otren~('cjC),CC.h',(o-OCH3)I2+) + OH-+products; AG: = TPTB assumption and the TATB assumption is used for all other I c': i (CB') solvent systems reported here.J. CHEM. iOC. FARADAY TRANS., 1994, VOL. 90 3297 16.0 c 12.0 I-E3 ! 8.0 +N 0,& d I ll* 4.0 m ?.2-u h 0 I-2.5 ' ICL,1 1 2.5 4.0 5.0f 102/D, Fig. 3 [At G"(CB+)'-A, Go(C2+)](,,,JkJ mol-' us. 102/D, at 25 "Cfor different aquo-organic solvent media: (a) MeOH, (b)EG, (c) EtOH), (4Pr'OH), (e) Bu'OH), (f)ME, (9)D, (h)AC, (i)DMSO. ( x ) water. media. The calculated values of [At G"(CB+)* -A, Go(C2+)](2tw)for different solvents at D,= 62.5 (25 "C) (see Fig.3) follow the order:AC > D > ME > Pr'OH > B Bu'OH x EG > EtOH > MeOH. If the electrostatic effect is constant, this result would mean that the solvation of the transition and initial states might be sensitive to solvent structure and hydrophobic interaction effects. c I 7Y -3 !---2.0 --5.5I I I 1 0 20 40 60 0 0.10 0.20 0.30 0.40 0.50 organic solvent (vol.%) XOW Fig. 4 [A,G"(i)],,+,JkJ mol-' us. vol.% organic solvent: (a), (b), Fig. 5 A.HS/kJ mol-' us. Xorl for different aquo-organic solvent MeOH; (c), (4, EtOH; open symbols, initial state; closed symbols, media: A, (a) MeOH, (b)EtOH, (c) PrOH, (a) Pr'OH), (e) Bu'OH, (f)transition state ME. B, (a)AC, (b)AN, (c) DMSO, (4D, (e) EG. J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 170 I A 150 130 110 90 T 9 0 9 0 0 0.10 0.20 0.30 0.40 0.50 0 0.10 0.20 0.30 0.40 0.50 XO rg XOW Fig. 6 AS*/J K-' mol-' us. Xorgfor different aquo-organic solvent media. Notation as for Fig. 5. Assessment of the transfer Gibbs energy of both the initial state and the transition state could be made for the MeOH-H,O and EtOH-H,O systems as follows. The trans- fer Gibbs energies of the dithionate salts of (afiS)-(o-methoxy- benzoatoXte tren)co bal t(m) in MeOH-H ,0 and Et OH-H ,0 media were determined from the measured solubilities of the complex using the relationship given in eqn. (5) A,G"(complex) = 2RT In (Sw/S,)(f~+lf~+) (5) where S, and S, denote the solubilities of the complex salt in aqueous and mixed-solvent media, respectively, and f;+ and f;+ denote the activity coefficients of the complex ion in aqueous and aquo-organic solvent media, respectively.The activity coefficient, f,+ (based on the assumption that f;+ = fi+ = 1 at I = 0), was calculated using the relationship: logf,, = -4AI'/'/(l + BuI"~)with A, B and a as described earlier. The Gibbs energy of transfer of the complex ion, [A, Go(C2+)](s+w), where A, G"(comp1ex) = A, Go(,'+)+ A,G"(S,062-), was calculated using the available data for [A, Go(S2062-)](stw).f-The transfer Gibbs energy of the t [A,G(S,0,2-)]F?;~/kJ mol-': 2.9, 2.1, 4.4, 7.5, 11.3, 14.0 at 10, 20, 30, 40,50, 60 vol.% MeOH (TPTB scale); 2.3, 5.1, 8.4, 12.9 at 20, 40, 60,80 vol.% EtOH (TATB scale).We are grateful to Dr. J. Burgess, University of Leicester, Leicester, UK, for communicating these data to us. complex ion, A, Go(C2+), along with A[A, GSlbtw) and [A,G"(OH-)],+,, [see eqn. (4)] enabled us to dissect the solvent effects associated with the initial and transition states of the complex ion for MeOH-H,O and EtOH-H,O media. Fig. 4 suggests that [A, Go(CB+)t],,+,, < [A, Go(Czf)](stw) is valid for all compositions of MeOH-H,O and EtOH-H,O (<40% v/v EtOH), thereby indicating that the stabilising influence of the mixed solvent is greater on the relatively more expanded transition state than on the initial state. Note that extrema are observed for solvation of both the initial state and the transition state (see Fig. 4), which demonstrates the importance of solvent structure.Thus, this work high- lights the solvent structure mediated solvation of the complex ions of varying charge and hydrophobic Variation of Activation Parameters (A@ and AS) with Solvent Composition The activation parameters (AH: and ASs) (see Fig. 5 and 6) exhibit non-linear variation with solvent composition for all mixed-solvent systems. Extrema in such plots are discernible for EtOH-H,O, Pr'OH-H,O, Bu'OH-H,O, Me-H,O, AC-H,O, DMSO-H,O and WH,O media. Note that the extrema observed for these mixed-solvent media are at rela- tively low values of Xorg.This might have a direct bearing on the solvent structural effects as it is known that a small addi- J.CHEM. 3OC. FARADAY TRANS., 1994, VOL. 90 120 110 "0r I-OQO W22 100 3O 3 0 90 I 1 I I 1808 100 120 140 160 180 2 A9/J K-' mol-' Fig. 7 AH*/kJ mol-' us. AS'/J K-' mol-' for all aquo-organic solvent media investigated. The sizes of the circles vary in order to accommodate a number of data points with their errors within a circle for different solvent compositions of the various solvent systems studied. tion of cosolvents modifies the structure of liquid water. The positions of the maxima in the AH* (or ASS)us. XPriOHplot (at XPriOHx0.05 and 0.18) match those of the sharp minimum in the relative partial molar volume of Pr'OH (v2 -VO,) 30-32 and the maximum in the ultrasonic absorption, respectively, for the Pr'OH-H,O ~ystem.~~.~~AHS (or AS*), however, tends to a maximum at X,,,, x0.20, at which point the relative partial molar volume of DMSO (v2-VO,) 30-32 in the DMSO-H,O medium tends to a minimum.If AHS is split into a reaction component and a solvation componentg (AH*= AHh + AS:), it is reasonable to expect that solvent structural perturbations causing hydrogen-bond formation or breakage will affect the solvation component of AHS,as this effect in the bulk phase will be passed on to the reaction site via the solvation shells of the transition and initial states. The fact that the solvent effects on AHSand AS* are essentially mutually compensatory (see Fig. 7) demon-strates this point. A.N.A. thanks the CSIR, New Delhi for the award of a Senior Research Fellowship. References 1 Y.Cheng, M. 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