Conductance of some Cobalt(II1) Complexes in Water at 25°CPart 2.-Conductance of Salts of Ethylenediaminetetra-acetatocobaltate(111)BY ALAN D. PETHYBRIDGE* AND DAVID J. SPIERS?Department of Chemistry, The University, Whiteknights, Reading, Berks. RG6 2ADReceived 4th April, 1975Results are reported for the molar conductivities at 25°C of aqueous solutions of lithium, sodium,potassium and tetraethylammonium ethylenediaminetetra-acetatocobaltate(m). They are analysedin terms of the Pitts conductivity equation and are best considered as slightly associated with associa-tion constants in the order Li < Na < K > Et4N. A value of 26.17f 0.10 S cm2 mol-’ is reportedfor P ( [ C o (edta)]-).In the preceding paper we reported studies of the conductance of aqueoussolutions of salts of trans- and cis-[Co(en),(NO,),]+ and found a significant amountof outer-sphere complex formation for some of the salts.We now report an extensionof our conductance studies to aqueous solutions of salts containing the [Co(edta)]-ion to see whether the same factors influence the formation of outer-sphere complexesby anionic inner-sphere complexes. There was no problem of conductance drift withtime with solutions containing this anion. Measurements could therefore be madeby the more usual concentration-run technique in which successive weighed aliquotportions of a concentrated stock solution were added to the solution in the conductancecell.EXPERIMENTALThe apparatus and thermostat used have been described elsewhere,” as have the techniquesfor preparing the cell and conductivity water.3a Other experimental details were outlinedin the preceding paper.’PREPARATION OF COMPOUNDSConductivity water and materials of the highest quality grade conveniently available(usually AnalaR) were used throughout.Full details of the preparation and purificationof these salts are given elsewhere.4K[Co(edta)], E, was used as the starting material in the preparation of the other salts ofthis anion. Early attempts to follow the preparation recommended by Weakleim andHoard were not completely satisfactory so an alternative procedure was adopted. Allthe samples of K[Co(edta)] were prepared by the latter technique and were purified once bydissolving the product in the minimum quantity of water and filtering into three times thevolume of acetone, which was stirred continuously. The sample used for conductivitymeasurements was precipitated in this way twice more.The product was the dihydratewhich was stable in air. Although the salt could be dehydrated by heating under vacuumat 100°C, the anhydrous compound was hygroscopic and conductance runs were made withthe air-dried sample of the dihydrate. The solubility at room temperature is approximately1.3 moldm-3. Analysis for dihydrate: found C 28.3, H 3.8, N 6.7; calculated C 28.4H 3.8, N 6.6 %.t present address : International Nickel Ltd., European Research and Development Centre,Wiggin Street, Birmingham B16 O N .774 CONDUCTANCE OF CO"' COMPLEXESNa[Co(edta)] was prepared by passing 1 dm3 of a 0.05 mol d ~ n - ~ solution of E through acolumn of cation exchange resin Amberlite IR-120 (analytical grade) in the sodium form.800 cm3 of eluate were collected and evaporated down to 40 cm3, obtaining the crude productby pouring this into 300 cm3 of acetone.The salt was purified twice by dissolving in a smallquantity of water and filtering into excess AnalaR acetone. The product, the tetrahydrate,was stable in air and was used for the conductance measurements. The anhydrous saltcan be obtained by heating at 100°C under vacuum but it is hygroscopic. The solubility atroom temperature is approximately 0.6 rnol dmW3. Analysis for tetrahydrate : found C27.3, H 4.5, N 6.4; calculated C 27.2, H 4.6, N 6.3 %.Li[Co(edta)] was also prepared by ion exchange from a 0.05 mol dm--3 solution of E.The product was extracted from the eluate and purified in a manner similar to that for Na-[Co(edta)].The salt was precipitated as the trihydrate but drying in vacuo at 100°C producedthe anhydrous salt (not hygroscopic) which was used for the conductance measurements.The room temperature solubility was estimated to be 0.5 mol dm-3. Analysis : foundC 33.6, H 3.5, N 7.8; calculated C 33.9, H 3.4, N 7.9 %.Et4N[Co(edta)] was also prepared by ion exchange from a 0.05 mol dm3 solution of E.Addition of a large excess of acetone to the concentrated eluate produced a viscous purpleoil. The acetone was decanted off and after stirring with excess fresh acetone and leavingovernight a purple product was obtained.This was recrystallised from boiling ethanol, butthe product was extremely hygroscopic, forming a purple solution on a few minutes' exposureto the atmosphere. The purified salt was dried in vacuo at 60°C for 48 h and a specialtechnique was devised for preparing the stock solution. A sample of the salt was dried on avacuum line overnight and, after filling the line with dry nitrogen, was added rapidly to aweighed flask containing a known mass of conductivity water. The flask was reweighedto obtain the mass of salt added. Because of the hygroscopic nature of the salt no micro-analysis of the salt could be attempted, but as will be seen later, the value of Am obtained wasconsistent with independent single ion molar conductivities.The preparation and conductance results for ~rans-[Co(en)~(NO&][Co(edta)J are reportedin the preceding paper because the A values are time-dependent.RESULTS AND DISCUSSIONThe molar conductivities of the four salts are given in table 1.All conductivityvalues have been converted to absolute units.In the discussion the results for all runs on a given salt are analysed together togive a more realistic idea of the accuracy of the parameters obtained. For the analysisof any single run the value of the deviation function a,( %)l was usually less than 0.01.Our method of fitting the data and our reasons for choosing the Pins (P) equation '* *rather than the Fuoss-Hsia (FH) equation have been given e1sewhere.l.It is possible to fit the data for these salts quite satisfactorily by assuming themto be completely dissociated in aqueous solution.However, the values of the contactdistance a shown in table 2 are unreasonably low, decreasing in the order Li > Na > K.Consequently we have fitted the results by treating the salts as being slightly associatedin solution. In contrast with the data obtained in the preceding paper by the single-point technique, all the data for single runs and the combined data for Li[Co(edta)]show two points of statistical best fit, one at a low d, frequently with a negativeassociation constant KA, and the other at higher d and KA values. There is a closecorrelation between these best-fit d and KA values, regardless of the salt, shown in fig. 1.In our reanalysis of literature data we have frequently observed 2 * lo this type ofbehaviour and suspect it is often due to small systematic errors in measurements madeby the concentration-run technique.Such double minima are seldom reported, prob-ably because other computer programs either do not search over a sufficiently largerange of d values or stop when the first minimum is obtained. We consider lo thatone way of avoiding this problem when comparing KA values for different salts oA . D . PETHYBRIDGE AND D. J . SPIERS 75the same charge type and in the same solvent is to compare them at a common valueof d. A convenient value is d = 4, the Bjerrum critical distance, which is 3.58 A fora 1 : 1 electrolyte in water at 25°C and is close to the values of d found in those caseswhere a unique point of best fit is obtained. Similar results are obtained if any otherreasonable value of d is used, e.g., 24, although the absolute values of KA are, of course,altered.104 clmol dm-37.600 414.50820.03423.93028.1988.030 714.85419.86624.64928.6566.662 811.13115.52918.54021.014104 CITABLE 1 .-MOLAR CONDUCTIVITIES AT 25°CA / S cm2 mol-1 104 c/mol dm-3 A / S cm2 mol-1 104 c/mol dm-3Li[Co(edta)lU62.607 27.168 60.973 73.80361.901 37.378 60.406 134.4561.446 51.483 59.742 200.8061.181 66.522 59.151 292.1560.901 90.629 58.343 376.96Na[Co(edta)]"74.066 31.335 71.959 60.51473.337 52.673 70.835 104.8172.894 69.691 70.124 159.1172.545 92.817 69.299 232.0272.27 1 124.89 68.333 300.55K[Co(ed tallu97.528 22.198 95.526 67.42396.873 34.149 94.620 97.47396.375 41.071 94.215 126.3496.072 53.939 93.378 152.4695.843 74.970 92.322 221.5299.571 91.250 290.40Et4N[Co(edta)]A,!S cmz mol-158.86957.14755.75754.27653.21770.54268.96867.5 1965.98564.80992.59291.24190.14689.28687.33685.744mol dm-3 18.729 29.373 36.335 45.002 55.447 64.045 72.015 82.874mol-* 55.329 54.592 54.180 53.733 53.243 52.879 52.566 52.171A/S cm2a Three independent runs in different cells.TABLE 2.-PARAMETERS GIVING THE BEST FIT WITH THE P EQUATION ; SALTS TREATED AS FULLYDISSOCIATEDsalt a/A Am fS cm2 mol- b*( %ILi[Co(edta)] 3.01 64.662 0.04Et 4N[Co(edt a)] 1.61 58.440 0.01Na[ Co(edta)] 2.28 76.210 0.07K[Co(edta)] 0.75 99.614 0.09TABLE 3.-PARAMETERS GIVING THE BEST FIT WITH THE P EQUATION WITH d = 4 ; SALTSTREATED AS ASSOCIATEDsalt Am/S cm2 mol-1 K*/dm3 mol- ' U K ~ @A( %) Am(Cco(edta)l-)Li[Co(edta)] 64.661 0.30 0.29 0.04 26.00Na[Co(edt a)] 76.217 0.67 0.25 0.06 26.14K[Co(ed t a)] 99.675 1.69 0.37 0.07 26.21Et4N[Co(edta)] 58.449 1.20 0.04 0.01 26.31Table 3 shows the parameters which give a best fit for the combined data for eachsalt when d is set equal to q.The association constants obtained from analysis ofsingle runs usually lie within a standard deviation of the KA value calculated for acombined run, unless the runs cover very different concentration ranges. Even so76 CONDUCTANCE OF CO"' COMPLEXESboth the P and FH equations give association constants for the [Co(edta)]- salts inthe order Li < Na c K > Et,N, although the values obtained from the FH equationare consistently 0.2 mol dm-3 lower than those in table 3.As discussed in the preced-ing paper the salt trans-[Co(en),(NO,),][Co(edta)] is quite strongly associated butas the A" value obtained is some 9 units too high not much reliance can be placedon the absolute value (8 dm3 mol-l at d = 4 for the P equation).If the values of KA at d = q are plotted against the sum of the crystallographicradii (fig. 2), the present salts fit in with the trend recorded in the previous paper,4t 0I2 xb2035 LA8X C0 223 x0 j-:, , , , , , ,-200 4 8 12 16dlAFXG. 1.-Values of KA and d giving statistical best fit of individual and combined runs using the Pequation.Numbers refer to individual runs, c denotes combined data. Salts are , 0 Li[Co (edta)],x Na[Co (edta)], A K[Co (edta)] and 0 Et4N[Co (edta)].although Na and Li fall slightly below the values for other salts. We note that anycorrelation of KA appears to be with the crystallographic radius of the anhydrouscation rather than the hydrodynamic radius of the solvated species. This seems toindicate that the solvating water is displaced when the cation and the [Co(edta)]-anion associate to form a non-conducting species.The deviation of Et,N[Co(edta)] is particularly marked, but unfortunately onlyone successful run was made with this salt as the experimental difficulties were greatowing to its extremely hygroscopic character.The low value Of KA for Et4N[Co(edta)]may well be due to the structure of the water round the two ions being so dissimilar thatthere is a kinetic barrier to the formation of a non-conducting outer-sphere complex.If this were so a study of the tetra-alkylammonium fluorides might be expected toshow similar results. For analysis with d = 2q the points are raised by about1 dm3 mol-1 and the order of association constants is Li < Na < K % Et4N.and ourrecommended value 3b for A"(Et,N+) = 32.13 S cm2 mo1-I with the An., values reportedin table 3 the values of A"([Co(edta)]-) in the last column are obtained. The meanvalue is 26.16f0.10 S cm2 mol-1 compared with the mean value for trans- and cis-[Co(en),(NO,),]+ of 27.45 * 0.3 S cm2 mol-l. A simple calculation using knownBy combining the published values of A"(M+) for the alkali metal ionA.D. PETHYBRIDGE AND D. J . SPIERS3+A+ A+AKE-A + ++0 + .*X A-NaEX 2"A-LiEEt4N E /I x x * x 0I I I 10 2 4 6 877&,s*lAFIG. 2.-Plot of KA values at d = q using the P equation, against the sum of the crystallographicradii of the ions. x alkali metal halides, 0 alkali metal oxosalts, + tetra-alkylammonium halides, A trans- and cis-[Co (en)2(N02)2]+ salts, salts of [Co (edta)]-.interatomic distances l 2 and angles and van der Waals radii l 3 shows that both ionshave a radius of 4.3 A.There is probably some slight hydrolysis of the [Co(edta)]- anion according to thescheme l4[Co(edta)]- f [Co(edta)(H,O)]- + [Co(edta)(OH)I2- + H+and definite evidence of similar behaviour with Cul* and several multidentate ligandshas been established by n.m.r.spectros~opy.~~ The pH of a 0.010 0 mol dm-3 solu-tion of K[Co(edta)] was 6.2 and in the preceding paper we ascribed the increased rateof aquation of the cation in trans-[Co(en),(NO,),][Co(edta)] to the hydrogen ionsproduced by the hydrolysis of this anion. From the point of view of the conductancemeasurements reported in this paper however, this hydrolysis is not important becausethe equilibrium concentration of the products on the extreme right-hand side is onlyabout 0.01 % of the total concentration of the [Co(edta)]- ion.Note added in proof. We have reanalysed our data using the latest Fuoss equation (J.Phys. Chem.,1975, 79, 525). The new equation with d = q yields ha values 0.04 S cm2 mo1-1 larger than thosefrom the P equation in table 3 but KA values 0.2, 0.4,0.6 and 0.4 dm3 mol-I lower for the Li, Na, Kand Et4N salts respectively.One of us (D. J. S . ) thanks the S.R.C. and the University of Reading for financialsupport.A. D. Pethybridge and D. J. Spiers, J.C.S. Furaduy I, 1976, 72, 64.E. M. Hanna, A. D. Pethybridge, J. E. Prue and D. J. Spiers, J, Solution Chem., 1974, 3, 563.A. D. Pethybridge and D. J. Spiers, (a) J. Electroanulyt. Chem., in press ; (b) to be published.H. A. Weakleim and J. L. Hoard, J. Amer. Chem. SOC., 1959, 81, 549.* D. J. Spiers, Ph.D. Thesis (Reading University, 1974)78 CONDUCTANCE OF CO"' COMPLEXESF.E.E.R.P. Dwyer, E. C. Gyarfas and D. P. Mellor, J. Phys. Chem., 1955,59,296.Pitts, Proc. Roy. SOC. A, 1953, 217, 43.Pitts, B. E. Tabor and J. Daly, Tram. Furachy SOC., 1969,65,849.M. Fuoss and K.-L. Hsia, Proc. Nat. Acud. Sci. U.S.A., 1967, 57, 1550 ; 1968, 58, 1818.l o A. D. Pethybridge and D. J. Spiers, Chem. Comni., 1974, 423.R. A. Robinson and R. H. Stokes, Electrolyte Solutions (Butterworth, London, 2nd edn. rev.,1959), p. 463.l 2 Interatomic Distances (Chemical Society Special Publications, London, No. 11, 1958; No. 18,1965).L. Pauling, % Nature of the Chemical Bond (Cornell University Press, 1940), chap. 10.l4 G. Schwarzenbach, Helv. Chim. Acta, 1949, 32, 839.F. J. C. Rossotti, K. B. Dillon, M. R. Harrison, D. J. Spiers and H. R. Sunshine, unpublishedwork.(PAPER 5/642