Complex Reactions in Analytical Chemistry Charles N. Reilley, Program Chairman, University o f North Carolina, Chapel Hill, N. C. o f Illinois, Urbana, 111.
I 12th Annual Summer Svmposiam, ACS Divisio; of Analytical Chemistry and Analgtical Chemistry, Urbana, Ill., June 1959
G. 1. Clark, local Chairman, University
Complex Titrations, Principles, and Methodology. Hermann Flaschka, Department o f Chemistry, Georgia lnstitute o f Technology, Atlanta, Ga. End Point Detection in Complex Titrations. Charles N. Reilley, Department o f Chemistry, University o f North Carolina, Chapel Hill, N. C. Gerald SchwarzenRelationships between Metal Complex Stability and the Structure of the Complexing Agents. bach, laboratorium f i r anorganische Chemie, Zurich, Switzerland. Current Developments in the Solution Chemistry of Complexes. David N. Hume, Department o f Chemistry, Massachusetts lnstitute of Technology, Cambridge, Mass. Reaction Rates and Stability of Metal Complexes. R. G. Pearson, Department o f Chemistry, Northwestern University, Evanston, Ill. Application of Chelates to Analytical Separation Processes. Henry Freiser, Department o f Chemistry, University of Arizona, Tucson, Ariz. Complexes in Cation Exchange Separations. James S. Fritz, Department o f Chemistry, Iowa State College, Ames, Iowa. Anion Exchange Separation of Metal Ions. Kurt Kraus, Oak Ridge National laboratory, Oak Ridge, Tenn. Coordination Kinetics Applied to the Separation of Metal Ions. Dale W. Margerum, Deportment o f Chemistry, Purdue University, lafayette, Ind. Increasing Selectivity by Masking. K. L. Cheng, Metals Division, Kelsey-Hayes Co., Ufica 4, N. Y. Polarography of Metal Complexes. Robert L. Pecsok, Department o f Chemistry, University o f California, 10s Angeles 24, Calif. Reactions and Potentiometric Titrations of Highly Charged Metal Ions with Multidentate Ligands. A. E. Martell, Department of Chemistry, Clark University, Worcester, Mass. Determination of Structure and Stability of Metal Complexes by Optical Rotation Measurements. Richard Juvet, University o f Illinois, Urbana, Ill. Metalfluorochromic Indicators. Theoretical Considerations and Applications. Donald H. Wilkins, General Electric Research laboratory, Schenectady, N. Y. Analytical Applications of EDTA in the Paint Industry. Claude A. Lucchesi, Sherwin-Williams Co., 7 1547 South Champlain, Chicago 25, Ill. Some Applications of Ion Exchange Resins and Chelometric Reagents to Metallurgical Analysis. Donald H. Wilkins, General Electric Research laboratory, Schenecfady, N. Y.
A summary of the symposium has been prepared b y Charles N. Reilley and i s given in place of the complete papers. For the record we list titles of papers, authors, and their addresses.
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Twelfth Annual Summer Symposium was held a t the University of Illinois, Urbana, Ill., June 10 to 13, 1959. Over three hundred chemists attended the three-day meeting and heard the 16 papers presented. I n addition, there were activp and detailed discussions. Hermann Flaschka presented the introductory lecture, accompanied by very effective demonstration experiments illustrating important considerations in complex titrations. He showed the various types of titration procedures (direct, back, and replacement), end point detection with color indicators, selectivity (by p H selection,) variation of titrant, masking, and demasking. Charles N. Reilley discussed the optimal use of metallochromic indicators 2
HE
ANALYTICAL CHEMISTRY
from a theoretical and practical vienpoint. As a number of competitive equilibria are present,. proper adjustment of solution conditions and judicious selection of various reacting species are necessary t o achieve adequate sharpness of the end point. The application of pM-pH or pY-pH diagrams for pictorial presentation of the significance of the equilibria involved was described. Such diagrams permit quantitative prediction of the end point sharpness and are also useful in arranging conditions for selective titrations and for testing the indicator response. Potentiometric titration n.ith the mercury electrode was also considered and compared briefly n ith end point detection via metallochromic indicators. The use of partial masking in detecting end points photomctricallp was also mentioned. Most photometric titrations take advantage of the inherent differences in the absolute stability constants of the metal complexes. However, through partial masking the effective stability constant of a photometric indicator ion can be altered a t will and thus allow selective titrations in numerous cases. For example, the addition of ammonia to
copyer ion reverses the normal order of titration and permits titrations of metals like calcium ahead of copper without interference from magnesium. David N. Hume discussed the present status of the solution chemistry of complexes. For most metals it may properly be said that the solution chemistry of the elements is the solution chemistry of their complex ions. The so-called “free” metal ions of most elements do not exist in solution as such but become associated to a greater or lesser extent ~ i t the h anions of acids or salts which may be present, n i t h ions derived from the solvent, or with the molecules of the solvent itself. It is the properties of these complexes-their structure, their thermodynamic stability, and their kinetic lability-which govern the behavior of the metals in solution. Whether it be in separation by extraction, precipitation, electrodeposition, ion exchange, or chromatography on the one hand, or in determination by spectrophotometry, potentiometry, conductometry, chronopotentiometry, or titrimetry on the other, n-hether or not the method will n-ork hinges ultimately upon the properties
of the complexes involved. It is, therefore, evident that quite apart from the special properties and applications of the popular chelating reagents, the complex chemistry of the metals in solutions of everyday acids and bases is a matter of great significance t o the analytical chemist. This significance is no less for labile systems, in which the complex species may be elusive and difficult to characterize, than for the more familiar robust complexes which can be isolated readily. The first great period of interest in solution chemistry of complexes was around the turn of the century, when many systems were studied and formation or dissociation constants calculated for the complexes which were assumed to be present. Unfortunately, many of the early results were in error as a result of the unfortunate assumption that only one complex species would be formed, by analogy with the robust complexes of cobalt and chromium. The most regrettable aspect of this situation is, however, that although better data are non- available, the older values are still frequently found in elementary textbooks. A major step forward resulted from the studiec of Bjerruni ( 1 ) who, on the basis of the study of a large number of metal-ammine systenis and a limited number of metal-anion compleses, developed a theory of consecutive stepnise complex formation. The essence of the theory is that when complexes are formed rerersiblg, all the struct urally possible intermediates are formed and are in equilibrium rTith each other A considerable number of workers have developed specialized methods for the study of consecutive complex formation ( l a ) . A very up-to-date and comprehensive conipilation of formation constants by the International Union of Pure and Applied Chemistry ( 2 ) brings out many striking things: Unthought-of complexes turn out to be the most prominent in some systems-for example, Pb(Oiic)A--and commonly accepted complexes turn out actually to be essentially nonexistent in otherse.g., CdC14-?. The description of the composition of solution containing metal ions and ligand ions or molecules is not a simple matter but may involve the simultaneous presence of two t o four comples species in significant amounts. I n the light of contemporary knowledge, the analytical chemist must revise his thinking about seemingly simple solutions and recognize that frequently they are mixtures of complexes in n hich the highest complex is not at all prominent and that their properties are as often dependent on kinetic as on equilibrium factors. Although metal ion complexes characteristically contain a single “central” metal atom surrounded by the coor-
dinating ligands, this is not necessarily the case. hlany instances of polynuclear complex formation are known, particularly when hydroxyl ion is involved, the hydroxyl being a n excellent “bridging” ligand. Sillen (11) has made extensive theoretical and experimental studies on the polymerization of partly deprotonated aquo ions and shown that for a considerable number of the metals, higher aggregates are obtained even in noticeably acidic solutions. Dimers are most frequently detected, but in some instances chain and sheet aggregates develop. T h a t this is not necessarily a n obscure phenomenon with little bearing on everyday matters is illustrated by the case of iron. In ferric perchlorate solutions, 0.04ilf a t p H 2.5, approximately GO% of the iron is present in the form of the Fe2(OH)2++ion (8). Although robust compleves containing several different ligands attached t o the same central atom have been known since the earliest days of compley chemistry, surprisingly little attention has been given to the possible formation of mixed ligand complexes in labile SJ Ftems. Recent studies a t hIIT indieate that the phenomenon may be a general one. An investigation of comples formation by bismuth in mixtures of chloride and bromide ions showed the formation of every possible species in the series BiCl6-*, BiC14Brd2.. . . through to BiBr6-2 (9))and similar results have been obtained in the mercury halide systems ( 7 ) . Indeed it may not be unreasonable to assume that when a metal is present in a mixture of complexing ligands, all the possible mixed ligand complexes can and nil1 be formed and in equilibrium with one another. Gerald Schn-arzenbach discussed the relationships between metal complex stability and the structure of complesing agents. His discussion follows this report (10). The role of reaction rates in the stability of metal complexes was considered by Ralph G. Pearson. While there is no absolute requirement that the rate of chemical reaction depend on magnitude of the over-all driving force, nevertheless linear free energy relationships involving rates and equilibria are common enough to be considered typical. Other cases may then be considered as atypical. By a simple electrostatic model for coordination complexes, a linear relationship for reactions with a common mechanism is expected. More elaborate models, including the effect of incomplete d shells of electrons, indicate that deviations can occur. Experimental results from numerous rorkers seem to confirm these conclusions. A parallelism between rate and equilibrium can be seriously disrupted by a change in mechanism. One reason for mechanism change is variation in the coordination numbers and geometries
of different metal complexes of a given ligand. For example, consider the following exchange reactions: Substance
Log K
Xi( CY),-* Fe( CY)6-4
22 37 42 44
2$);$;
Rates Fast
Very slow Fast
Very slon-
I n general, tetrahedral complexes (reacting by an S,*-2 displacement) are more rapid than octahedral compleses (which usually react through dissociation by a n SN1process). The proton, considered as a metal ion, gives the greatest effect of this kind and slitcia1 mechanisms for proton transfers are considered. I n a n unsymmetrical complex cc 11taining different ligands, the effect of one ligand on reactivity of another ligand can be very large. This effect need not always parallel stability or instability. A. E. Martell turned attention to the chemistry of the compleses of highly charged metal ions with multidentate ligands. He pointed out that metal ions having charges of +3 and f 4 generally have strong tendcncic,s to hydrolyze and that this somewhat complicatc s the preparation of solutions for titrat ion procedures. This tendency also finds its expression in the hydrolysis of the chclate compounds of these ions n-ith niultidentate ligands-Le., they form hydroso chelates which may then polymerize t o form polynuclear complexes. Thus multidentate chelating agents such as EDTA n-hich give simple titration curves with divalent metal ions intcract n-it!i metal ions of higher chargc, t o give relatively complex titration curl-es. I n the case of iron(III), zirconil.1ni(Il-), and thoriumjIV), the shar;) iiiflcctions v-hich characterize the end points for the formation of normal chelate compounds are interrupted b)- buffer regions resulting from further interactions with base, these reactions being particularly characteristic of chelatr compounds formed with highly charged metal iocs. EDTA and CDTA (cyclohexanedianiinetetraacetic acid) chelates of thorium(1V) form binuclear complescs. n-hile t!w thorium(1V) chelate of DPTA (diethylenetriaminepentaacetic acid) forms only a nionohydroso chelate, I n contrast, the thorium(1V) HEDTA (-Y-hyrlroxyethylethylenediaminetriacetic acid) chelate undergoes olation t o form polynuclear chelates. Some metals, such as zirconiuni(1Y) can react with one or t,n-o additional equivalents of a second multidentate ligand. Particularly stable are the mised chelates with one mole of E D T A with one mole of Tiron, chromotropic acid, 8-quinolinol sulfonate, and acetylacetone. RIartell concluded that fivemembered chelate rings are the most stable in zirconium chelates, and that VOL. 32, NO. 1, JANUARY 1960
3
zirconium(1V) usually attains a characteristic coordination number of eight in its aqueous complexes under favorable conditions. Richard S. Juvet discussed the application of optical rotation measurements to the determination of the structure and stability of optically active metal chelates. A plot of observed optical rotation us. p H for various molar ratios of metal ion to ligand (keeping the total concentration of ligand species constant) is a n especially useful method for establishing the p H range in which each chelate exists in appreciable amounts. In general, the free ligand, salts of the ligand, and metal chelates have different characteristic molar rotations. Equations of the optical rotation-pH curve have been derived. These equations are useful in determining not only the ionization constants of metal chelates and of optically active weak acids and bases, but also the number of hydrogen ions liberated per mole in chelate formation. A continuous variation study in which observed optical rotation is plotted against mole per cent metal ion, maintaining the p H constant, was shown t o be useful in establishing the empirical formula of spectrophotometrically transparent, optically active metal chelates. Chelates of lead, calcium, and magnesium with gluconic acid and of lead and cadmium with N-methylglucamine have been studied by this technique. I n addition t o the 1 t o 1 calcium gluconate species which exists in the p H range 2 t o 11, two other calcium gluconate complexes were shon-n t o existone in the p H range 11 t o 13; the other, a t p H ralues greater than ca. 13. These complexes have not previously been detected by other methods. Polarography of metal complexes was presented by Robert L. Pecsok. The usefulness of complex ions in polarographic analytical methods is well known. RIost commonly, complexes are used to shift half-wave potentials of metals so t h a t mutually interfering waves can be separated. Often addition of complexing agents drastically improves the shapes of poorly defined irreversible wives. They are also useful in preventing the precipitation of certain metal ions a t high pH. Through indirect methods, certain nonpolarographically active substances can be determined. Polarographic data are also useful in establishing optimum conditions for large scale electrolysis at macroelectrodes. Analytical chemists have not overlooked the possibility of using polarographic data to study the nature of complex ions, their properties, structures, stabilities, and reactions. Diffusion currents can be used analytically to measure the concentration of one or more species in a n equilibrium mixture. 4
ANALYTICAL CHEMISTRY
Limiting currents not controlled by diffusion can be useful in examining reaction mechanisms and rates. Nore generally, the shift in half-wave potential is studied as a function of concentration of various species participating in the electrode reaction. Often a simple interpretation of the data yields the nature and formation constants of the complexes present. Recently more complicated examples have been explained, involving kinetic currents, film adsorption, and secondary reactions and nonadditive currents. Recent examples illustrating most of these ideas were presented with particular emphasis on E D T A and hydroxy acid-type complexes. Henry Freiser reviewed the applications of chelating agents to various types of separation processes involving metals. It is helpful both in understanding current applications of these reagents and in predicting future trends of chelate utilization t o outline the factors governing the formation and properties of metal chelates. The role in chelate formation of such structural parameters of chelating agents as basicity, electronegativity of bonding atoms, and steric considerations, as well as those pertaining to the metal ions, such as ionic potential and availability of bonding orbitals, was discussed. The classification of chelating agents on the basis of charge type and the effects of structural modifications were reviewed in the light of the applicability of these agents to the various types of separation processes : precipitation processes, solvent extractions, partition chromatography, and ion exchange separations. The complexes in cation exchange separations were discussed by James S. Fritz. The requirements for using complexing agents t o effect three important types of ion exchange separation of metals were reviewed: displacement chromatography, elution chromatography, and selective displacement. I n the latter, the column is eluted with a complexing agent under conditions such that one metal or group of metals is quickly eluted from the column without appreciably affecting the other metal ions adsorbed on the column. The advantages of this method vere discussed; the following are examples of some separations using cation exchange resins and selective displacement techniques. Eluted Mo Pb Fe
U Th Ti, Fe
Agent Citrate Citrate Pyrophos-
phate Oxalate EDTA EDTA
Retained Fe, Cr,Ni
Ba Mn
pH 1 5.6 3.4
R. E., Cd R. E.
2.1 2.1
Zr
0.6
To effect separations by selective
displacement, eluting agents must be selective in their complexing action. Although one way t o improve selectivity is by careful control of pH, constant p H is often difficult to maintain during passage through the ion exchange column. “pH of the aqueous phase and the p H of the resin” are maintained by buffers. The relative affinity of cation exchange resins for various cations plays a n important role in separations. Cations of higher charge are usually much more strongly retained by the resin than those of lower charge. This prevents some separations that would otherwise be possible using the EDTA method. However, the valence effect is used to advantage in a chromatographic method whereby metals of different charge are separated by elution with an ethylenediammonium salt. Separation of metals by anion exchange was discussed by K. A. Kraus. The method in a sense is complementary t o the cation exchange method, discussed by Fritz, where elution of metals is achieved by forming nonadsorbable complexes. I n the anion exchange method, the complexes (negatively charged) are adsorbed by the exchanger and eluted by changing the medium sufficiently to cause dissociation of the complexes or a decrease in the fraction of the metal as anionic complex in solution. For reasons not clearly understood, the commercial anion exchange resins show enormous selectivity for some complexes and separations are feasible a t very high ionic strength nhere many other methods fail. Adsorption functions were presented for a large number of metals in a variety of media, from which it is feasible t o deduce conditions for adsorption and concentration of traces and for separation of metals from each other. A number of such separations were described. For typical examples, see
(sJ4 )*
Adsorption of a metal by an anion exchanger does not establish that it carries a predominant negative charge in solution and studies of adsorbabilities as a function of ligand concentration are necessary to determine the fraction of the metal converted to negatively charged complexes. Conversely, these studies permit deductions regarding the species existing in solution and the pertinent solution equilibria ( 5 ) . D. H. Wilkins described some applications of ion exchange resins and chelometric reagents to metallurgical analysis. By combining the polver of anion exchange separations and the sensitivity of metalfluorechromic indicators in EDTA titrimetry, Wilkins was able t o analyze several rather complex metal ion mixtures. The impressive separations and analyses given included:
Li. Me B,‘ME A1 Cu, Pt Fe, Pd
Ni; Au Mn, Ru Cu. Pd a , ’ f i < C o , Mn All Zn, Cu, Fe
Ni. Co., Fe., Zn in fer-
Ates Fe, Co, V Fe, Co, Mn Co in steel Mn in ferromanganese Trace Co in Ni All Nil Co, Cu, Fe All Ti Ni, Cr, Co, Fe, Mo, W,
Nb,Ta
Dale W. Nargerum reported the possibility of separating metal ions through coordination kinetics. The inertness or lability of complexes may allow separation of one metal from another, whereas such separations may not be possible on the basis of thermodynamic stabilities. To effect this, the actual physical separation of different complex ions must be fast compared to the coordination rate'. Liquid extraction, the batch use of adsorbents, or rapid flon adsorption columns can provide the necessary fast techniques. Ion exchange resins are particularly adaptable to separations of this type because the change of ligand coordinated to a metal ion may reverse the charge of a complex. Such a gross change does not require a n adsorption material hich distinguishes subtle differences in properties and makes rapid separations possible. To achieve reactions which are sufficiently slon~ with common metal ions, it is necessary to choose strong complexing ligands such as 1,lO-phenanthroline, (ethylenedinitri1o)tetraacetate ion, cyanide ion, triethylenetetramine, and other polyamines. The polydentate ligands are of particular interest because of their increased tendency toward sloa er reactions. Both the formation of complexes from the aquated metal ions and the dissociation of complexes in acid have been used. Exchange reactions between other neutral and negative ligands are useful. To slon~ down formation reactions, prior equilibria with labile Complexes or ~ i t hion exchange resins can be used. Acid catalyzrs the dissociation rate of many polydentate complexes, making this approach to separations of limited usefulness. Similarly, some ligands speed up thr removal of other complexed ligands. While these effects complicate the prediction of separations, they also aid in selectivity. I n addition, the adsorption material may also catalyze or inhibit reactions. Thus the adsorption of the 1,lOphenanthroline complexes on a cation exchange resin inhibits their acid dissociation rate. Xery definitions for masking and demasking were proposed by K. I,. Cheng. Masking may be defined as a process in which an element or compound is changed to such an inactive form without separation that certain
reactions are prevented; demasking, as a process in which a masked element or compound is released and regains its activity to enter into certain reactions. A new term, “antimasking,’, was proposed; it may be defined as a process in which an element or compound is made resistant to masking action by its change to a more stable form. Another new term, “antimasking ratio,” was proposed for evaluating or predicting the equilibrium of masking action and antimasking action or demasking action involved in a system. It may be defined mathematically as follows: Antimasking ratio (4MR)
)2 = (Ph& __
PPYI,
where pi& and p& are negative logarithms of the metal ion concentration dissociated from a metal-antimasking agent, and a metal-masking agent in standard state a t a concentration of 1mole per liter, respectively. The theoretical treatments for masking and antimasking were made. The equations permit explanation and calculation of the amounts of masking agents, demasking agents, or antimasking agents required for the purposes. According to the calculated ilhIR values and the results shown in two tables, a n ARlR value of 7 or above is required for obtaining proper antimasking action. A number of examples of masking, demasking, and antimasking were illustrated in periodic table forms and other tables. The precipitation of metals in the presence of EDTA a t various p H with common inorganic precipitants such as hydroxide, halides, sulfate: phosphate, tellurite, and ferrocyanide was reported. Other results employing organic precipitants or color-forming agents in the presenceof EDTA, tartrate, or cyanide were also reported. Several new specific reactions were accomplished through the use of masking actionfor example, thallium gave a yellow precipitate n-ith Bismuthiol I1 in the presence of EDTA, tartrate, and cyanide a t p H 6 to 8; manganese(I1) formed a yellowish precipitate with 2thenoyltrifluoroacetone (TTA) in the presence of tartrate a t p H around 10; and only silver gave a yellowish brown color reaction with dithizone in the presence of EDTA and cyanide a t p H 4 to 6. The masking and demasking actions of reactions of zirconium and hafnium with Xylenol Orange were reported. The fluoride, which masks the reactions of zirconium or hafnium with Xylenol Orange, could be demasked by addition of beryllium. A remarkable masking action was illustrated: Hafnium may be determined in the presence of limited amounts of zirconium n ith Xylenol
Orange using hydrogen peroxide as a masking agent. Some conditions for obtaining successful masking, demasking, or antimasking were discussed. Among them, the selection of suitable complexing agents and pH, and the combined use of more than one masking agent were stressed. Donald H. Wilkins discussed the application of fluorescent indicators to chelometric titrations. Previously the development of nerv indicators has been confined principally to the modification of visual p H indicators by the addition of a chelating functional group. This concept has now been extended successfully to fluorescent p H indicators. The intense color of the complexes formed by inany metal ions and the chelometric reagent inhibits the use of visual indicators. I n contrast to visual indicators, the fluorescent indicators are unaffected by the color of the solution during the titration. These indicators have now been applied in many chelometric titrations of metals which form the highly colored complexes, such as macro amounts of copper, nickel, cobalt(11) and (111), and chromium. The fluorescent indicators are also advantageous for microtitrations. Because the eye is more sensitive to the presence or absence of light than to changes of color, one may use much smaller amounts of indicator and consequently obtain a n insignificant blank. An early observation was made that tn o kinds of metal complexes are formed n-ith these indicators. I n the “normal complex” the fluorescence of the free indicator is quenched by the formation of a metal-indicator complex. I n the second type of reaction a metal-indicator complex is formed (indicator reversal complex) which fluoresces a t a high pH, n-hereas the free indicator does not fluoresce. TKO commercially available indicators are Calcein W and Calcein Blue. Calcein IV is a condensation product of fluorescein, iminodiacetic acid, and formaldehyde. Calcein Blue is a condensation product of 4-methylumbelliferone, iminodiacetic acid, and formaldehyde. The wave length of optimum activation of fluorescence has been used to determine the acid ionization constants and the metal indicator stability constants for metal fluorechromic indicstors. These indicators have also been used as fluorescent reagents; mole of metal ions can be easily detected by visual observation of the fluorescence. Claude A. Lucchesi reported that the use of E D T A as a universal titrant has particular application in the paint industry because many important materials contain only a single determinVOL. 32, NO. 1, JANUARY 1960
5
able element. It was felt that in the near future EDTA will be virtually the only titrant necessary for the quality control testing of driers and pigments. Elements in mixed pigment systems are determined without prior separation by proper choice of p H and masking agents. The most important example given involved the production control testing of paint driers, metallic salts of carbosylic acids. The organic acid salt is dissolved in alcohol-benzene solution and escess aqueous E D T A is added. The unreacted EDTA is then backtitrated with zinc t o the Eriochrome Black-T end point. The metal contents of the octoate and naphthenate salts of calcium, lead, cobalt, manganese. and zinc can be determined in 10
minutes with the same accuracy obtained by the much more time-consuming ASTM methods (6). The new method is one of the few examples in which the EDTA titration is used in a largely nonaqueous medium. LITERATURE CITED
(1) Bicrrum. J.. “Metal AiniminPFormx‘ b o g in Aqueous Solution.” P. Haase and Son, Copenhagen, 1941: Chenz. Reis. 46,
381 (1950).
( 2 ) Bjerrum, J., Schwarzenbach, (3.. Sil-
len, L. G., “Stability Constants. Part 11. Inorganic Ligands.” The Chemical Society, Eondon, i958. (3) Kraus, K. .4.,Selson, F , “Anion Exchange Studies of the Fission Products,” Proceedings of International Conference on Peaceful Usee of Atomic Energy,
Geneva 1955, Vol. 7, pp. 113, 131, United Nations, 1956. (4) . , Kraus, K. A., Nelson, F., “Metal Separations by Anion Exchange,” Symposium on Ion Exchange and Chromatography in Analytical Chemistry, 1956: .4m. SOC. Testing Materials. Spec: Tech. Publ. 195 ( 195%);d (5) Kraus, K. A., Nelson, F., Structure of Electrolytic Solutions,” W. J. Hamer, ed., p. 340, Wiley, New York, 1959. (, 6,) Lucchesi, C. A , , Hirn, C. F., A N ~ L . CHEX 30,’1877(i958). ( T ) Marcus, Y., Acta Chem. Scand. 11, 329, 610, 811 (1957). (81 Mulay, L. Y., Selwood, P. W., J . A??&. Cherri. SOC.77, 2693 (195.5). ( 0 ; Xexmnn, I,., Hume, D. Ti.Ibid., , 79, 4571. 4.581 (1957). (10) Pchn-arzenbach, G.! .i;\..i~. CHEX 32, ti (1880r. (11) Gillen, L. G., Acta Chein. Scand. 8, 299 (1954). (12) Sullivan, J. D., Hindman, J. C., J . Ani. Cheni. SOC.74, 6091 (1952).
Relationships between Metal Complex Stability and the Structure of the Complexing Agents GEROLD SCHWARZENBACH EidgenGssische Technische Hochschule, 6 Universitufstrasse, Zurich, Switzerland With the exception of oxidation and reduction, the reactions shown by metal cations in aqueous solution are all substitution reactions by which water molecules from the solvation shell are replaced by other ligands. Unidentate (ammonia, acetate), chelating (ethylenediamine, oxalate, glycinate), and bridging ligands (hydrazine, OH-, SF2, C03-*, are distinguished. The last ones are rnultidentate like the chelating ligands because of steric reasons; however, they are unable to satisfy several coordination sites on the same individual metal ion. Instead, they add more than one cation, tying them together; they form polynuclear complexes and finally, insoluble precipitates.
T
most important points for the characterization of a ligand are the nature and basicity of its ligand atom. For aqueous solution chemistry, only the atoms of the halogens (ligands: F-, C1-, Br-, I-), oxygen (ligands and ligand groups: OH-, C03-*, SOC-’, Po4+,aliphatic and aromatic R-OH and R-0-, ether oxygen, keto oxygen, carboxylate oxygen), sulfur (ligands and ligand groups: HS-, S+, mercaptan sulfur R-SH and R-S-, ether sulfur, keto sulfur, mono- and dithiocarboxylate sulfur), nitrogen (ligands and, ligand groups: ammonia and organic amine nitrogen, nitrogen of Schiff HE
6
ANALYTICAL CHEMISTRY
bases, amides, nitroso groups, and azo groups), and carbon (cyanide) are iniportant. Comparison of Coordination Tendencies. The tendency of the Larious metal ions t o substitute a molecule of n a t e r with a ligand group can be inyestigated a t present n i t h the aid of complex stabilities. The values 1 of log K 1 or ;log Pn can be compared with K1 as the stability constant of the 1 to 1 complex and Pn the over-all stacomplex bility constant of the 1 to with an unidentate ligand. Chelate complex stabilities may also he conipared if due consideration is given to the chelate effect and the possible strain within the chelate rings. I n the case of bridging ligands, solubility products of precipitates having the same type of crystal lattice can be used for comparison. Furthermore, with precipitates in equilibrium there are usually mononuclear complexes existent within the solution. Their stability constants can be obtained with the aid of solubility measurements and then it is possible to compare again the log K1 values so obtained. General a n d Selective Complexing Agents. A comparison shows t h a t oxygen donors and fluorides are general complexing agents, combining with a n y metal ion with a charge more t h a n one. Acetates, citrates tartrates, and 8-diketones sequester all metals in general, and hydroxides,
fluorides, carbonates, and p1iosphntc.s are common precipitating agents. T11~ strength of the coordinating bond formed increases enormously with the charge of the metal ion and decreases with its radius, which is known as elecB>- comparing trovalent behavior. various oxygen donors the observation is made that bond stability increasts regularly with the basicity (measured by proton addition) of the ligand atom. Cyanide, heavy halides, sulfur donors, and to a smaller extent the nitrogen donors, in contrast to oxygen donors, are selective complexing agents. These ligands do not combine with the cations of A metals (having a noble-gas electronic structure). Only B-metal cations (having 18 outer electrons) and transition-metal cations are coordinated to carbon, sulfur, nitrogen, chlorine, bromine, and iodine, and the solution stabilities of the complexes show that charge and radius of the metal ion no longer are the dominant factors for bond strength. A nonelectrovalent behavior exists, comprising the formation of covalent bonds and possible crystal field stabilization. n hich is most pronounced Kith cations of noble metals of low charge (the electrovalent behavior being as small as possible), such as CuT, Ag-, and du+. The largest of these three ions forms the most stable bonds, and by comparing the stabilities of the halogen complexes the series is found to be F-
