Translator
Heinrich Ley
George 6. Kauffman
California State University, Fresno Fresno, California 93710
On Inner Metal-Complex Salts.
The copper salts of aliphatic amino acids, e.g., of glycine NH2.CH2.COOH and its homologs, are well known as characteristic salts of these acids. Because of their remarkahle r o l d in aqueous jolutlon and h ~ r a u s eof their r)artiallv fa\.orahle solubility relationships,' these salts have long been used for the detection and separation, respectively, of the hydrogen compounds.3 In this communication we are to investigate such compounds somewhat more closely from a physicochemical viewpoint and a t the same time to give a plausible explanation of their constitution. As is the case with most salts of heavy metals, the dissociation of the copper salts is very strongly influenced hy the strength of the anion.* As will he shown in a later communication in another place, the electrolytic dissociation of the comer .. salts clearlv decreases with decreasing strength of the acids as, e.g., in the series: CIZCH.COOH; ClCH..COOH. , CH.COOH. CH.CHvCOOH. ~ h u i as , in other cases, the dissoc~ationof the salt runs parallel to that of the acid. It was therefore to he expected that the copper compounds of the amino acids a& poor electrolytes sinre the hydrogen rmnpounds such as glyrine are extremely u,eak aridi ( 1 1. 'l'hat copper g1yc;nste [(;I\ c o c < ~ / /.k.l . r.~ fcan < r l ~be m l y very. ilinhtly .~dissoc~ated in aqueous solution may be -deduced, among other things, from measurements of freezing point depressions in aqueous solutions of these substances, which originate with Curtios (2) and which I reproduce here in his presentation Percentage Strength of Solution
Freezing Point Deoression
I
the imide hydrogen atom would he replaced by the metal ,NHCH&OOH
Cu\
NHCH,COOH
This conception of the constitution of copper glycinate is preferred by Curtius (4), most probably on the basis of the observation that upon alkylation of this salt amino acids substituted on the nitroeen are formed. But this conception does not seem to h e probable if only because the amino acids which have been substituted twice on the nitrogen, e.g., RzN.CHzCOOH, yield copper compounds (R2N.CH2COO)zCu which are completely analogous to the salts of the nonsnbstituted acids, e.g., in aqueous solution they possess an equally intense dark hlue color and also resemble copper glycinate with respect to their dissociation relationships. I have investigated somewhat more closely the copper salt of diethylaminoacetic acid (C2Hs)zNCHzCOOH (51,which was obtained from barium diethylaminoacetate and copper sulfate and dried over phosphorus pentoxide. The determination of the freezing point lowering, calculated for undissociated salt, gave values about 15% too high, a fact which, however, can he satisfactorily explained a s due to experimental error-only a small amount of material was available. Determination of the electrical conductivity proved that only a n extremely small electrolytic dissociation can be present.
Molecular Weieht
Molecular weight of Cu(C0zCHzNHz)z: 195.6.= As may be expected, the electrical conductivity of the salt is also slight. The equivalent conductivity (reciprocal R)? a t 25°C is as follows
This salt too has a deep hlue color in aqueous solution; as observation of the absorption spectrum shows, the l/32 equivalent solution in a l-cm thick layer transmits more red rays than a solution of copper glycinate under the same conditions. Translation of "fiber innere Metall-Komplexsalze.1," Zeitschrift f ~ rElektraehernie, 10, 954(1904). Communication from Chernisehes Institut der Uniuemitiit L e i p i g . All footnotes are
At higher dilutions, the A values change markedly with time. which orohahlv indicates a slight decom~ositionof the At. In addition to the most obvious formula for this salt
with a metal-oxygen bond, which K. Kraut (3) in particular has favored, another constitutional picture can he considered. It would he conceivable that upon salt formation not the hydrogen atom of the carhoxyl group but rather 698
/
Journal of Chemical Education
translator's notes. IIn aqueous solution, the color of these compounds is deeper and more intense than ordinary capper(I1) salts. 2Many of these eompounds are extremely insoluble in water. It is an irony of history that the compound discussed in this paper, which led Ley to propose the concept of inner complex salts, is not typical of the majority of eompounds in this class in that it is freely soluble in water. That is, the parent amino acids. More correctly,the strength of the parent acid of the anion. In modem nomenclature, bis(glyeinato)eopper(II). The correct molecular weight is 211.7. 'When a potential difference of 1 V is applied to a centimeter cube af a conductor, the current in amperes which flows is equal to' the specific conductance, x , in ohm-' em-' units. For electrolytes, the equivalent conductance, A = m,/lMH), where u, is the dilution in liters/equivalent. represents dilution in liters/equivalent; A represents equivalent conductivity.
The copper salt of piperidoacetic acid- C5Hlo.N.CHzCOOH9 seems to behave quite similarly. Accordinelv. the assumntion of a metal-nitroeen bondlo in copper &inate is highly improbable a n d i s directly contradicted bv the followine observations. If ammonia is added to copper glycinate in aqueous solution. scarcelv anv chanee in the blue color can he ohserved; only with a - g e a t excess of ammonia does the color become somewhat darker. However. a distribution e x ~ e r i ment can easily prove that bonding with ammonia has taken place. The distribution coefficient of ammonia hetween water and chloroform a t 20°C and a t ammonia concentrations C which varied between 0.76 and 0.2 molar" was found to have the value 25.2 = C H ~ O I C C H C I ~ Now if copper glycinate, which is insoluble in chloroform, is added to the aqueous phase, then the distribution relationship is displaced in favor of the aqueous phase, as the following experiment shows Total Concentration of NHa:0.766.
-
Definitions: C1 = moles of copper glycinate; C2 = moles of NH3 bonded in the aqueous phase; c l = ml 111 NNHs contained in 50 ml of water; CI = ml 111 NNH3 contained in 50 ml of chloroform; k = C2/C1 = males of NH3 which are bonded by one mole of copper salt. The formation of an ammonia compound is of course comprehensible on the basis of both conceptions of the constitution of copper glycine. If a copper-nitrogen bond were present, then the ammonium salt of a copper amidoacetic acid could be formed
secondary valences) [Nebenualenzen]. Now the assumption is obvious that in copper glycinate too the amino groups are bonded to the copper atom by secondary valences
according to which copper glycinate appears specifically to be the analog of copper-ammonia acetate [Kupfemcetat- Ammoniak]
0.CDCH1.NH3 'Whereas, according to its composition, copper glycinate seems to be an ordinary ternary salt such as copper acetate Cu(CzHsOz)z, according to its properties, especially its color in aqueous solution, it is a complex salt. Since the group which determines the complexity of the metal salt, in our case NHz, is a constituent of the anion of the salt COzCHzNHz, the copper salt of aminoacetic acid can be viewed as an inner metal complex salt [inneres Metallkomolexsalzl.l5 The ohservatibn already mentioned, uiz., that on addition of ammonia to c o p ~ e rnlvcinate the color of the solution shows no substan&l change, proves that undissociated copper glycinate Cu(0COCHzNHz)z and the ion [Cu(NH3)n]--l3influence the light absorption in a similar manner. I found another example which illustrates the relationships between the salts W R, Cu. / RNH, and the actual complex salts R', ,.NH,
'..
According to the other conception, in the ammoniacal solution of the salt there should he formed a complex salt whose anion would he [ N H Z C H ~ C O Zand ] ~ .whose ~ ~ cation would he [Cu(NHa)n].. (6).13 That the last conception is the only correct one was proved by qualitative transference experiments. The experiments were carried out in the well-known Nernst apparatus (7). Two solutions of 0.87 g and 0.40 g of copper glycinate in 50 ml of ammonia (C = 0.766), over which less dense solutions of potassium sulfate in ammonia were arranged in layers. The electrolysis was performed a t about 25" at a potential of 110 V; the amperage fluctuated between 35 and 40 mamp. In both cases, after only a few minutes the blue layer was seen to migrate to the cathode, from which it is definitely to be concluded that the cation [Cu(NH3)nI13 is present in the ammoniacal salt solution. The number n, which expresses the composition of the complex cation, may be calculated by well-known methods, e.g., from a large number of distribution experiments as described above (8). As has been emphasized repeatedly, copper glycinate, with respect to the color of its aqueous solution, exhibits a remarkable similarity to the copper-ammonia salts, in which the light absorption in aqueous solution is determined by the presence of the complex salt Cu(nNH3)Xz as well as of the c o m ~ l e xion ICu(nNHn)l--.I3 In accor-. . dance with Werner's (9) concel;tion,'4 the complex cop~er-ammoniac o m ~ o u n dCul2NH?)X2 has the constitu-
c u:
"NH, in copper phenylglycinatel6 and the aniline complex salt of copper acetate.'T Copper phenylglycinate, which is best R,'
'OThat is, a copper-nitrogen primary valence (Hauptualpnz) hnnd
"The GermanMole (moles)is here translated as "molar." lZ [NH&H2COO]l3 [CU(NH~)~]Z+ l4 For an Enelish translation of the . oaDer . referred to bv Lev. see Kauffman, G . B . , "Classics in Coordination chemist&, PHA 1. The Selected Papers of Alfred Werner," Dover Publications, New York, 1968, pp. 5-88. Inasmuch as this type of compound is a neutral species rather than a salt. Marcel Delepine later suggested the shorter and more accurate term "inner complex" [Taube, H., Chern. Reu., 50,69 (1952)l;some authors prefer the term, "neutral complex." 16 r
in which the ammonia molecules are thought of as honded to the metal atom by secondary affinity forces (so-called Volume 50, Number 70,October 1973
/
699
obtained as a difficultly soluble salt from the barium salt of phenylaminoacetic acid and copper sulfate, dissolves in water to give a grass-green color. A similar color appears when a dissociated copper salt is treated with aniline; under certain conditions it is also possible to obtain from copper acetate and aniline a well-defined solid complex salt which dissolves in water to give an intense green color. For the preparation of the salt, pure, freshly distilled aniline is added to a cold saturated solution of copper acetate; the first drop immediately produces an intense green coloration, and on further addition of aniline the solid salt precipitates. It is quickly filtered off by suction. since otherwise it decom~oseswith formation of a dark coloration; it is probably to he assigned the compositton C U ( C ~ H ~ O ~ ) ~ ~ C ~ H ~ K H H ~ ~ ~ I H ~ O l8The position
of these numbers should be reversed. 1BLev was later to write a monoeraoh entitled "Beziehuneen " zwisehen Farbe und Konstitution bei orgonixhen Verbindungen"
".
(Relations between Color and Constitution among Organic Compounds), S. Hirzel, Leipzig, 1911. ZoAtti R. Accnd. Lineei, 13, 5, ii 26 (1904). Bruni and Farnara prepared and examined the capper and nickel salts of glycine, aalanine,, o-aminoisobutyric acid, leucine, aspartic acid, and o-, m-, p-aminohenzoic acids. They concluded that the capper salts of aliphatic amino acids differed from mmt other capper salts in possessing their own bluish violet color that is not changed by addition of ammonia. Since the solutions gave few of the reactions characteristic of Cuz+, they assumed that this ion was present only in extremely low concentrations, and they proposed as alternative structures for the glycinate
For discussions of Giuseppe Bruni's life and work see Rollier, M. A,, Scientia (Milan), 82, 23(1947); J. Amer. Chem. Soc., 71, 381(1949);Schidrawitz, P., India Rubber J , 117, 1031(1949);Rubber Chem. and Technol., 23,303(1950).
700
/
Journal of Chemical Education
0.2844 g of substance: 0.0565 g of CuO 0.4984 g of substance: 0.0936 g of CuO calculated 15.7% Cu, found 15.01.18 15.89%'8 Cu Over sulfuric acid, the salt apparently loses aniline and water. Accordingly, certain properties of some salts, e.g., the color, can be explained by the assumption that certain radicals present in the anion of the salt exert a n influence on the metal etom similar to that exerted by molecules found coordinated to the metal atom among actual complex salts. (In addition to the NHz- and NHR- groups, respectively, of the above-mentioned examples, still other radicals mav he involved.) At the same time. the develooments represent a n extension of the universally fruitfh Werner theorv and simultaneouslv a contrihution to the relatively lir;lc investigated relationships hetween the constitution of chemical cwnpnunds and their color.'" (Received: December 6) Addendum to the proof. Herr Professor [R.] Abegg has kindly called my attention to the work of G. Bruni and [C.] Fomara which appeared in July of this year (Rend. d. R. Acad. d. Lincei Roma, XIII, 5a, 26),20 in which a completely analogous constitutional formula is given for copper glycinate, without further proof, to he sure. I have already presented the most essential results of my work a t a session of the Chemische Gesellschaft in Wurzhurg about 1%yr ago. Herr Dr. E. Holzweissig has already, a t my request, determined the conductivity of copper glycinate (Dissertation Wiirzburg). Ley's Citations (1) See espaeislly Bredig. (0.1,2. Ekktrochem.. 6, 34 (33-37(189911:Winkelblech. [ K ) , 2.phyaik. C h m . , 3 6 . 6 4 11190111. (21 [Curtius.T..and Sehulz. H..lBer. 23.3041 l(189011. (3) [Goldberg. B.. Kunz.P., andKraut. K.,IAnn.,266,304[1189111. 1" )ICurtius.T..lJ. p m k l Chem.. 121 26.16311188211. (5) A"".. 145. 222. [This r e f e m m ia inemreet and should probably read Erlenmeyer. Jr., E..and Bade, F., Ann., 377, 222(1W4l.1 Details of the preparation. ete.. of thesalt inquestion will lollwin another place. (61 The masiblo formation of basic or com~lieatadcomplex sale is not being considered in thisgeneral diseu33ion offhesituation. (7) [Nemst, W . l Z Eisktioehrm.. 3.3(18/(1897)1. (8) For copper-ammonia complexes. M P Oawson. [H.MI. and MeCrac, IJ.1, J Chem. Sot. 1771, 1239119Ml;Banadofl. W.12 A w r g Chem.. 41, 132[(1W411. 191 IWerner.A..JZ. o m r g Chem., 3. [267(189311.
.
