The Effect of Ligands on Hydrolysis
Jack I. Morrow The City College of CUNY New York, New York 10031
Constants of Transition Metal Ions
For many years, introductory general chemistry books have failed to emphasize the effect that ligands (in the inner coordination sphere) have upon the chemical behavior of transition metal ions. Indeed, most books fail to recognize the existence of the inner coordination sphere except in those sections on amphoterism and instability constants. With the advent of ligand field theory and its inclusion in some of the newer general chemistry textbooks, the shortcomings of these earlier textbooks have in large measure been overcome. The student, for the most part still remains unaware of the nature of the various species present in aqueous solution particularly, polynnclear complexes. The results obtained with Cr(CIOa)lin solution clearly reveal that treatment of Cr(H20)?+ as a monobasic Brhsted Acid is inadequate to account for the quantitative experimental results. Therefore other species must be present, the principal ones being the polynuclear complexes. The reactions that occur may be summarized as follows c~(H,o)~" I c~(H,o~,oH* + H+ K~ (1)
subject (3) he compares thc titration curves for a typical weak monoprotic acid with an aquo-ion acid and ultimately states that "it is difficultto escape the conclusion that polymcrs must be present in solution." Reaction (4), the overall dimerization reaction, is the combination of reactions (1) and (3) with KO(= Kh2K,) = 2 X 10W4. That bridging occurs through two hydroxo- groups a t the experimental conditions has been shorl-n by G. Thornson (5) and Morrow and Levy (6). Cr(II1) does form an homologous series of polynuclear complexes conforming to Sillhs hypothesis of Cores and Links (7) with adjacent chromiurns being bridged by two hydroxo- groups. Higher polynuclear complexes beyond the binuclear complex (reaction (4)) are formed. This was shorn by G. Thomson (5) using ion exchange. If one neglected all reactions but reaction (I), the first hydrolysis, then the apparent hydrolysis constant, K,, is calculated by eqn. (5)
where [H+] is the hydrogen ion concentration determined with a pH meter, and T , is the total Cr(II1) concentration. Values of Kh obtained by this equation are approximate since the other reactions have not been taken into account. However, if these approximate (or apparent) K, values are plotted against T , and the value of Khat T M = 0 is obtained by extrapolation, then the effectof reactions (3) and (4) are eliminated and the correct Khvalue is obtained. The contribution of reaction (2) can be neglected under our experimental conditions because of the large separation in value of K, and Kz . Experimental Determination of TM
2H,O
+
2 ~ ' K,
(4)
Reactions (1) and (2) are the first and second hydrolysis reactions respectively with K, = 1.5 X 10W4and Kz = 1.5 X lo-" (1, 2 ) . I n partially hydrolyzed solutions the equilib[C~Z(H~O)~(O> H>) ~[Cr(Hz0)50H2+], ~+] rium constant for the t m c dimerization reaction is K , = lo4 ( I ) . It isinteresting to note that when mrtal ions do form polynuclear complexes the value of K , is usually large being in excess of K, = lO"5')). This minimizes thc presence of any species containing OHgroups not involved in bridging to form higher polynuclear complrxes when an rquilihrinm distribution of species is achieved. This point of view was also put forward by L. Pokras (4). In an excellent revien. of the 748
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Journal o f Chemical Education
The Cr(II1) concentrations were measured by acidifying an aliquot portion of a stock solution, (from which all dilutions were made) with 2 M HCIO, to prevent the formation of polynuclear complexes, and measuring its absorbance. Its concentration wes then determined fram a Beers Law curve obtained from the reduction of Cr042- with H102 in 2 M HCIO,. The Beers Law curve was prepared at 408 nm using a Beckman DU Spectrophotometer.
pH An L and N p H meter was used to measure the pH of the solutions with no acid or base added. This gives p H values to 0.01 pH units with an accuracy of 1 0 . 0 1 pH units. Preparation of Aged Cr(ll1) Solutions
Chrominm perchlorate (obtained fram K and K laboratories) solutions were prepared having concentrations of about 0.2 X 10-= M, 0.4 X 1 0 V M, 0.8 X Af, 1.5 X 1% 2.3 X 10-% and 3.2 X 110%M all concentrations being accurately known. Neither acid nor base was added to any of these solutions.
The ionic strength in all sollltions was set equal to p = 1.0 M using NaCIO,. These solutions were then allowed to age for about three months at 25%, as the Cr(II1) ion is slmost inert in its substitution reactions.
The figure shows the results of plotting K , (apparent) The extrapolated value of K , (= 1.7 X versus T,. is in good agreement with that obtained by Postmus and King (S), Kh = 1.5 X Literature Cited
Plot of
Kh lopparent)
TM.
(1) EARGET. J. E.. AND CANNON.K. D. in "Transition Metal Chemiatry. A Series of Advaneea," Val. 1 (Editor: C*nmw, R . L.),Marcel Dekker Ino.. New York. 1965. (2) POSTMTIB, C., A N D KIN., E. L., J . Phys. Chem., 59, 1208 (1955). (3) P o s n ~ s L.. . J . CnEnr. Eono.. 33,223 (1956). (4) P o a n ~ sL ,.,J . C x e ~Eonc.. . 33, 152 (19561. (5) Tnonraon. G.. AEC Aooession. No. 35255. Robert No. UCRL-11410 University of California. Berkeley, 1964. (8) Monnow. J . I.. AND Levr. J . , J . P h y s . Chem.. 72,885 (19681. (7) SIFFEN,L. O., At1.z Chem. Scond., 8, 299, 318 (19541.
Volume 49, Number 1 I
, November 1972
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