COORDINATION DISPROPORTIONATION EQUILIBRIA IN SOLUTION
between these and other molecules in the pigment complex in vivo. However, our results-in particular, the absence of a noticeable band shift even after 90 min of sonication-suggest that the effect of such “chemical” changes upon the spectrum are minor compared to that of the “geometrical” change (the [(sieve effect”). More precise evaluations of the sonication effect will
3549
have to take into account also the other changes caused by sonication. Acknowledgments. We are thankful to Dr. Govindjee for his helpful discussions during this investigation. Thanks are due to Dr. L. AI. Black for permission to use his ultracentrifuge and to Xrs. Ruth St. John for her aid in centrifugations.
Coordination Disproportionation Equilibria in Solution. I. Aluminum Chloride in Acetonitrile
by W. Libus and D. Puchalska Department of Physical Chemistry of the Technical University of Gdahsk, Gdahsk, Poland Accepted and Transmitted by The Faraday Society
(January 6 , 1067)
+
+
Coordination disproportionation equilibria of the type ( k Z)MA, l l l A z - k k + IclIA,+Lzare proposed as a general explanation of the electrolytic properties of some metal halogenides in nonaqueous solvents. The magnitude of the coordination disproportionation constant is shown to depend critically upon the relative stabilities of tetrahedral and octahedral complexes formed in the system. Solutions of AlC13in acetonitrile were examined conductometrically. A complete coordination disproportionation of the solute, according to the reaction 2hlC&(s) 4L -P [A1Cl2L4]+ [illC14]-, was inferred from the limiting slope of the equivalent conductance curve.
+
+
The dissolution of anhydrous CO(NCS)~in acetonitrile had previously been shown‘ to be accompanied by the coordination disproportionation reaction CO(NCS)Z(S)$. 6L
+
[Co(NCS)Lb]+
+ [Co(NCS)3L]-
L being the solvent molecule. The complex electrolyte thus formed consists of a six-coordinate octahedral cation and a four-coordinate tetrahedral anion. Further experiments2 showed that coordination disproportionation, though not complete, also occurs when CoC12 is dissolved in the same solvent. The equilibrium taking place in the resulting solution is
+ 2L
3 [COC12L*]
[COLG]2+
+ 2 [CoC13L]-
The formation of t,etrahedral anionic species [ptlC13L]in dimethylformamide solutions of MnClz, FeC12, CoC12, KiCl2, and CuClz was also shown by Katzin3 and, using his original notation, ascribed to the coordination disproportionation reaction 2°CtAfC12
OCtdIC1+* tet11C13-
Matzin also predicted that solvents of a low base strength but a high dielectric constant should, in general, favor coordination disproportionation of metal halides. (1) W. Lib&, Roctniki Chem., 35, 411 (1961). (2) W‘. LibuB, ibid., 36, 999 (1962).
(3) L. I. Katzin, J . Chem. Phys., 36, 3034 (1962).
Volume 7 1 , Sumber 11 October 1967
3550
W. LIB&
Essentially different equilibria were frequently suggested by a number of author^^-^ in their interpretation of similar systems. Janz, Marcinkowski, and Venkatasetty, for instance, ascribed the spectroscopic and conductometric properties of the system CoC12CH3CN, in contrast to our findings, to the system of equilibria6 2C0C12
+ 6L
[CoC12*3L]
[c0Ls12+ +
[c0c14]2-
Considering these divergent interpretations, we recently extended our earlier experiments to other systems and higher concentration ranges. Several other instances of coordination disproportionation equilibria were additionally found and will be reported in the present series of papers. It seems evident now that coordination disproportionation, as suggested by Katzin, is a reaction frequently occurring in solutions of metal halogenides in polar solvents. In our opinion the properties of many solutions of these compounds may be rationalized in terms of that type of equilibria. I n the present paper a general discussion of factors affecting coordination disproportionation will be given. A presentation of experimental results regarding A1C13 solutions in acetonitrile will follow.
Definitions and Basic Relations Octahedral and tetrahedral complexes of the metal ion were found to be involved in coordination disproportionation equilibria occurring in solutions of cobalt(11) salts. I t is conceivable, however, that other configurations might also be responsible for the occurrence of that phenomenon. Coordination disproportionation involving tetrahedral and octahedral complexes must, obviously, in some way depend upon the relative stability of these complexes. It is necessary, therefore, prior to discussing that problem, to introduce some definitions and relations regarding configurational equilibria. If we assume that mononuclear tetrahedral and octahedral complexes may be formed in a solution of a metal salt MA,, then two series of complexes, namely, [1\Ih,L6+] (octahedral) and [JIA,L4-,] (tetrahedral), must be taken into account, Id denoting the solvent molecule. U'hile some of them may exist in the form of only one structural modification, others may be equilibrium mixtures of two or more isomers, e.g., cis and trans modifications of [;\lA2L4]. The present discussion will disregard the configurational equilibria that may occur between the various possible structural modifications of an octahedral or a tetrahedral complex of a given composition, as they, most probably, play an insignificant role in the coordination disproportionaThe Journal of Physical Chemistry
AND
D. PUCHALSKA
tion. We shall, using the shorter notations "XA, and 'MA, in place of 34A,L6-, and MAnL4-,, define the stability constants O b n and t'oPn of the octahedral and tetrahedral complexes, respectively, as
OPn
i"RfAn1 = ("R4](A]"
,
t/o
'
=
{'MA,] ("A1)(A]"
(1)
braces denoting activities. I n these definitions, the solvent activity is assumed to be constant. The configurational equilibrium constant, K,, will, under the assumed conditions, be defined as x, =
{ tRIA,f {"AlA,)
___
(2)
That quantity is related to the equilibrium
[i\lIA,L4-,]
[RlA,Ls-,]
+ 2L
(3)
and may be used as a measure of the relative stability of tetrahedral and octahedral modifications of the empirical complex RIA,. It can thus be seen that O ~ n x n= " O P n
(4)
It is obvious that the stability constant, P,, of a complex in solution, as determined by standard methods, relates to the sum of complexes, all of which have the same empirical formula MA,. I n order to derive the relation between the conventional stability constant, P,, and the stability constants of individual modifications of the complex MA, for a system in which octahedral and tetrahedral complexes are formed, the condition of constant solvent activity must be assumed. An unequivocal definition of Pn would otherwise not be possible. Thus
Let us assume that infinitely dilute solutions of the species NA,, 11, and A are the corresponding reference states for the activities. For a solution sufficiently dilute to be treated as ideally dilute, we shall have Pfl
=
[;\lh,]* [bl]*([A]*)"
asterisks denoting concentrations relating to the assumed conditions. For a system where both octahedral and tetrahedral complexes are formed, we shall have (4) P. A. D. de Maine and E. Koubek, J . Inorg. Nucl. C h m . , 1 1 , 329 (1959). (5) D. W. Meek and R. S. Drago, J . Am. Chem. SOC.,8 3 , 4322 (1961). (6) G . J. Janz, A. E. Marcinkowski, and H. V. Venkatasetty, EZectrochim. Acta, 8 , 867 (1963).
3551
COORDINATTON DISPROPORTIONATION EQUILIBRIA IN SOLUTION
[MAn]* = ['XA,]*
+ ['RIA,]*
[SI]*= ["MI*
+ [tn~]*
(7)
and
log K ,
where "21 and t,lI denote the AIL6 and ML4 solvo complexes, respectively. Combining eq 6 and 7 the relation [0?11A,I *
[tAJ ]
['MA,]* (8) [ORI]*([A]*)"
"(5 ') = ["lI]*([A]*)"-k
is obtained which, if definitions 1 and 3 are applied, can be given the form of
+ 1)
P,(HC
=
'6,
+
t'o~n
(9)
or, if eq 4 is taken into account, that of @,(KO
+ 1) =
+ 1) +
[ R I L ~ I ZL
(11)
and its value is likely to be very small for the majority of real systems. A reversible coordination disproportionation reaction may be formulated in the general may
(12
+ Z)?tlA,
A
ZIIAz-k
+ k;LIA,+i
=
log K 1 - 6(n - 1)
(18)
where 6 is a constant for a given system. That relation applies very closely to such a system as ?;i(II)-?IJH3, C O ( I I ) - N H ~or, ~Al(II1)-I;- in aqueous solutionSwhere, most probably, only octahedral complexes are formed. The dependence of log K , upon n, on the contrary, becomes irregular for systems where, in the process of stepwise complex formation, there occur changes in the configuration of the complex or in the electronic ground state of the metal.s It seems justified to assume, on the basis of these findings, that eq 1s applies to the stepwise complex formation involving equivalent coordination positions of the metal ion. Taking into n
(10)
The quant
