Entropy Effects in Chelation Reactions Chung-Sun Chung National Tsing Hua University, Hsinchu, Taiwan 300, Republic of China I t is instructive to use a thermodynamic cycle to show the atomic and molecular factors which may influence an observable chemical property (I). A Born-Haher-type analysis,
indicates that the entropy change of a chemical reaction is related to the entropy terms
The standard states are as follows: entropies of solution are for 1m ideal solutions, entropies of ideal gases are at 1atm. AS,,I,., the solvational entropy factor, is defined as the difference between the entropy of reaction in aqueous solution and that in the gas phase. ASlo is the entropy of reaction in aqueous solution, which can be measured experimentally. AS3' is the entropy of reaction in the gas phase, estimated theoretically as a sum of AStrms~.tiOm,ASi.t",,i, ,htion, ASwmmetW,Mi,,, AS,ib..ti,., and Asi.h,,.~ .,,ti,. Thus, the entropy change for a reaction in aqueous solution can he evaluated as a combination of entropy factors by the relation
Valuable insight or understanding can he obtained from a detailed examination of these factors. Several entropy effects of inorganic chemical reactions are discussed as examples. Some of the ligands included in this paper are shown below.
I1 trien
formerly thought to be caused by the increase in translational entropy, due to the fact that the formation of the chelate resulted in an increase in the number of species in solution. However, the article by Myers ( 4 ) showed that the entropy of solution of ligands was an important contributor to the position of equilibrium, as well as the entropy of solution of the coordinated metal ions. The article by Chung (5) made a significant contribution, in that it showed how to calculate the entropy factors in chelation reactions, which Myers computed only partially and empirically. The enthalpy change in chelation reaction was previously thought to be nearly zero. However, the article by Myers (4) showed that differences in enthalpy of solution of ligands were significant contributors to AGO, as well as differences in bond strength of the various ligands. In the present paper, ail that is being considered is the effects of entropy changes on the position of equilibrium, and that the enthalpy change can have significant effect on AGO in chelation reaction. In order to understand more thoroughly the entropy concepts of chelate ring formation and the nature, significance, and magnitude of each of these entropy factors, a typical example (eqn. (3)) has been studied in detail (5).
where en is ethylenediamine. The factors influencing the entropy change of chelation for octahedral complexes in aqueous solution are listed in Table 1. Combining these contributions yields the entropy change, which agrees satisfactorily with the experimental values (6) as given in Table 2. Because of the hindered internal rotations around the M-N bond, the inTable 1. Factors lnfluenclngthe-Entropy Change of Chelatlon for Octahedral CornDlexes In Aaueous Solution at 25%' Contributionfrom free ligands, cab nmleC'.deg-'
factor solvation translation intrinsic mtation symmetry isomer vibration internal rotation
-20 34.1 14.4
Table 2.
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Entropy Data for Chelation Reactions In Aqueous Solutlon at 25%
-
Reaction
The Chelate Effect The term "chelate effect" refers to the enhanced stability of a complex system containing chelate rings as compared to the stahility of a system that is as similar as possible except that it contains no rings or fewer rings (2,3). This effect was
.contribution from complexes, cal. mol-'deg-'
+ +
Cd(NH2CHJ)22+ en 2 NH2CHs Cd(NH&H3)42+ 28" 4NH*CH3
+ +
F r m ref. (6).
Cd(en)2+
-
Cd(enh2+
A9,pta.
ASO-s,
cal.mole-'. deg-'
cd.mole-'. deg-'
7
7
19
>14
ternal rotational factor in the case of n = 2 isnot double that of n = 1,so the entropy change of chelation a t n = 2 is more than double that a t n = 1. The large negative solvational entropy factor is mainly due to the difference in solvational entropies of the ligands and the different standard states employed; entropies of solution are for 1rn ideal solutions, whereas entropies of ideal gases are at 1atm. The difference in solvational entropies of the ligands is probably due to the fact that the hydrophobic exterior of two uncoordinated methylamines is larger than that of an uncoordinated ethylenediamine. For similar systems, we may expect a negative &ational entropy contribution in chelation reactions in aqueous solution for the same reasons. The solvation factor is oreanic solvents will be less sienificant than in aqueous solution. Thus, the entropy change of chelation in the eas phase . or in an oreanic solvent will he lareer than that in aqueous solution. The other factor that eives a laree contrihution to " neeative .. AS" in chelation reactio& is the internal rotational term. Not only the internal rotationsof the free lieandli hut also the internal rotations in the complex containing monodentate lieands are lost in the chelation vrocess. There are five deerees of freedom of internal rotatibn lost for each chelate'ring formed in thii reaction. The loss of internal rotational entropy explains the decreasing magnitude of the entropy term with increasing ring size. The net increase in the number of unbound ligands in the chelation process explains positive contributions to the ASo term due the favorable t
