Thermodynamics of bis-β-diketonato complexes of Cu(II) and Ni(II)

J. inorg,nucl.Chem.,1971,Vol.33, pp. 2919to 2932. PergamonPress. Printedin Great Britain

THERMODYNAMICS OF BIS-/J-DIKETONATO COMPLEXES OF Cu(II) A N D Ni(II)* B A L A C H A N D R A R A O t and H, B. MATHUR$ National Chemical Laboratory, Poona-8, Poona, India (Received 30 September 1970)

A b s t r a c t - T h e thermodynamic functions of formation of Cu(lI) and Ni(lI) complexes of some/3diketones, viz. acetylacetone, benzoylacetone, anisoylacetone and dibenzoylmethane have been determined in 75 vol. % d i o x a n e + 25 vol. % water. The enthalpy changes increase in the order: HBZBZ > H A Y A C > HBZAC > HACAC. This is also the order of stability or free energy change. The thermodynamic functions have been separated into temperature dependent and temperature independent components. The increase in the magnitude o f - A G ° and AS° and a decrease in the magnitude o f - - zlH-, in water-dioxane medium as compared to water, results from the difference in electrostatic interaction between metal ion and ligand anion in the two media. The thermodynamic parameters have been discussed in relation to the nature of bonding and the symmetry of the metal complexes. INTRODUCTION

A NUMBER o f studies[ 1-10] have been made of the/3-diketonato metal complexes, in relation to the nature of the substituents in the/3-diketones, but data on the relative stability of these complexes have been found to be somewhat contradictory. Nakamoto e t a/.[3] suggested on the basis of i.r. studies that substitution of the methyl group in acetylacetone by the phenyl group increases the ~ C and M - - O stretching force constants and hence the stability of the bis-/3-diketonato metal chelates should follow the order: dibenzoylmethane > benzoylacetone > acetylacetone. On the other hand, Holtzclaw and Collman [11] concluded from polarographic reduction studies that phenyl substitution weakens the M - - O bond and that the stability of the chelates follows the reverse order, viz. acetylacetone > benzoylacetone > dibenzoylmethane. If a substituent changes the electronic properties of a ligand, such that its basicity is increased, then in general, the stability of its complexes should also :~Communication No. 1504 from the National Chemical Laboratory, Poona-8, India. +Part of the Ph.D. Thesis submitted to Indian Institute of Technology, Powai, Bombay. STo whom all the correspondence should be addressed. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

G. A. Guter and G. S. Hammond, J. A. chem. Soc., 81, 4686 (1959). D. F. Martin and B. B. Martin, lnorg. Chem. 1,404 (1962). K. Nakamoto, Y. Morimoto and A. E. Martell,J.phys. Chem. 66,346 (1962). N. K. Dutt and B. K. Sen, Sci. and Culture 26,286 (1960). H. F. Holtzclaw, A. H. Carlson and J. P. Collman, J. Am. chem. S oc. 78, 1838 (I 956). W. G. Borduin and G. S. Hammond, U.S.A.E.C., Rep. ISC-946-28 (1957). J.J. Fritz and R. G. Taylor, J. Am. chem. Soc. 80, 4484 (1958). L. G. VanUitert, W. C. Fernelius and B. E. Douglas, J. A m. chem. Soc. 75, 2736 (1953). L. G. VanUitert and W. C. Fernelius, J. Am. chem. Soc. 76,379 (1954). I_,.G. VanUitert and W. C. Fernelius, J. Am. chem. Soc. 75, 3862 (1953). H. F. Holtzclaw and J. P. Collman, J. Am. chem. Soc. 79, 3318 ( 1957). 2919

2920

B. R A O and H. B. M A T H U R

increase. Any anomalous change in the stability of a particular complex can easily be traced to steric or solvation factors, if information is available on the enthalpy and entropy changes associated with the metal ligand coordination reaction. In this paper we report a thermodynamic study on the interaction of Cu(II) and Ni(II) ions with the fl-diketones: acetylacetone, benzoylacetone, anisoylacetone (p-methoxybenzoyl acetone) and dibenzoyl methane. Since these /3diketones, with the exception of acetylacetone, are insoluble in water, the experiments were conducted in an aqueous dioxane medium containing 75 vol. % dioxane. EXPERIMENTAL Materials Acetylacetone (AR grade, BDH) was distilled before use. Benzoylacetone and dibenzoylmethane were of A R grade (BDH). Anisoylacetone was prepared in the laboratory and recrystallised from methanol. Dioxane was purifiedll2] and freshly distilled over sodium metal before use. Solutions of tetramethyl ammonium hydroxide of the desired concentration in 75 vol. % dioxane + 25 vol. % water were made from a 10% aqueous solution of tetramethyl ammonium hydroxide (E. Merck). Copper perchlorate was crystallised from a solution prepared by dissolving spectroscopic pure copper in perchloric acid. It was twice recrystallised from water and its solution was standardized by electrogravimetric method [ 13]. The solution ofnickelous sulfate hexahydrate (Baker and Adamson Products, U.S.A., A.R.) was also standardized by an electrogravimetric method[14]. Tetra-nbutylammonium iodide and tetramethylammonium chloride (AR grade, BDH) were used as inert electrolytes and since the latter was hygroscopic, its solution was standardized by estimating the chloride ion concentration [ 15]. Procedure Potentiometric titrations were carried out as described earlier[16]. The ionic strength was maintained at 0.02 with tetra-n-butylammonium iodide in the solution of Ni(II) ion and with tetramethylammonium chloride in the solution of Cu(II) ion. The mixing of dioxane with water is accompanied by a decrease in volume. The apparent volumes of 75 vol. % dioxane+ 25 vol. % water mixtures were therefore multiplied by appropriate volume correction factors to get the real volume. The volume correction factors had the values of 0-984, 0.984 and 0.982 at 15°(2, 25°C and 40°(2 respectively[16]. The ionic product of water PKw in the above solvent media were calculated to have values of 18.95, 18-62 and 18.13 at 15°C, 25°C and 40°C respectively [ 16]. The hydrogen ion concentration was calculated following the procedure of Van Uitert and Haas [17] --log (H +) = B + l o g Ua

(1)

where B is the pH meter reading and log U , is the pH correction factor for a fixed temperature and composition of the medium and is related to the activity coefficient by the relation: log Un = log U~t - log l / y .

(2)

The values of log Un for a fixed ionic strength of 75 vol. % aqueous dioxane and at.a fixed temperature 12. A. I. Vogel, A Text Book o f Practical Organic Chemistry 3rd Edn. p. 714. Longmans, London (1966). 13. A. I. VogeI,A Text Book o f Quantitative lnorganicAnalysis 3rd Edn. p. 608. Longmans, London (1964). 14. Ref. [13]. pp. 613. 15. Ref. [13]. pp. 260, 266, 460. 16. U. B. Rao and H. B. Mathur, Ind. J. Chem. 7, 1234 (1969). 17. L . G . Van Uitert and C. G. Haas, J. A m. chem. Soc. 75, 451 (1953).

Thermodynamics of bis-/3-diketonato complexes

2921

were found by comparing pH meter readings with the stoichiometric concentration during a titration of HCIO4 solution with tetramethylammonium hydroxide in 75 vol. % dioxane+25 vol. % water. The values of log [1/3"_+]were determined by interpolation from plots of log [1/3'±] vs. mean molarity (m._) in 75 vol. % dioxane+ 25 vol. % water from the data given by Harned and Owen for HCI[18]. The values of log U~ in 75 vol. % dioxane + 25 vol. % water were thus determined by us at 15°, 20°C, 25°C, 35°C and 40°C and found to be linearly dependent on temperature as given by expression (3): log U

o

H

(3)

(0'007406) t + 0.828

where t is the temperature in °C. It was further confirmed by potentiometric titration in media of different ionic molarities that the value of log U~ is independent of ionic strength [ 16]. Stoichiometric dissociation constants qo were determined by potentiometric titration of the B-diketones (tz = 0.02M) with a solution of tetramethylammonium hydroxide, since an earlier report [19] had indicated that the /3-diketones also formed metal complexes with Na + and K ÷ ions. The values of qo were then calculated from the following expression: --log qD = --log (H +) + log [ (Cach) [[(CH3)4NOH] + ( H + ) - ( O H -)

1]

(4)

where (Cnch) is the concentration of the diketone taken. The thermodynamic dissociation constant kD was determined from the relationship: pkD = Pqo + 2 log [ 1/3'_+]

(5)

It was assumed that 3'nch = 1 and 3,8 + = 3'Oh = 3'-+ (1 : I) or 3'_+.The following equations were used to express the formation of bis-/3-diketonato metal complexes in 75 vol. % dioxane + 25 vol. % water. (MCh+) M 2 + + C h - ~ MCh+;q, = (MZ+) (Ch_)

MCh+ + Ch - ~ MCh2; q2

(MChz) (MCh +) (Ch-)"

(6)

(7)

The following additional equilibrium was also considered for the Ni(II)-acetylacetone system in water medium. (MCh~-) M C h ~ + C h - ~ MCh~-; qa = (MCh2) (Ch-)"

(8)

The formation curves were constructed from the values of h and -log Ch- which were calculated from the following expressions: C h - = ~ qo

rl C HCh+ C H o - [ N ( C H a ) , O H ] - (H +) - ( O H - ) ]

h = [Cnch-Ch-{~-~+

1}] I/CM

(9)

(10)

where h is the degree of complex formation and Cu is the total metal concentration in all forms. Stepwise concentration formation constants log qn were calculated by the least square treatment of Irving and Rossotti[20] on CDC-3600-160A computer at the Tata Institute of Fundamental ! 8. H. S. Harned and B. B. Owen, The Physical Chemistry o f Electrolytic Solutions p. 684. Reinhold, New York (1958). 19. W. C. Fernelius and L. G. Van Uitert,Acta chem. scand. 8, 1726 (1954). 20. H. Irving and H. S. Rossotti, J. chem. Soc. 3397 (1953).

Jinc Vc& 33 No. 9G

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B. RAO and H. B. M A T H U R

Research, Bombay. The thermodynamic stepwise formation constants were then determined from the relationships: log kt = log q~ + log 3`MCh+ 3`M'+3`Ch-

(1 1)

3`MCh2

l o g k s = log q2 + log 3`MCh TC'----"-h+ '

(12)

The mean activity coefficient 3`. of the titrate was calculated from the Debye Huckel equation [21]. IZ1Z211'29 × 1 0 - s V ~ / (1 -t 3 5 . 5 6 a % / ~ log[I/y.-]=

(DT)3/2

/

(sT) ~/2 /

(13)

where/~ is the ionic strength, T is the absolute temperature, Z~, Z2 are charges on the positive and negative ions and a °, the mean ionic diameter in Angstrom units was taken as 10A[22, 23]. The values of the dielectric constant ¢ of 75 vol. % dioxane + 25 vol. % water were taken as 1,1.50, 13.75 and 12.50 at 15°(2, 25°(2 and 40°(2 respectively and were obtained by interpolation from the data of Harned and Owen [24]. It follows directly from Equation (13) that for a 2 : 1 electrolyte at a fixed ionic strength y_.(2: 1) = [3`.-(1: 1)] 2

(14)

The mean activity coefficient y_. for an electrolyte is defined as [25]: 3`.- = (y+'+y-~-)~/~++~-)

(15)

where v+ and v_ are the number of positive and negative ions produced by an electrolyte. Here T± (2: 1) = (yz+~y~_2)~/3

(16)

or

T2+ =

[y_.(2 : 1)] 3 y_2

(17)

Substituting the value of y±(2: 1) in terms of y_.(l :1) from Equation (14) into Equation (17) and putting y±(1 : I) = y_., we have, 3`~+= (3`*_)'

(18)

The Equations (11) and (12) can then be rewritten as log kl = log ql + 4 log [1/3,_.]

(19)

log k 2 = log q2 + 2 log [ 1/y_*]

(20)

The log k values reported here are accurate t o ---0.03 log units and the values of AH ° and'AS ° are reliable to ___0.5 kcal mole -1 and 2 cai mole -1 deg. -~ respectively. 21. H. S. Harned and B. B. Owen, The Physical Chemistry of Electrolytic Solutions p. 66. Reinhold, New York (1958). 22. I. D. Chawla and C. R. Spillert, J. inorg, nucl. Chem. 30, 2717 (1968). 23. R. M. lzatt, C. G. Haas, B. P. Block and W. C. Fernelius, J. phys. Chem, 58, 1133 (1954). 24. H. S. Harned and B. B. Owen, The Physical Chemistry of Electrolytic Solutions p. 713. Reinhold, New York (1958). 25. S. Glasstone, Introduction to Electrochemistry p. 138. D. Van Nonstrand, Amsterdam (1954).

Thermodynamics of bis-/3-diketonato complexes

2923

RESULTS AND DISCUSSION

The values of the thermodynamic ionization constants of acetylacetone (HACAC), benzoylacetone (HBZAC), anisoylacetone (HAYAC) and dibenzoyimethane (HBZBZ) at 15°C, 25°C and 40°C are summarized in Table I. Table 1. Ionization constants (pk) of/3-diketones in 0.02M N(CH3)4CI Temp. Ligand HACAC* HACAC HBZAC HAYAC HBZBZ

15°C

25°C

40°C

8.97 12.46 12.67 13-18 13-55

8.88 12-41 12.60 13"11 13"41

8.77 12.38 12.53 13.00 13-18

Medium Water 75 vol. % dioxane + 75 vol. % dioxane + 75 vol. % dioxane + 75 vol. % dioxane +

25 25 25 25

vol. % water vol. % water vol. % water vol. % water

* In 0.02M KNO:~.

These pk values determined from experiments at fixed ionic strength (~ = 0.02M) agree within +__0.2log units with those determined where the ionic strength was not kept constant during the potentiometric titrations. The constant small differences in the pk values of a particular ligand at 15°C, 25°C and 40°C given in Table 1 and those reported earlier[16] can be directly traced to the differences in the value o f - l o g k resulting from the difference in the actual values of log [I/y_+] of experimental solutions and those derived from equation (13) at different ionic molarities in the two experiments. Predictably, this does not introduce any difference in the values of heats of ionization obtained by the temperature coefficient of these ionization constants and those in the earlier publication [ 16]. Table 2 and Table 3 summarize the stepwise thermodynamic formation constants of bis-/3-diketonato complexes of Cu(II) and Ni(II). The stepwise changes in the enthalpy and entropy accompanying the formation of bis-/3-diketonato complexes of Cu(ll) and N i(l i) are given in Table 4. It may be observed that the formation of copper and nickel complexes of acetylacetone in 75 vol. % dioxane + 25 vol. % water is accompanied by smaller changes in enthalpy and larger changes in entropy as compared to that in 100 per cent water. The increased stability or free energy change for the formation of these complexes in dioxane water medium arises as a result of difference in the standard states chosen for the two solutions and is due to a larger value of the entropy change which more than compensates the decrease in the magnitude of-AH. It is convenient to divide any thermodynamic function AX ° (X = G, H, S etc.) into two parts: (a) AH c a temperature independent component intrinsic to the molecules or ions and arising out of short range or covalent forces insensitive to the environment. (b) AH°z, a temperature dependent part, owing to the interaction of the dipoles or ions with long range electrostatic forces in the solvent medium. Then, AX ° = AX~ + AX~z

(21 )

2924

B. RAO and H. B. M A T H U R

Table 2. Stepwise formation constants of bis-/3-diketonato Cu(ll) complexes in 0.02M N(CHa)4CI in 75 vol. % dioxane + 25 vol. % water

System Cu(II)-HBZBZ

Cu(II)-HAYAC Cu(II)-HBZAC

Cu(II)-HACAC

Cu(II)-HACACf

Formation constant

15°(]

Temp. 25°C

40°C

log kl log k2 log k* log k~ log k2 log k log k~ log k2 log k log k~ log k~ log l log k~ log ks log k

13-58 12.04 25.62 13.15 11.41 24"56 12-60 11.00 23.60 12.19 10.56 22.75 8.50 6.95 15-45

13.32 11.79 25-11 12.93 11.24 24.17 12.42 10"81 23"23 12.06 10.43 22.49 8.29 6.70 14"99

12.99] 11.48~ 24-47J 12.59] 10.90[ 23"49J 12" 19] 10.60[ 22"79J 11.93] 10.30[ 22-23J 8.07] 6.48[ 14"55]

Medium 75 voi. % dioxane + 25 vol. % water 75 vol. % dioxane + 25 voi. % water 75 vol. % dioxane + 25 vol. % water 75 vol, % dioxane + 25 vol. % water Water

*Logk = log kl + log k2. i'In 0.02M KNO3. Table 3. Stepwise formation constants of bis-/i-diketonato Ni(II) complexes in 0.02M tetra-n-butylammonium iodide in 75 vol. % dioxane + 25 vol. % water

System Ni(II)-HBZBZ

Ni(II)-HAYAC

Ni(II)-HBZAC

Ni(II)-HACAC

Ni(II)-HACACt

Formation constant log kt log k~ log k* log k~ log k2 log k log ks log ks log k log k~ log k~ log k log kl log ks log k

15°(] 9.99 8"84 18-83 9.29 7.89 17.18 9.19 7.84 17.03 8.87 7-28 16.15 5.81 4.56 10.37

Temp. 25*(3 9.93 8.73 18.66 9.23 7.79 17.02 9.13 7.76 16.89 8.77 7.21 15.98 5.69 4-47 10.16

40°(2

Medium

9.79] 8-58[ 18.37J 9.13] 7.68[ 16-81J 9.03] 7-69[ 16.72J 8.74] 7.14[ 15.88J 5-54] 4.23~ 9.77J

75 vol. % dioxane + 25 vol. % water 75 vol. % dioxane + 25 vol. % water 75 vol. % dioxane + 25 vol. % water 75 vol. % dioxane + 25 vol. water

Water

*Log k = log k~ + log ks. tin 0.02M KNOs.

The separation of the thermodynamic parameters into temperature independent and temperature dependent components was first suggested by Gurney[26] for proton ionization reactions and has recently been extended to metal complex 26. R. W. Gurney, Ionic Processes in Solution. McGraw Hill, New York (1953).

Thermodynamics of bis-fl-diketonato complexes

2925

formation by Anderegg[27] and Nancollas[28, 29]. The nature of the temperature dependence of AXe, can be approximated from the Born's model[30] of an ion as a rigid conducting sphere of radius r and charge ze in the solvent which is treated as a structureless medium of dielectric constant e. In the Born model, the change in the Gibbs free energy AG~, is the electrostatic work We, done in creating the charge on an ion fiom zero to ze and is given by z2e 2

AG~t= W~I=

2,r

(22)

Hence for any ionogenic reaction, the Gibbs free energy change per mole may be expressed as

Ne~[ z~ aG°'= 2, Z r products

z~] Z r

(23)

reactants

where N is the Avagadro's number. The variation of ¢ with temperature can be expressed with a precision of--- 0.2% by an empirical expression [26, 31,32] , = ,oe -rj°

(24)

where 0 is the temperature characteristic of each solvent, , a d ¢o is a constant. If we denote by 4) the expression within the square brackets in Equation (23), we have Ne 2

AG o, = - ~

= Ne~2, o

4)

(25a)

q~erl°

(25b)

The electrostatic entropy change is then given by Equation (26). a s o, =

-o --Ne2[

a {1~+1(~] (26)

27. 28. 29. 30. 31. 32.

G. Anderegg, Heir. Chim. Acta 51, 1856 (1968). G. Degischer and G. H. Nancollas, J. chem. Soc. A, 1125 (1970). G. H. Nancollas, Interactions in Electrolytic Solutions. Elsevier, Amsterdam (1966). M. Born, Z. Physik 1, 45 (1920). E. J. King, Acid-Base Equilibria. McMuUar, New York (1965). H. S. Harned and B. B. Owen, The Physical Chemistry of Electrolytic Solutions p. 161. Reinhold. New York (1958).

2926

B. RAO and H. B. M A T H U R

The derivatives of ~b are expected to be small and are generally neglected[31]. Equation (26) reduces to: hSe°t=

--N e 2

2c0 ~ber/°(1/0)

(27)

The electrostatic enthalpy change AH°z is then given by AHot = AGot + TAS°t

(28a)

= ( Ne2/2¢o )~ber~°(1 -T)

(28b)

The contribution of the temperature dependent components AG°~, AS°t and AHe°t to the total change in AG °, AS ° and AH ° cannot however be quantitatively evaluated directly from Equations (25), (27) and (28) without a knowledge of the value of ~b. It is convenient to express ~b in terms of a harmonic mean radius (3 l) r* which for Equations (6) and (7) may be expressed as:

d# ~-

1

4

"rMCh+

rM*+

--1 ~b= r~ch+

1 .]

--4

rCh- = r---~-

(29)

1 .] --2 MCh_ = r*

(30)

The values of ~b are therefore negative for reactions which are accompanied by charge neutralization. The values of 0 and ~o for water in Equation (24) are 219°K and 305.'} respectively[26]. For 75 vol. % dioxane, 0 and Eo have been determined to be 183°K and 70 respectively from the values of ~ at 15°C, 25°C and 40°C [16]. The following general correlations can be obtained from Equations (25-30), for reactions such as represented by Equations (6) and (7). 1. AG°~ has a negative sign. Its magnitude will increase as the dielectric constant of the medium decreases. Hence as shown in Table 4, the decrease in AG ° = AGc°+AG°~ accompanying the formation of the bis-acetylacetonato complexes of Cu(II) and Ni(II) in 75 vol. % dioxane medium is higher than that in 100% water medium owing to a larger contribution from AGe°c 2. ASe°t is positive and its magnitude will increase with a decrease in the dielectric constant. The higher values of the entropy changes (Table 4) in the dioxane-water medium as compared to that in aqueous medium, arise as a result of the increased positive contribution from AS°t to the total AS °. If the "iceberg" concept of Evans and Frank[33] is applied to the dioxane-water medium, the interaction between solvent dipoles and a metal ion will be of two types; (a) between those "frozen" water dipoles which are replaced by ligand molecules during complex formation, (b) between more distant water and dioxane dipoles 33. M. S. Evans and H. S. Frank, J. chem. Phys. 13, 507 (1945).

Thermodynamics of bis-/3-diketonato complexes

2927

Table 4. - A H ° (kcal mole-l), - A G o (kcal mole-l), and AS° (cal mole-J deg -~) values for the formation of bis-/~-diketonatocomplexes of Cu(lI) and Ni(lI) at 25°C

1 2 l 2 1 2 1 2 1 2

n[Ch--]

--AG°

(HBZBZ) (HBZBZ) (HAYAC) (HAYAC) (HBZAC) (HBZAC) (HACAC) (HACAC) (HACAC) (HACAC)

18.18 34.27 17.64 32.98 16.95 31.70 16.46 30-69 11.31 20.45

Cu(ll) --AH° 9.7 18.9 9-2 17.7 6.8 13-4 4.1 8.3 7.1 14.8

AS°

--AGo

Ni(I1) --AH°

AS°

Medium

28 51 28 51 34 61 41 75 14 19

13-55 25-46 12-60 23"23 12.46 23-05 11.97 21.81 7.76 13"86

3.4 7.5 2.7 6.1 2"6 5.1 2.1 4.4 4.6 9-9

34! 60' 33! 57 33 60 33 58 II 14

75 vol. % dioxane + 25 vol. % water

Water

w h i c h a r e o r i e n t e d a s a r e s u l t o f t h e e l e c t r i c field o f t h e m e t a l ions. F o r a m e t a l c o m p l e x d i s s o l v e d in m e d i a o f d i f f e r e n t d i e l e c t r i c c o n s t a n t s , e.g. 1 0 0 % w a t e r o r 75 vol. % d i o x a n e , t h e i n t e r a c t i o n (a) will n o t c h a n g e v e r y m u c h a n d a s t a t e o f s a t u r a t i o n will e x i s t a s f a r a s t h e e n t r o p y c h a n g e s a r e c o n c e r n e d . T h e " f r o z e n " d i p o l e s c o n s t i t u t i n g t h e s o l v a t i o n s p h e r e o f a n ion c o n s i s t e n t i r e l y o f t h e m o r e p o l a r w a t e r m o l e c u l e s ; b u t t h e effect d u e to (b) will c a u s e m u c h g r e a t e r e n t r o p y change, on neutralization of the charges on the cations and anions during complex f o r m a t i o n ; in d i o x a n e - w a t e r m e d i u m as c o m p a r e d to 1 0 0 % w a t e r . 3. A t t e m p e r a t u r e T > 0, AHe°t is p o s i t i v e a n d this t e m p e r a t u r e d e p e n d e n t c o m p o n e n t o f t h e e n t h a l p y c h a n g e m a k e s a n e n d o t h e r m i c c o n t r i b u t i o n to t h e t o t a l c h a n g e in e n t h a l p y . A l o w e r i n g in t h e d i e l e c t r i c c o n s t a n t will i n c r e a s e t h e m a g n i t u d e o f this e n d o t h e r m i c c o n t r i b u t i o n . I t m a y b e s e e n f r o m T a b l e 5 t h a t t h e e x o t h e r m i c h e a t o f f o r m a t i o n is l o w e r in 75 vol. % d i o x a n e t h a n in w a t e r f o r t h e bis-acetylacetonato complexes of both Cu(II) and Ni(II). Table 5. The values of AH~z and A H c ° (in kcals mole-1) calculated for bis-B-diketonato complexes of Ni(ll) and Cu(II)

Ligand HBZBZ ] HAYAC t HBZAC HACAC HACAC

Medium

75 vol. % dioxane + 25 vol. % water Water

-AH ° 18-9 17'7 13'4 8'3 14-8

Cu(II) AH°z

-AHc °

-AH °

Ni(ll) AHo~

-AHc °

7.3 7.3 8.4 10.0 2.8

26.2 25-0 21 '8 18.3 17.6

7-5 6-1 5' 1 4.4 9-9

8.3 8.0 8'3 8.1 2"4

15.8 14.1 13-4 12.5 12.3

I t m a y b e r e c a l l e d t h a t t h e v a l i d i t y o f E q u a t i o n s (25), (27) a n d (28) p r e s u p p o s e s t h a t t h e v a l u e o f e in t h e v i c i n i t y o f i o n s m a y b e t a k e n a s e q u a l to t h e b u l k d i e l e c t r i c c o n s t a n t . T h e i n t e n s e e l e c t r o s t a t i c field n e a r a n i o n c a u s e s a s a t u r a t i o n o f t h e d i e l e c t r i c [ 2 9 , 31]. T h e p o l a r i z e d s o l v e n t c a n b e e x p e c t e d to

2928

B. RAO and H. B. M A T H U R

have an effective dielectric constant less than that of the pure solvent[34]. This however does not pose any difficulty, since any change in • will bring about a change in ~b (see Equation 25) and we may regard e and ~b as mutually adjustable parameters. While writing the association constants for metal ligand reactions represented by Equations (6) and (7), the concentration terms for water have been omitted. In order to compare the free energy changes in complex formation reactions involving different numbers of solvent molecules, it is necessary to include a "cratic term"[26, 29] A n R T I n 5 5 . 5 so that the equilibrium constant for the reaction is rendered dimensionless. Here, An is the change in the number of solute molecules during the reaction and 55"5 are the moles contained in 1000g of water. The free energy change should therefore be expressed as - A G °' = R T In k + A n R T In 55.5

(31 a)

instead of the expression --AG ° = R T In k

(3 lb)

Putting AG ° = A G c o + AG°~, we have AG °' = A G e ° + AG°z - A n R T In 55.5

(3 lc)

Hence AS o =--~-O (AGo,) 0T _

N e 2 dpertO1 + A n R In 55.5.

(32)

2Co

Rearranging we have, Ne2th e r/° = --0[AS ° - A n R In 55.5] 2E O

(33)

Substituting the value of [ ( N e 2 $ ) / ( 2 ~ d ) ] e rl° from Equation (33) in Equations (25), (27) and (28), we have AG°~ = ---0 (AS ° - A n R In 55.5)

(34)

AS~°I= (AS ° - A n R In 55"5)

(35)

AH°t = ( T - O )

(AS°-AnR

34. R. M. Noyes, J . A m . chem. Soc. 84, 513 (1962).

In 55.5)

(36)

Thermodynamics of bis-/3-diketonato complexes

2929

and the temperature independent parts of the enthalpy, entropy and free energy changes are given by: A H c o = A H o - A H o t = AH ° - ( T - - O ) ( A S ° - A n R

In 55.5)

(37)

ASc° = AS° -- ASg~= A n R In 55.5

(38)

A G e ° = AG ° - AG°z = AG ° - 0 (AS ° - A n R In 55.5)

(39)

An = - - 2 , in the formation of bis-diketonato complexes of Ni(II) and Cu(II) (see Equations 6 and 7), and hence the temperature independent part of the entropy change, ASc ° becomes negative (Equation 38) and reflects the loss of translational entropy of the ligands on complex formation as well as changes in the rotational and vibrational motion which are normally small and can be considered to make equal contributions when we compare complexes of a series of similar ligand with a single metal ion. The observed entropy changes (AS °) are however positive due to the large positive contribution of the temperature dependent part AS°t which reflects the release of bound water molecules from the hydrated ions on complex formation. The introduction in Equation (37) of AS ° which increases with the charge on the metal ion results in an increased endothermicity in AH°t and hence a lower observed exothermic change in AH °. A set of expressions similar to (37), (38) and (39) for A H c °, ASc ° and AGc° respectively can be derived for a solvent medium of 75 vol. % dioxane. Here the cratic term is " A n R T In 22"75" instead of A n R T In 55"5 for the water medium, 22.75 being the number of moles contained in 1000g of 75 vol. % dioxane+ 25 vol. % water. The value of AHc ° in this medium is therefore given by Equation (40). A H c ° = AH ° - ( T - 0) (TAS ° - A n R In 22.75).

(40)

The values of AH°z and AHc ° calculated for bis-/3-diketonato complexes of Ni(II) and Cu(II) are given in Table 5. It may be observed that there is a close agreement between the calculated values of AHc ° for the formation of bis-acetylacetonato Ni(II) in water and in 75 vol. % dioxane. The enthalpy changes accompanying the formation of /3diketonato complexes of Ni(II) and Cu(II) in 75 vol. % dioxane increase in the order: H B Z B Z > H A Y A C > H B Z A C > HACAC. This is also the order of stability or the free energy change. The stepwise enthalpies of chelate formation AH1° and AH2° are equal within the limits of experimental error in all cases (Table 4). This is in general agreement with the results reported in the literature [35] and in agreement with the thermochemical cycle of Yatsmirskii and Grinberg [36], in which the main contributing effect which remains constant is the difference between the heats of vaporization of ligand and of displaced water. The only change is in the charge of the ion which will bring about a change in AS °. 35. J. Lewis and R. G. Wilkins, Modern Coordination Chemistry. Interscience, New York (1960). 36. A. A. Grinberg and K. B. Yatsmirskii, Bull. Acad. Sci. U.S.S.R. Disc. Chem. Sci. 239 (1952).

2930

B. R A O and H. B. M A T H U R

The small difference in the value of A H c ° for the formation of bis-acetylacetonato Cu(II) in dioxane-water medium and in water is of the order of the experimental error, but can also possibly arise from a difference in the tetragonality of the copper complex in the two media on account of the Jahn-Teller effect[37]. It has been shown by N M R studies [38] that in dibenzoylmethane and benzoylacetone, the phenyl ring is coplanar with the hydrogen bonded enolic ring and hence it is able to increase the electron density on the adjacent oxygen atom by resonance. These more basic donor oxygen atoms will therefore form more covalent 0 - - M bonds by virtue of their higher electron density and lead to the following order of increasing negative values of A H c ° and AH°: H A C A C < H B Z A C < HBZBZ. The presence of a strongly electropositive--OCHa group in the phenyl ring of anisoylacetone ( H A Y A C ) further increases by induction the donor property of the oxygen atoms in this ligand as compared to benzoylacetone. The former ligand therefore should form stronger (F--M bonds in its complexes than the latter and this is consistent with the observed increasing order of--AH °, viz. H A C A C < H B Z A C < H A Y A C < HBZBZ. It may also be noted that the entropy changes for the formation of bis-/3-diketonato Ni(II) complexes with all the fl-diketone ligands studied by us are constant to __+2 e.u. in the dioxane + water medium. This indicates that the stereochemical arrangement of the ligand around the Ni(II) ion in the above series of complexes in dioxane-water medium is similar. The formation curves for all these systems became flat at ~ = 2 indicating that the maximum number of ligands that can be coordinated in the dioxane-water medium is 2. It is reasonable to assume, as indicated earlier, that the solvation sphere of metal ions and the metal complexes in 75 vol. % dioxane (which still contains more moles of water than of dioxane) is the same as that in the water due to more polar nature of the water dipoles as compared to the dioxane dipoles and the low concentration of the solutes. The Ni(II) complexes of fl-diketones are known to crystallise from aqueous organic solvents as dihydrates[39], e.g. Ni-benzoylacetone. 2H20 and Ni-acetylacetone.2HzO. Single crystal studies[40] on bis-acetylacetonato Ni(II).2H.,O show it to have an octahedral symmetry with the central Ni atom, surrounded by four oxygen atoms of the two acetylacetonate radicals in the xy plane and the two oxygen atoms of the two moles of water of crystallisation forming a slightly distorted octahedron with N i - O distances of 2.021A and 2.014,~ and N i - O ( H 2 0 ) distances of 2.139/~. The N M R studies[41] on the base adducts of anhydrous bisacetylacetonato Ni(II) dissolved in organic solvents also show that the two acetylacetone radicals lie on the equatorial plane of the metal ion and the two moles of added basic ligands (e.g. pyridine) occupy the axial positions to this equatorial plane. This suggests that the bis-fl-diketonato complexes of Ni(II) 37. 38. 39. 40. 41.

L. E. Orgel, A n Introduction to Transition Metal Chemistry. Methuen, London (1960). R. L. Lintvedt and H. F. Holtzclaw, lnorg. Chem. 5,239 (1966). R, Nast and H. R. Rukemann, Chem. Ber. 93, 2329 (1960). H. Montgomery and E. C. Lingafelts,Acta Crystallogr. 17, 1481 (1964). R.W. Kluiber and W. D. Horrocks, J. Am. chem. Soc. 87, 5350 (1965).

Thermodynamics of bis-/3-diketonato complexes

293 !

ion have two water molecules a trans-octahedral symmetry in the dioxane water medium. The formation curve of bis-acetylacetonato Ni(II) in water however shows no sign of flattening at h = 2 (Fig. 1) indicating a possibility of the formation of the tris complex although we have not been able to study the complex formation beyond h = 2, due to the onset of hydrolysis. 20

18

16--

14

I 2

I¢

I 0

08

06

04

02

, 02

[ 8

! 9

-Io 9 Ch-

Fig. 1. Formation curves of bis-acetylacetonato Ni(ll) at 25o C. Q - In 7_ vol. % dioxane + 25 vol. % water: O - In water.

The formation curves of bis-/3-diketonato complexes of Cu(II), both in water and 75 vol. % dioxane, became flat at h = 2. These Cu(II) complexes unlike the corresponding Ni(II) complexes do not have equal entropies of complex formation. These differences can be attributed to the different degrees of tetragonal distortion of the trans octahedral symmetry of the copper complexes, which in the extreme case can lead to a square planar configuration with 4 shorter 0 - - M distances [37] than in the corresponding Ni(II) complexes. Confirmatory evidence is available from the fact that the Cu(II) complexes of acetylacetone, benzoyiacetone and dibenzoylmethane crystallise from solution in the anhydrous form. The crystal structure studies on copper complexes of acetylacetone[42,43], benzoylacetone[44] and dibenzoylmethane[45] confirm that the Cu(ll) is surrounded by four oxygen atoms at a shorter distance of 1.91 ___0.01,~,. 42. 43. 44. 45.

H. Koyema, Y. Saito and H. Kuroya, J. Inst. Polytech. Osaka 4C, 43 (1953). L. Dahl, Communication to T. S. Piper and R. L. Belford, Molec. Phys. 5, 516 (1966). P. K. Hon. C. E. Pfluger and R. L. Belford, Inorg. Chem. 5, 516 (1966). G. A. Barclay and A. Cooper, Private communication to D. P. Graddon, Coord. Chem. Rev. 4, 1 (1969).

2932

B. RAO and H. B. MATHUR

Our results therefore confirm the conclusions of Nakamoto e t a/.[3] who showed by i.r. studies that phenyl substitution increases the strength of O - - M bond in the/3-diketone complexes. The contradictory conclusions of Holtzclaw and Collman [5, 11] from i.r. and polarographic reduction studies of the metal chelates can be traced to, (1) the error in the use of the perturbed ( C ~ O ) stretching frequency to infer indirectly the strength of the M - - O bond, (2) the unreliability of using halfwave potentials in irreversible polarographic reductions as a measure of chelate stability.