Acid catalysis of the hydrolysis of acetato-pentammine complexes of cobalt(III), rhodium(III) and iridium(III)

1. Inorg. Nucl. C h e m . , 1962, Vol. 24, pp. 1241 to 1250. P e r g a m o n Press Ltd. Printed in N o r t h e r n Ireland

ACID CATALYSIS OF THE HYDROLYSIS OF ACETATOPENTAMMINE COMPLEXES OF COBALT(III), RHODIUM(Ill) A N D 1R1DIUM(III)*t F. MONACELLI, F. BASOLO a n d R. G. PEARSON Department of Chemistry, Northwestern University, Evanston, Illinois (Received 29 March 1962; in revised form 23 April 1962)

Abstract--The syntheses of [M(NH3)5(RCOO)](CIO~)o. where M = Rh(lll) or Ir(lll) and RCOO -CHzCOO-, (CHz)aCCOO or CF3COO- are described. The rates of hydrolysis of these complexes as well as the corresponding Co(Ill) systems were determined in acid, [H ~] -- 0-01~-1 M. In all cases the rates of hydrolysis increase with increase of acid concentration. The mechanism of acid hydrolysis of these systems is discussed. KINETIC studies o f the hydrolysis o f acetato a n d s u b s t i t u t e d a c e t a t o p e n t a m m i n e c o b a l t (111) ions, O

iJ [(NH3)sCo__O__CR]~ + H 2 0 __~ [(NHa)sCo__OH2]~a

~_ R C O O ,

(1)

were r e p o r t e d earlier. Ij) T h e rates o f hydrolysis were f o u n d to be the same, w i t h i n e x p e r i m e n t a l error, at H ~ c o n c e n t r a t i o n s o f 0.004 a n d 0.008 M. It was c o n c l u d e d that in this acid r e g i o n there was n o detectable acid catalysis. Yet it is k n o w n t h a t the h y d r o l y s i s o f c e r t a i n types o f c o m p l e x e s are acid catalysed. These c o m p l e x e s seem to fall into either o n e or the o t h e r o f two different categories. (1) C o m p l e x e s c o n t a i n i n g l i g a n d s t h a t are s t r o n g l y basic or have a l a r g e t e n d e n c y to h y d r o g e n b o n d , e.g. [Co(NHa) a CO3] +121, [Co(en)2F~] ~(a) a n d [Fe(CN)~] 4-,(4). (2) C o m p l e x e s c o n t a i n i n g flexible m u l t i d e n t a t e basic ligands, e.g. [Fe(bipy)z] 2~,(51, [ F e ( E D T A ) ] -,16/, [ N i ( E D T A ) ] 2--,Iv), a n d [Cr(C204)313-,¢8). * T h i s p a p e r reports the results o f a kinetic i n v e s t i g a t i o n o n hydrolysis reactions o f type ( l ) where the m e t a l s are c o b a l t (1II), r h o d i u m (III), a n d i r i d i u m (111) a n d the n e g a t i v e ligands are C H 3 C O O , a n d (CH3)~COO , a n d C F 3 C O O . In all cases the rates • This research was supported by the United States Air Force through the Air Force Office of Scientific Research of the Air Research and Development Command, under Contract No. AF 49(638)315. Reproduction in whole or in part is permitted for any purpose of the United States Government. t This paper was presented by F. M. at the meeting of the Italian Chemical Society in Naples in May, 1962. ~: The symbols used are: e n - ethylenediamine, bipy - 2,2'-bipyridine, EDTA = ethylenediaminetraacetate ion. ~' F. BASOLO,J. G. BERG~ANNand R. G. PEARSON,J. Phys. Chem. 56, 22 (1952). ~-'J A. B. LAMBand K. J. MVSELS,J. Amer. Chem. Soc. 67, 468 (1945). ~' F. BASOLO,W. R. MATOUSHand R. G. PEARSON,J. Amer. Chem. Soc. 78, 4883 0956). ~P A. W. ADAMSON,J. P. WELKERand W. B. WRIC;HT,J. Amer. Chem. Soe. 73, 4789 (1951). i:,, j. H. BAXENDALEand P. G. GEORGE, Trans. Faraday Soe. 46, 736 (1950). ~'~'S. S. JON~S and F. A. LONC;,J. Phys. Chem. 56, 25 (1952). ~7~ D. W. MAR(;ERUM,J. Phys. Chem. 63, 336 (1959). i~, K. V. KRISHNAMURTYand G. M. HARRIS,d. Phys. Chem. 64, 346 (1960). ~'~ A. WEANERand N. GOSHNC,S, Ber. Dtsch. Chem. Ges. 36, 2380 (1903). 1241

1242

F. MONACELLI,F. BASOLOand R. G. PEARSON

depend on the concentration of H + between acid concentrations of 0.01-0.10 M. A t e n t a t i v e m e c h a n i s m f o r h y d r o l y s i s is discussed. EXPERIMENTAL

Preparation of complexes. The complexes of Co(III) were prepared by the reaction of ICo(NHs)5 CO3]NO3with the desired acid according to the methods described previously (x,9). The only difference was that the desired complexes, [Co(NH3)5(RCOO)] 2+, were isolated as the perchlorate salts by the dropwise addition of conc. HCIO4 to the reaction mixture surrounded by an ice-bath. The aquo complexes, [M(NH3)sH20] (CIO4)3, of Co(IlI), ~1°) Rh(llI) cnl and Ir(III) 112} were also prepared by the methods described in the literature. Noreportwasfoundforthesynthesisofsaltsof[lr(NH3)~(RCOO)] ~+. The compound [Rh(NHs)5 (CHsCOO)] Br2.H~O is described as being prepared by grinding the hydroxo compound in a suspension of acetic anhydride, c18~ Our results by this method were not satisfactory so that the methods described here were used. The Rh(IlI) and Ir(IlI) complexes were prepared by the reaction [M(NH3)sH20] 3+ + R C O O - ---*[M(NH3)~(RCOO)] ~+ + H20

(2)

This is essentially the same reaction as that used for the synthesis of the Co(III) complexes but the experimental conditions are different. Since the Rh(III) and Ir(III) compounds had not been prepared previously, the methods of synthesis are described. However, it should be remembered that the primary purpose was to obtain the pure compounds so that no attempt was made to find the optimum conditions for maximum yields. Detailed procedures are given only for the preparation of the acetato complexes of Rh(III) and Ir(III). The trifluoroacetato complexes were prepared in the same way whereas slight modifications were necessary for the pivalato systems. These modifications, due to the insolubility of pivalic acid in water, are described. [Rh(NHa)s(CHsCOO)] (C10~)2. An excess ( ~ 3 g) of Ag20 was added to a hot (~70°C) solution of 2.25 g of [Rh(NH3)sC1]CI~ (~4~ contained in 150 ml of water. The mixture was stirred for a few ( ~ 5 ) minutes and then filtered. The filtrate, which is perhaps largely [Rh(NHa)~C1] (OH)z, was refluxed for one hour. At this point a second excess of Ag20 was added and the mixture was again stirred for a few minutes. The insoluble material was removed on a filter and the filtrate was refluxed for one hour. This same procedure was repeated a third time. Such a tedious procedure was used because prolonged refluxing of [Rh(NH2)sC1] C12 in an aqueous suspension of Ag~O---AgCI results in the apparent loss of NHa and some decomposition of the complex. The final filtrate containing [Rh(NH3)5OH] (OH)~ was acidified with 25 ml of CH3COOH and the solution was evaporated to dryness on a steam-bath. Care must be taken to remove the solid residue from the steam bath as soon as it becomes dry in order to prevent decomposition. The solid residue was then dissolved in 50 ml of water and conc. HCIO4 was added dropwise ( ~ 1 0 ml) with constant stirring. Small white crystals of the desired product, [Rh(NHs)6(CH3COO)] (C104)~ separated. These were collected on a filter, washed with alcohol and ether, then air dried at room temperature. The compound prepared in this way was sufficiently pure but further purification was possible by recrystallization from a minimum amount of water. The pivalato complex was prepared in the same way also starting with 2'25 g of [Rh(NH3)sCI]C12. The final filtrate containing [Rh(NH3)5OH] (OH)2 was heated on a steam bath and 5 g of (CH8)8 CCOOH was added along with sufficient (10 ml) ethanol to put it in solution. The reaction mixture was then concentrated to about 50 ml and 3 g of (CH3)sCCOOH was added plus enough ethanol to dissolve it. After heating on the steam-bath for an additional hour, the solution was cooled to room temperature and transferred to a separatory funnel. The solution was twice extracted with 50 ml of ether to remove the excess acid. The water solution was then cooled in an ice-bath and conc. HC104 was added ( ~ 1 0 ml) dropwise. The desired crystalline salt was collected on a filter, washed with alcohol and ether, and air dried at room temperature. Since the salt is very soluble in water, it was recrystallized from boiling ethanol to which was added just enough water to dissolve the solid. [Ir(NHs)5(CH3COO)] (C104)2. The Ag20 procedure described above was not used for the Ir(III) system which is known ta~) to generate the hydroxo complex by reaction with NaOH. A solution of 2 g of [Ir(NH~)sCI] CI~TM in 70 ml of 1M NaOH was refluxed for 8 hr. The reaction mixture was then acidified by the addition of 30 ml of conc. CHsCOOH after which it was evaporated to 2 ml i~0~ GMELrNS'Handbueh der Anorganischen Chemic, 58 B, 94 (1932). (~) GMELINS'Handbueh der Anorganischen Chemie, 64, 109 (1938). (~z) GMEL[NS' Handbuch der Anorganischen Chemic, 67, 144 (1939). ix~) B. E. DIXON, J. Chem. Soe. 781 (1935). its) GMELINS'Handbuch der Anorganischen Chemic, 64, 134 (1938). its~ GMELtNS' Handbueh der Anorganischen Chemic, 67, 157 (1939).

Acid catalysis o f the hydrolysis o f acetatopentammine complexes

1243

on a steam bath. A n o t h e r 20 ml o f conc. C H a C O O H was added and heating was continued for 1 hr. E n o u g h water was added to give a volume o f about 50 ml and the solution was cooled in an ice-bath. The addition o f conc. H C I O 4 ( ~ 1 0 m l ) dropwise with stirring yielded thedesired whitecrystalline product. This was collected on a filter washed with ethanol and ether, and then air dried at r o o m temperature. Further purification by recrystallization from an ethanol-water mixture was possible. The pivalato complex was prepared by refluxing 1.9 g o f [Ir(NHa)sHo.O] (CIO~)a with 40 ml of a saturated solution at r o o m temperature o f (CHa)aCCOONa for 10 hr. This reaction mixture was then cooled to r o o m temperature and 12 ml o f conc. HCIO4 was slowly added. After transferring to a separatory funnel the mixture was extracted with 150 ml o f diethyl ether. The solid containing the desired c o m p o u n d contaminated with pivalic acid was collected on a filter. This solid mixture was then dissolved in 10 ml o f hot water and extracted again with 50 ml o f ether. The water layer was then cooled in an ice-salt bath and conc. HC104 ( ~ 5 ml) was added dropwise. The crystalline product which separated was collected and recrystallized from absolute ethanol. The analyses o f all o f these c o m p o u n d s are reported in Table 1. The microanalysis determinations TABLE 1.--ANALYSESOF THE [M(NHala(RCOO)](CIO,)_, COMPOUNDS Compound

C

Per cent calculated H N CIO~

C

Per cent found H N

CIO 4

[Co(N Ha)sH 20](CIOa) a [Co(N Ha)aCH aCOO ](NOa) ~ [Co(N Ha)s(CHa)aCCOO ](C104) 2 [Co(N Ha)aCFaCOO](CIO4) e

-7"37 13"5 5"25

-5"50 5"40 3"28

-30"0 --

64"8 37"9* 44'8 43"6

. . . . . . . . 7"02 5"66 30"3 13'9 5'84 .... 5'35 3"54

64"5 38'0* 45'6 43"6

[Rh(N H a)aH 20](C104) a [Rh(N Ha)sCH aCOO](CIOa) 2 [Rh(N Ha)s(CHa)aCOO ](C1002 [Rh(NHa)sCFaCOOI(CIO~)2

5'36 12"3 4"80

-4'03 4'90 3"00

13"8 -14"3 14"0

59"2 44'6 40"8 39'8

5'36 11"7 4'70

4'21 5"15 3'74

13"8 -14"0 14"3

59"5 44"4 40"8 39"7

[lr(N Ha)sH20](CIOt)a [Ir(NH a).~CHaCOO](CIO~)z [lr(N Ha)s(CHa)aCCOO](CIOa) 2 [Ir(N H a)sC FaCOO ](C104) ~

4"46 10"4 4' 06

3"35 4'15 2"54

11"9 13"0 12"1 11"8

50"2 37"1 34"4 33"7

4"93 9"91 4"03

3'79 4"09 2"91

12"1 13"3 12'2 12"0

49"6 37"3 34"5 33'6

* This is % NO a . o f C, H and N were made by Miss H. BECK. The percentage of anion, either CIO4- or N O s - , was determined by the replacement o f C1 from an anion resin. Approximately 50 mg of the complex salt was dissolved in about 5 ml o f water and this solution was passed through IRA-400 in the chloride form. The chloride ion eluted was then titrated with 0.01 M A g N O s using K2CrO~ as an indicator. For the aquo complexes it was necessary to first remove the complex on a cation resin, IR-120 in the sodium form. Unless this is done there is apparently an interaction between the aquo complex and c h r o m a t e ion which makes it difficult to get reliable results. It was also necessary to remove all Co(Ill) complexes in this manner, since the pink colored solutions make it difficult to see the visual end point. Analytical data for all of these c o m p o u n d s are reported in Table 1. Kinetic studies. The rates o f reaction were followed spectrophotometrically. In all cases the wavelength used was that k n o w n to give a large change in optical density during hydrolysis. These wavelengths (shown for each system in Table 2) were determined by a comparison o f the spectra o f the acetato complexes, [(NHs)~(RCOO)] 24, with the corresponding aquo complex, [M(NH s).~(H~O)] a ~. These spectra were measured using a Beckman DK-2 recording spectrophotometer. The spectra for M =- R h ( l l l ) are shown in Fig. 1. Those for Co(III) and Ir(llI) are similar except that they are shifted towards longer and shorter wavelengths, respectively. Except for the very slow reactions o f the Ir(lI1) complexes, the experimental procedure used to follow the rates o f hydrolysis was always the same. In general aqueous solutions were prepared containing approximately 1-1'5 mg/ml of complex, 0.01-0.1 M HCIO~ and sufficient NaCIOa to give an ionic strength o f 0.100 7: 0"005. Some o f the solution was placed in a 1 cm stoppered quartz cell which was then inserted in a Beckman D U cell compartment. This was maintained at constant temperature, :~:0.1 °, by circulating water from a thermostatic bath. Because very high temperatures were required, it was necessary to separate the heated spacers from the rest o f the instrument. This was done by placing a half inch insulator layer next to the heated spacers and then water cooled spacers between the insulator and the instrument. In this way it was possible to maintain temperatures as high as 90 ° in the cell c o m p a r t m e n t whilst keeping the rest of the instrument at r o o m temperature. The quartz cell containing the reaction mixture was positioned in the light path only during the time o f actual measurements o f optical density. The reference cell contained either water or a NaCIO~ ~ HCIO4 solution.

1244

F. MONACELLI, F. BASOLO a n d R . G . PEARSON TABLE 2.--RATES OF HYDROLYSES OF [ M ( N H a ) s R C O O ] 2+ COMPLEXES*

Complex [ C o ( N H a ) s C H a C O O ] 2+

[ C o ( N H a ) 5 ( C H a ) a C C O O ] ~+

[ C o ( N H 3 ) s C F a C O O ] ~+

[ R h ( N H a ) s C H a C O O ] 3+

[ R h ( N H 3 ) 5 ( C H 3 ) a C C O O ] 3+

[ R h ( N H a ) s C F a C O O ] 3+

[ I r ( N H a ) s C H a C O O ] 3+

[ I r ( N H a ) 5 ( C H 5 ) a C C O O ]3+

[ I r ( N H a ) s C F a C O O ] 3÷

2 (mff)

Temp. (°C)

[H + ] (M)

292

55"0 69"9 78"8 78"7 78.8 78'8

0'0485 0"0480 0"0097 0'0194 0.0291 0'0485

9'8 5'4 4"3 7"1 9'6 1.4

X x x X x X

10 6 10 5 10 -5 l0 -s 10 -5 10 -4

70"9 78'6 78"6 78-6 78.6 88-3

0'0480 0'0097 0'0485 0-0730 0-0970 0"0465

1"7 2-6 5'0 6.4 7'7 1'6

X X x x x x

60"0 70"0 78'8 78"9 78.9

0'0192 0.0096 0'0097 0.0485 0-0960

1'8 5.7 1.5 1"6 1'7

53'8 62'7 78'8 78'9 78"7

0"099 0"096 0"0194 0'0291 0"0485

68'6 78'4 78'6 78"6 87'6

310

268

230

245

223

230

225

240

kobs (sec 1)

k]:[20 (sec 1)

kH + (see-1 M - l )

1"4 X 10 -~ 8"0 X 10 -~

1'7 x 10 -4 9"3 x 10 -4

2.0 X 10-5

2"5 x 10-5

10 -5 10 -s 10 5 10 -5 10 -5 10 -4

5'2 x 10 -5

2"5 X l0 4

2'1 x 10 -s 7"7 X 10 5

5.8 X 10-4

x X X X x

10-5 10 -s 10 4 10 4 10 4

1'8 x 10-5 5.7 x 10 -5

~3

x 10 -5

1'5 x 10 4

HI

X 10 4

2'0 5'2 5"5 8"2 1'3

X X X × X

10 5 10 -5 10 -5 10 5 10 -4

0'098 0"0194 0'0485 0"0970 0'0470

3"2 1"9 4'6 8'8 1"1

X X × X X

10 -5 10 -5 10 s 10 -5 10 -4

64'0 72"8 78"9 78"9

0'0195 0"0190 0"0097 0'0485

3'3 8"4 1"5 1"6

X X X ×

10 -5 10 -5 10 -4 10 -4

79"9 79-9 80'0 80"0 91"5

0"0194 0"0485 0'0776 0'0970 0"0465

1'6 4"1 5'9 7"2 1-1

x X x × ×

10 -5 10 -6 10 -3 10 -3 10 5

< 1 × 10-6t < 1 × 10 61"

7"7 x 10 -5 2-4 x 10 -4

80'6 80-6 80.6 95"5

0"0388 0-0582 0-0873 0"055

3"9 6"2 8-4 3'6

X X x ×

10 -7 10 -~ I0 -v 10 -~

< 1 x 10 ~l" , ( 1 × 10-71"

1.0 X 10 -5 6'0 X 10 5

74"0 80"7 80.7 80-7 80"7 92.5

0'0475 0-0194 0'0388 0.0680 0"0970 0.0185

7.3 6-3 1"0 1-5 2'0 1.7

X × x x x x

10 -5 10 -5 10 -5 10 -5 10-5 10 -5

1.8 X 10 -5

1"2 X 10 -4

3'1 x 10 -~ 1.1 x 10 -5

1.8 x 10 4 3"5 x 10 -4

1.6 X 10 a

2"0 × 10 4 5"3 × 10 4

, ~ 4 X 10 -e

2"6 X 10 3 3"3 X 10 -4

~2 ~6

X 10 -5 × 10 -6

8'9 X 10 4 2'1 X 10 -3

3"1 X 10 -5 8'2. X 10 -5

~ 8 X 10 -5 ~ 1 X 10 -4

1"5 × 10 -4

~1'5

X 10 -4

* Ionic strength o f 0.10 (NaCIO4). t T h e s e values are so small t h a t only limiting values can be assigned. F o r t h e s l o w r e a c t i o n s o f t h e I r ( I I I ) c o m p l e x e s , t h e r e a c t i n g s o l u t i o n s in a l u m i n u m foil c o v e r e d v o l u m e t r i c flasks, w e r e k e p t in a t h e r m o s t a t i c b a t h . A l i q u o t s o f t h e s o l u t i o n s w e r e r e m o v e d a t v a r i o u s t i m e s a n d t h e r e a c t i o n w a s q u e n c h e d b y c o o l i n g t o r o o m t e m p e r a t u r e . T h e o p t i c a l densities w e r e determined for each aliquot. B o t h techniques g a v e g o o d results. T h e d a t a used for kinetic plots w e r e those o b t a i n e d after e q u i l i b r i u m t e m p e r a t u r e h a d b e e n r e a c h e d , w h i c h r e q u i r e d a b o u t 10 m i n . F r o m t h e o p t i c a l d e n s i t i e s

Acid catalysis of the hydrolysis of acetatopentammine complexes

1245

observed and the known initial concentrations and the molar absorbancy of the aquo and acetato species, it was possible to calculate the extent of reaction at various times. Fig. 2 shows that good first order plots of the data were obtained over a period of approximately one half-life. The values of kobs have a precision of :?:5 per cent or better. The absorption spectra of the reaction mixture of

250 i

[R h(NH3)5CH3COO]+2

I >LP

I \

z 200 < ffa rr o co m 150 <

~

/

(

~

*"f"~

', '1

}

....

~'" \.

. ~,,

"',~X

,¢;/

-~,,\

.....

--~.(,,,

~.". .....

\..

" 100

/I

\.

',

rr

o

\

L

<

[~hC~)~c(c~)~ c°°]+~ [~h(~)~% COO]+~

~ "{,

x~'

".,\

/ ,

50

0

I

I

I

I

I

i

I

I

220

240

260

280

300

320

340

360

WAVELENGTH (m~) FIG. l . - - T h e absorption spectra of rhodium(Ill) complexes.

0 300

Co(Ill)

Rh(.lII)

¢%.~0o o 0.100 Ir(III)

I

0 0

20 2

40 4-

60 6

B0 8

100 120 140 160 180 200 MINt~Co-Rh 10 12 14- 16 18 20 HR. ( I t )

FIG. 2.--The rates of hydrolyses of [M(NH3)~CH.~COO] 2+. Temperatures: Co, 78.7°; Rh, 78.8°; lr, 79"9 °. Acid concentration of 0.0197 M H ~. [Rh(NH3)sCH3COO] +~ at infinite time was the same as that of the corresponding aquo complex. Determinations for Co(Ill) and Rh(llI) were made at three different temperatures in order to estimate the enthalpies and entropies of activation. Only two temperatures were used for the slowest reactions of the lr(lI1) complexes. Oxygen-18 studies. The method used to determine the x80/1~O ratio in the aquo product [Rh(NHa)sHzO] Br3 was the same as that described by ANBAR and GUTTMANN(16L We are indebted (161 M. ANBAR and S. GUTTMANN,J. Appl. Rad. Isotopes 5, 233 (1959).

1246

F. MONACELLI,F. BASOLOand R. G. PEARSON

to Professor H. TAUBEfor permitting us to perform these experiments in his laboratory. We wish also to thank Mr. J. B. HUNT for his assistance with some of the experimental details. The general procedure was to dissolve ,-,-0.2 g of [Rh(NH3)s(CH3COO)] (C104)2 in 10 ml of 1 M HCIO,, of water of approximately 1"8 per cent oxygen-18, which was at 80°. After 20 min. (-E95 per cent reaction), the solution was transferred to a centrifuge tube contained in an ice-bath. The mixture was centrifuged and the mother liquor decanted from the crystalline solid. This was then dissolved in 7 ml of normal water and 5 ml of conc. HBr was added. The mixture was cooled in an ice-bath and then centrifuged. The bromide salt was collected and dried in a vacuum desiccator over CaCI~ for 5 hr. A yield of approximately 80 per cent was obtained. The salt had the correct ultra-violet spectrum, bromide analysis and weight loss when heated at 100 ° for the compound [Rh(NH3)sHzO] Br3. In a control experiment the same procedure was followed starting with 0"2 g of [Rh(NH~)sH20] (C104)a.

22

CoCN H3)5CH3COOl~'2 20

18

16

0 m12

L.,c~.~)~c%)3ccoo] ÷~

10

CCO0]

+2

0

~

)

s

(CH3)3CCOO]*2

FIG. 3--Rates of hydrolyses of [M(NH3)5(RCOO)] ÷2 vs. [H÷] at ~ 8 0 ° and/~ = 0.10. RESULTS T h e o b s e r v e d rate constants kob s, for the hydrolyses o f [ M ( N H a ) s ( R C O O ) ] z+ as a f u n c t i o n o f [H +] are given in T a b l e 2. F r o m plots o f kobsVS. [H +] as shown in Fig. 3, it is possible to estimate the u n c a t a l y s e d rate constants, kn,o, as well as the acid catalysed rate constants, ku+ (see E q u a t i o n 3). The a p p a r e n t i n d e p e n d e n c e o f the rate at low [H +] r e p o r t e d in Reference (2) is n o t in d i s a g r e e m e n t with Fig. 3. The v a r i a t i o n in rate expected is so small as to be within e x p e r i m e n t a l error. The rate constants at various t e m p e r a t u r e s are, also i n c l u d e d in T a b l e 2. S o m e o f the values o f kK, o are so very small t h a t o n l y a r o u g h a p p r o x i m a t i o n is possible. Similarly the kri+ values for [M(NHa)s(CF~COO)] 2+ o f C o ( I l l ) a n d R h ( I I I ) are only given as an o r d e r o f m a g n i t u d e . T a b l e 3 contains the available enthalpies a n d entropies o f activation.

Acid catalysis of the hydrolysis of acetatopentammine complexes

1247

Oxygen-18 experiments show that the aquo product obtained by the hydrolysis of [Rh(NHz)5(CHaCOO)] +2 has an Oxs/o 16 ratio corresponding closely to that of the heavy water solvent. Under the same conditions there is complete water exchange with the complex [Rh(NHa)sH20] z+. TABLE 3 . - - H E A T S AND ENTROPIES OF ACTIVATION klt20

Complex

. . . . . . . . . . kcal/rnole

[Co(NH3)sCH3COO] ~+ [Co(NH3).~C(CH3)zCOOI 2[Co(NH3) sCF3COO]2+ [Rh(NHa),c, CH3COO]2+ [ R h ( N H s ) s C ( C H 3 ) : ~ C O O I 2~ [ R h ( N H s ) s C F : ~ C O O ] 2-,[ I r ( N H : 0 , ~ C H s C O O ] 2~ [Ir(NH2)sCF:~COO] ~ * Calculated

25 -26 --25 -24

from rate constants

e.u.

k I[ v

kcal/mole 8 2 --6 14

25 26 -23 23 . . 25 * 14

e.u. 2 .6 -4 6 .

. 8 36

at only two temperatures.

DISCUSSION

The results given in Table 1 and shown graphically in Fig. 3, demonstrate that the O II

rates of hydrolysis of [(NHz)sM--O--CR] 2+ complexes depend on the H ~ concentration. Under the conditions of these experiments the observed pseudo first-order rate constant, kobs, is found to fit the two term expression k,,l~ = k m o -r- kn+[H i]

(3)

where kn2 o is the extrapolated rate constant for the uncatalysed rate and k . . is the acid catalysed rate constant. This suggests that two reaction paths are involved and the results can be explained by the reactions O II ku~o • [(NHa)sM--O--CR] 2+ [- H20 [(NH3)sM--OH2] 3+ + RCOOO O I[ K~ H II [(NHa)sM--O--CR] 2+ + H + , . [(NHz)sM--O--CR] a+ O H II [(NH3)sM--O--CR] 3+ + HzO

k6

:~ [(NHz)~M--OHo.]a+ + RCOOH

(41

(5)

(6)

k n + = K~k6

The same scheme was used to account for the acid catalysis of the replacement of monodentate ligands from other systems32,3,41 This requires that the protonated species be more liable than if it were not protonated, that reaction (6) be faster than (4). Of course it must be noted that only at high acid concentrations will there be

1248

F. MONACELLI, F. BASOLOand R. G. PEARSON

enough of the protonated species for it to make an appreciable contribution to the overall rate of hydrolysis. Evidence for the protonation of [Co(NH3)sCHaCOO] ~+ was recently reported by VL~EK.~17~ The acid catalysis observed for the hydrolysis of these complexes containing monodentate carboxylate ligands is of interest with regards to the acid catalysed dissociation of certain metal chelates, e.g. [Fe(EDTA)]- and [Cr(C20~)a]3-. For such systems containing multidentate carboxylate ligands, it was suggested that the acid catalysis may be due to protonation of the freed carboxylate ion.~S,7,s~ This in turn could prevent its returning to the metal and reclosing the opened chelate ring. Kinetic evidence alone does not distinguish between this mechanism and an electrophilic attack by hydrogen ion on a coordinated carboxylate in the chelate, ~7~as proposed in (5) and (6). It is not known which process is involved. However the electrophilic attack path is the only one available to systems containing monodentate carboxylate ligands. Thus this path may well be responsible for or at least make a contribution to the acid catalysed dissociation of certain metal chelates. Previous studies on various acetatopentamminecobalt(III) complexes showed that at low acid concentrations their rates of hydrolyses are almost independent of the steric requirements of the carboxylate ligands, but the rates increase with decreasing base strength of the ligands anions. I1) The present investigation was initiated in order to determine whether the analogous rhodium(Ill) and iridium(Ill) systems have the same behaviour. The values of ku~o for the uncatalysed hydrolysis in Table 2 show that the rates for the acetate and pivalate complexes of Rh(llI) and Ir(llI) respectively seem to be of the same order of magnitude. Furthermore they are approximately one or two orders of magnitude slower than the rates for the analogous trifluoracetate complexes. Unfortunately the rates are so small that the extrapolated values, except for the trifluoroacetate systems, can only be given to an order of magnitude. However the results do show that the Rh(III) and Ir(llI) complexes follow the same general pattern as was observed previously for the complexes of Co(Ill). The values for the acid catalysed hydrolysis constants, kH+, in Table 2 show again that the three metal systems seem to behave in a similar manner. In each case the rate decreases by a factor of approximately five in going from the acetate to the pivalate complexes, whereas a decrease of about twenty occurs in going from acetate to trifluoroacetate for the Co(Ill) and Rh(III) systems. The slower rate for the trifluoroacetate complexes can be explained in terms of a lower concentration of the protonated species. Thus equilibrium (5) is not so far to the right, due to the trifluoroacetate ion being the weakest base. Since pivalate ion is only a slightly stronger base than acetate ion, the slower rates of hydrolysis of the pivalate complexes are due to Steric effects. It is not clear why the value of kH+ for the trifluoroacetate complex of Ir(III) is larger than for the corresponding acetato complex. These complexes have at least a formal relationship to organic esters, the difference being that the pentamminemetal(IlI) in essence takes the place of the alkyl group derived from the alcohol in the ester. Various techniques have been used to demon!O strate that in the acid hydrolysis of esters, acyl-oxygen bond cleavage, R--O]-CR, ~17~A. A. VL~EK,Advances in the Chemistry of Coordination Compounds (Edited by S. KmSCHNER) pp. 590--603. Macmillan,New York (1961).

Acid catalysis of the hydrolysis of acetatopentammine complexes

1249

occurs instead of the alkyl-oxygen bond. as) With this formalism in mind, it is of interest to consider whether the hydrolysis of these complexes proceeds by metaloxygen (7) or acyl-oxygen (8) bond fission. O tl [(NH3)sM__O__CR ] ~z ,

~ [(NHa)sM

O ]I H,.,o O__CR]! z , 1[ ~"~

[(NH3)sM--OH2] ~3 ~ RCOOH

(7)

O [(NH3)sM__O__CR ]~-2 .

~: [(NHa)sM--O

-

CR]~a = - ~ ]i O [(NH3)5M__OH,2 ]!3 _ RCOOH

(8)

BUNTON and LLEWELLYN(19) have performed oxygen-18 experiments, at low acid concentration (pH ~ 4), on the hydrolysis of the cobalt(Ill) complexes [Co(NHa).~ RCOO] z+. Using oxygen-I 8 water as the solvent, they found no isotropic enrichment in the reaction product CHaCOOH, but CF3COOH contained oxygen-18. For the acetato complex this establishes that the uncatalysed hydrolysis path (4) proceeds by metal-oxygen bond cleavage (7). The result for the trifluoroacetato complex suggests acyl-oxygen bond fission. However this was somewhat inconclusive because under the same experimental conditions there is extensive oxygen exchange between CFaCOOH and water. I n an attempt to determine the position of bond rupture for the acid catalysed path (6), oxygen-18 experiments were performed on [Rh(NH3)sCHaCOO] a4. Isotopic enrichment was found in the product [Rh(NH3)sH20] 3+ but under the same experimental conditions, water in this complex was observed to exchange with the solvent. Therefore the isotopic enrichment found does not provide information as to the position of bond fission during hydrolysis. That the rate of water exchange of the aquo complex is similar to the rate of acid catalysed hydrolysis of the acetato complex may at first seem unexpected. Since the protonated acetate ligand, CHaCOOH, is a weaker base than H20, it is expected to be replaced more readily than water. However one must remember that the concentration of the protonated acetato species (5) even in l M H v is small, whereas the aquo complex is present in 100 per cent concentration. No attempt was made to examine the oxygen-18 content of the hydrolysis product CH3COOH, because at these conditions of high acid concentration it would appear (2°~ that there will be extensive oxygen exchange between acetic acid and water. Although appropriate oxygen-18 experiments have not yet been designed to establish the position of bond fission for the acid catalysed hydrolysis, the kinetic data in Table 2 provide some basis for speculation. First it should be noted that the rates of reaction for complexes of these metals are generally known to decrease in the order Co(Ill) > Rh(lll) > Ir(lll). Thus the estimated ~ relative rates of hydrolysis for the ~1~)j. HINE,Physical Organic Chemistr.v, Chap. 12. McGraw Hill, New York (1956). i1~,1C. A. BUNTONand D. R. LLEWELI_YN,J. Chem. Soc. 1692 (1953). ~2~,~1. ROBERTSand H. C. UREV,J. Amer. Chem. Soe. 61, 2580 ( 1939); 1. ROBERTS,J. Chem. Phys. 6, 294 (1938); M. L. BENDER,R. R. STONEand R. S. DEWEY,J. Amer. Chem. Soe. 78, 319 (1956). ~2, F. BASOLOand R. G. PEARSON,Meehanisn2s o f Inorganic Reactions, pp. 121-122. J. Wiley, New York (1958).

1250

F. MONACELLI,F. BASOLOand R. G. PEARSON

complexes [M(NH3)sBr] 2+ are 4,000: 1000:1, respectively. This is definitely not the case for the acid catalysed hydrolysis of [M(NH3)~RCOO] ~+. The values of kI~ for corresponding Co(Ill) and Rh(III) complexes are very nearly the same and those for analogous Ir(III) systems are only about fifty times smaller. It is further important to note that this is also the case for the kn2 o values of the trifluoroacetato complexes. These results suggest an acyl-oxygen bond fission path in which the metal ion, one atom removed from the position of bond rupture, has a small effect. For a reaction path involving metal-oxygen bond cleavage, a larger effect of the metal ion on the rate of reaction is expected. On the basis of the available data (Table 2), this appears to be the case for the water reaction, kn2 o, of the acetato and pivalato complexes. Such a result is in agreement with the oxygen-18 experiments of BUNTON a nd LLEWELLYN.119)

Additional support that the acetato and pivalato complexes undergo uncatalysed hydrolysis by metal-oxygen bond cleavage is afforded by the observation that the ka,o values for the two complexes of the same metal ion are approximately the same. Instead some steric retardation is observed for the rates of acid catalysed hydrolysis, ka+, which appear to involve acyl-oxygen bond fission. These observations are consistent with the expected results, since the pivalato group offers essentially no steric hindrance at the metal ion but does hinder attack at the acyl carbon atom. That the hydrolyses of trifluoroacetato complexes may proceed by acyl-oxygen bond rupture was suggested earlier on the basis that the more positive acyl carbon atom is more susceptable to nucleophilic attack than it is in the corresponding acetato systems. ~19) Likewise a protonated acetato ligand contains a more positive acyl carbon atom than does the parent acetato group. This then may be responsible for the preferred acyl-oxygen bond cleavage in the acid catalysed process. Although the conclusions reached with regard to the position of bond rupture in these reactions are in accord with all of the present experimental facts, there is as yet no direct proof that these conclusions are correct.

Acknowledgement--F. M. wishes to thank the Consiglio Nazionale delle Ricerche for a travel grant to return to the University of Rome.