Polyhedron Vol. 2, No. 12, pp. 1267-1272. 1983 Printed in Great Britain.

0277-5387/83 $3.00 + .oO 0 1983 Pqamon Press Ltd.

THE PRODUCTION OF Cr(NI&)~+ FROM THE ACID CATALYSED HYDROLYSIS

OF Cr(NH&(NCO)*+

DAYID YANG and DONALD A. HOUSE* Department of Chemistry, University of Canterbury, Christchurch, New Zealand

(Received 15 February 1983; accepted 18 March 1983)

Abstract-The rate of the reaction

Cr(NH3)5(NCO)2+ + H30+ - Cr(NH3)63+ + CO2 + other Cr(III) products

has been investigated at 40-65”C with [HClOJ varying from 0.04 to 0.6 M 01 = 0.6 A4, NaClO.,). The observed rate law has the form: - d[Cr(NH3),(NCO)2+]/dt = k0,,JCr(NH3),(NCO)2+] where kob, = a[H+]‘{ 1 + b[H+]‘}-’ and at 550°C a = 0.36 M-’ se2 and b = 6.9 x 10e3 M-’ s-‘. The rate of loss of Cr(NH3)&NCO)‘+ increases with increasing acidity to a limiting value (at [H+] N 0.5 M) but the yield of Cr(NH3)63’ decreases with increasing [H+] and increases with increasing temperature. In the kinetic studies the maximum yield of Cr(NH3)2+ was 35% but a synthetic procedure has been developed to give a 60% yield.

The major acid hydrolysis product of M(NH3)#lCO)2+ is M(NH3)2+ and the kinetics of this reaction have been measured for M = C0(111)‘*~ Ru(III)~ and Rh(III).’ At low acid concentrations (< 0.1 M) the rate law - dw]/dt = k p][H +] is observed,1-3 but at higher acidities, some deviation from this is found.

Recently, the synthesis of Cr(NH3)s(NC0)2+ has been described,4 and this too, hydrolyses in acid to

give Cr(NH,), 3+.4 In our hands, however, the yield hexammine from

&NH3),(NCO)](N03)2 in HEO, rHt%, ~1: never greater than 20%. In this paper we describe kinetic data for the loss of Cr(NH3),(NCO)2+ in acid media and present conditions to produce a reasonable yield (60%) of Cr(NH3)63+.

EXPERIMENTAL

[Cr(NH,),(NCO)](NO,), was prepared as de- scribed by Schmidtke and Sch6nherr.4 The bright orange crystalline nitrate salt had visible absorp- tion and IR spectral parameters similar to those

*Author to whom correspondence should be ad- dressed.

described in the literature. Cr Calc, 17.2% Cr Found, 17.1%.

Kinetic studies and product analysis Solutions (6.6 mM) of Cr(NH3),(NCO)2+ were

prepared by dissolving 100 mg of the nitrate salt (F.W. = 303) in water (20 cm3) and adding appro- priate volumes of HC104 and NaClO, solutions to give 50 cm3 p = 0.6 M, [H+] = 0.04 - 0.6 M. The rate of complex decomposition was measured spec- trophotometrically (1 cm cells) at constant tem- perature either by repetitive scans or fixed wave- length (490 nm) techniques. No isosbestic points were maintained in the spectral scan envelope (700-3OO’nm), but satisfactory “infinity” readings (7-8 half-lives) were obtained.

Pseudo-first-order rate constants were calculated from the expression

kt =lne , m

where A,, A, and A, are the absorbances (490 nrn) at time zero, time, t, and at the end of the reaction (> 7 half lives).

At the end of the reaction, the sample used for

1267

1268 D. YANG and D. A. HOUSE

spectrophotometric analysis was returned to the bulk flask (which had been maintained at the appropriate temperature for the duration of the spectrophotometric measurements) and the solu- tion was stored at - 5°C for at least 24 hr. During this time, all the Cr(NH,),3+ present crystallised as the perchlorate salt and 10 cm3 aliquots of the now pink coloured solution were removed for total Cr analysis.’ The difference between the [Cr]ikhal (6.6 mM) and the [Cr] in the mother liquor allowed an estimate of the amount of Cr(NH3),3+ pro- duced. The [Cr] in the mother liquor of a control containing 100 mg of [Cr(NH,),](ClO,), dissolved 50 cm3 of 0.04 M HC104, 0.56 M NaClO, was too small to be of any significance (co.1 mM).

Synthesis of [Cr(NH,),](ClO~), A solution of NaClO,.H,O (45 g) in water

(500 cm3) with 5.0 cm3 of 60% HC104 added was heated to 85°C. 3 g of [Cr(NH,),NCO](NO,), were rapidly added to the stirred solution and the temperature was maintained for exactly 6.0 min before rapid cooling in ice. After 24 hr at - 5°C the yellow crystalline product (2.7 g, 60%) was re- moved from the faintly pink coloured mother liquor by filtration, and washed successively with 2-propanol and ether, and air dried. Calc for [Cr(NH,),](ClO,),: Cr, 11.5%. Found: Cr, 11.5%.

Synthesis of cis-[CrCl(NH,),(OH,)]Cl~ [Cr(NH,),(NCO)](NO,), (2 g) was dissolved in

HCl (6 M, 30 cm3) and warmed (40°C) until pink crystals deposited (20 min). The solution was cooled to room temperature, and the filtered prod- uct (1.5 g, 93%) washed with isopropanol and then ether and air dried. Calc for [CrCl(NH,)~(OH,)]Cl,: Cr, 21.3%. Found: Cr, 21.4%.

L4

65

0 H' ,

0.1 0.2 0.3 0.4 0.5 0.6

Fig. 1. Plots of kob vs p+] for the decomposition of Cr(NH,),(NCO)*+ in HCIO, (JI = 0.6 M) at 40, 55 and

65°C.

RESULTS AND DISCUSSION

Tables 1 and 2 list values for the pseudo-first- order rate constants (k,,,J for the loss of Cr(NH,),(NCO)*+ in acid conditions at various temperatures, together with yields of Cr(NH3)63+. Figure 1 shows a plot of kobs vs [H+] at various temperatures. The non-linearity and limiting val- ues at high [H+] suggest that the [H+] dependence on the reaction rate is unique for H+ catalysed decompositions of M(NH3),(NC0)2+ complexes.‘” Activation energy plots for In (k,,) vs T(K)-’ at any particular [H+] are linear and Table 3 presents the calculated kinetic parameters, with a systematic trend to lower activation energies (and more nega- tive activation entropies) with increasing acidity. The data in Table 2 indicate the decomposition reaction is characterised by a positive salt effect and linear plots of k;, vs [H+]-’ (Fig. 2) give slope (k,-,‘) and intercept ([ktiK]-‘) (Table 4) for the reaction scheme:2,3

Cr(NH3),(NC0)2+ + 2H+ + H,O &

Cr(NH3)5(NH2C(OH)2)4+ (1)

Cr(NH3),(NH,C(OH)2)4+ -% products (2)

where

kobs = kbK[H+12{ 1 + K[H+12}-‘. (3)

Values of K (eqn 1) increase with increasing tem- perature and the associated thermodynamic para- meters are given in Table 4.

The yield of Cr(NH3)2+ (Tables 1 and 2) in- creases with increasing temperature, and de- creasing [H +] and is independent of ionic strength.

Fig. 2. Plots of [k& vs [H+]-* for the decomposition of Cr(NH&,(NCO)*+ in HC104 01 = 0.6 M) at 40, 55

and 65°C.

Tab

le

1. P

seud

o-fi

rst-

orde

r ra

te

cons

tant

s (1

03k,

,,, s

-‘)

for

the

loss

of

C

r(N

H3)

,(N

C0)

*+

in H

C10

4 (p

= 0

.6 M

, N

aC1O

,Yb

and

% C

r(N

H3)

63+

pro

duce

d

cH+l

4cl.o"c

55.0°c

65.0°C

[Ml

0.04

0.68'0.03(27.0)

1.71'0.40(30.8)

[0.52]

Cl.161

0.05

0.26+0.01(22.3)

0.95+0.07(30.9)

2.5210.32135.3)

[0.211

co.773

L1.711

0.10

0.55t0.04

2.17*0.18

3.69t0.36

CO.601

c2.141

L4.601

0.20

l.OOt-0.10

3.54+0.27(20.0

6.69iO.41

(Il.131

r3.841

[8.001

0.30

1.13+0.11(9.2)

4.32'0.13(11.0)

7.40t0.64(16.8)

El.351

14.521

(8.701

0.50

1.42kO.14

4.39'0.21(5.0

7.8310.50(9.1)

Cl.501

c4.971

E10.11

0.60

1.39t0.09

3.96kO.16'

7.06t0.05G(12.7)

L1.521

L5.053

LlO.21

a

Mean * the standard

deviation

of at least three determinations.

Numbers

in parenthesis

are the % Cr(NH3)63+

formed.

[Cr(NH3),(NCO)12+

= 6.6 mM.

b -

Numbers

in webrackets

are 103k+bs(calc)

usi

ng

the

expression

k +bs(calc)

= (&lim~C~+12111

+

K[H

+I~

I-~

with values of

klim and K taken from Table 4.

5

One determination.

Tab

le

2. P

seud

o-fi

rst-

orde

r ra

te

cons

tant

s (l

o’k,

,, s-

‘)

for

the

loss

of

Cr(

NH

3)5(

NC

O)2

+

in

HC

lO,

at

vari

able

io

nic

stre

ngth

an

d in

itial

co

ncen

trat

ions

, T

= 5

50°C

”

[a+

] b

IJ-

complex

i

lo3$bs

Ref.

M

M

mM

s-l

0.1

0.6

6.6

2.17tO.18

c

0.1

0.6

9.9

1.84 (25.8)

d

0.1

0.6

13.2

1.85 (27.0)

d

0.1

0.6

16.5

1.57 (30.5)

d,e

0.04

0.04

6.6

0.36 (?)

d.f

--

4

0.04

0.6

6.6

0.68?0.03(27.0)

c

::

0.1

0.1

6.6

1.14 (?)

d.l

ti

r=.

0.1

0.6

6.6

2.17'0.18(25.8)

c

g

0.2

0.2

6.6

2.90 (14.4)

d

0.2

0.6

6.6

3.54+0.27(20.0)

c

i X

0.3

0.3

6.6

3.34 (11.5)

d

"v

-+"

0.3

0.6

6.6

4.32+0.13(11.0)

c

0.5

0.5

6.6

4.03

(5.2)

d

0.5

0.6

6.6

4.39+0.21(5.0)

c

5

Numbers

in parenthesis

are % Cr(NH3)63+

formed.

h Total

[ClO,-1.

c From Table

1.

d

Single

determination

with an estimated

error

of ?5%.

c

[Cr(NH3)61(C104)3

deposited

during

the kinetic

run.

i

NO

[cTc

(NH

~)~

I(c

~o

~)

3 deposited

at these low C104- concentrations.

1270 D. YANG and D. A. HOUSE

Table 3. Calculated kinetic parameters for the acid hydrolysis of Cr(NH~)JW!O)*+ in HClO, 01 = 0.6 M) at 298.2 K

Ea AS'

(k.7 mol-') (J K-l mol-1)

0.04 0.31 co.343 83.4?12 -60+24

0.05 0.553 LO.511 79.2?3 -69+6

0.10 1.57 [1.553 67.5*3 -99?6

0.20 2.71 E3.121 68.3+2 -92t4

0.30 3.13 [3.86] 68.3+4 -91+8

0.50 4.45 [4.373 60.7+2 -133+4

0.60 4.56 C4.481 57.8?2 -123+4

5 See footnote b_, Table 1.

Although the maximum yield of Cr(NHJ:+ under the reaction conditions used was only 3574, we have extrapolated these trends to develop a synthetic procedure where yields of 60% of the hexaammine can routinely be obtained from the easily prepared [Cr(NH,),(NCO)](NO,),.

Unfortunately we have been unable to charac- terise the other Cr(II1) complexes formed. Absorp- tion spectra of the resulting pink solutions (after precipitation of Cr(NHS)a3+) indicate a complex mixture of Cr(NH3),(OH,)~+_X (x # 5) but the ratio of the products (x = l-4) could not be determined. We can exclude Cr(NH3),(OHJ3+ as a product on three accounts:

(a) The C104- salt of this complex is reasonably

insoluble, but no [Cr(NH3),(OH,)](C104)3 was de- tected.

(b) Treatment of the pink solution with concen- trated HCl did not produce any of the very HCl insoluble [Cr(NH3).&l]C12.

(c) The major product of the reaction between

Cr(NH3)4NCO)*’ and 6 M HCl is cis-

[C~W-&k(OWICL Table 5 lists the reported visible absorption

spectral parameters for various Cr(NH,),(OH,)~+_ * species, together with selected data for the red Cr(NH,),(OH&+_, complex mixtures produced in the reaction between Cr(NH3),NC02+ and H+. Inspection of these data suggest that di- and tri-ammines are the most likely products and that,

Table 4. Calculated limiting values for rate and equilibrium constants

4 a,r b d

T [Kl '9 Ilim K- AHoc ASo- K e talc

(OC) (s-l) (M-l s-l) (kJ mol-l)(J K-1,1-1)

25.0 298.2 4.76 48.2 9.6 64.4 48.2

40.0 313.2 16.0 59.5 58.1

55.0 328.2 52.6 68.7 68.7

65.0 338.2 106.0 76.8 76.3

d Estimated from the intercept of k+bs-' vs [H + -2 plots. 1

b - Estimated from the slope of Lbs-' vs CH+Ie2 plots. 5 Estimated

from the slop of the 1nK vs T -l plot. a Calculated from the

expression RT In K2g8 = TASO-AHo. 5 Calculated from the expression

RT In KT = TASO- AH0 using the values of ASO.and AH0 in the Table.

f Kinetic activation parameters for klim are'Ea = 65.3 t 1 -1 kJno1 ,

AS' = -98 f 2 J K-'mol-1.

The production of Cr(NH,),)+

Table 5. Visible absorption spectral parameters for CI@IH~)AOH&‘_~ species’

Product Mixture b_,c,fi

x A x MX max lo6k+4(700c)Ref. [H+l Imax lmax T

(s-l) @) (OC)

6 465 350 47.G e 0.04 525 387 55

(40) (33) (15.9) (14.9)

5 480 359 41.2 B 0.10 517 381 55

(35.1) (30.3) (21.0) (17.0)

c-4 595 366 24.2 f 0.20 517 381 55

(35.8) (26.8) (19.9) (16.3)

t-4 470 369 44.4 f 0.30 515 397 55

(21.5) (32.1) (22.1) (17.4)

fat-3 513 374 7.6 f 0.50 512 376 55 -

(36.6) (22.6) (24.3) (18.6)

mer-3 502 376 23.5 0.60 510 374 55 - g

(26.3) (26.8) (26.5) (20.4)

c-2 526 386 3.4 g 0.05 523 385 40

(27.0) (21.3) (18.3) (15.1)

t-2 522 387 17.5 grh 0.05 526 388 55

(21.3) (24.6) (17.1) (15.4)

1 547 396 1.1 CJ 0.05 525 387 65

(19.9) (18.6) (16.2) (15.4)

0 574 407 9

(13.2) (15.5)

5 A in nanometers, values in parenthesis are molar extinction

coefficients (E, M -1 -1) cm . 5 = C&-, &= a-.

b [Cr(NH3)6](C104)3 precipitated by storage at -5OC for at least

24 hours before spectral analysis. c [Cr(III], determined

spectrophotometrically by alkaline oxidation

Mean of 3 determinations (E f 0.2 M -1 cm-l).

e Ref. 9. g 8. Ref. 2 Ref. 6. !!

to Cr04 2-

with H202.

a )I = 0.6 M_ (NaC104).

Ref. 7. i Kinetic

1271

data from Table 4, ref. 9.

like the production of Cr(NH,),3+, the product ratio depends on both [H+] and temperature. In addition, Table 5 also lists rate constants for the thermal rupture of successive Cr-NH, bonds in HClO, media.““” If di- and tri-ammines are in- deed produced, it is obvious that their rate of formation from the parent pentaammine must be strongly catalysed with respect to thermal Cr-NH, bond rupture. It has been observed previously,*‘*‘* that weakly acidic leaving groups (e.g. NO*-) can catalyse Cr-NH3 bond rupture in acidic media and it is possible that the CO2 produced in the acid

decomposition of Cr(NH,),(NCO)Z+ behaves in a similar fashion.

CONCLUSION Comparison of the data obtained here with

those obtained for other M(NH3)S(NCO)2+ sys- tems is difficult because of different rate laws, but at a constant [H+] the order of reactivity is Rh3 > CO’~ > Ru3 % Cr.

In the case of Rh, Co and Ru, there is good evidend’ for an intermediate N-bonded car- bamate complex and the mechanism proposed here

1272 D. YANG and D. A. HOUSE

(eqns 1 and 2) incorporates such a protonated species for the Cr(II1) system. It is possible that the rapid loss of NH, could be attributed to chelation of the carbamate (cj oxalate)13 and this could account for the isolation of cis-[CrCl(NH,),

(OH,)]Cl,. Indeed Cr(NH,), /NH,\ , o ,C=O >

‘+ or

CrPJH3h /o\ 2+ ( ,. //C-NH,

> could also be in-

cluded as potential components of the unknown red reaction products.

Acknowledgements-We thank the University Grants Committee for providing funds to purchase instruments used in this research. We also thank the University of Canterbury for the research grant 81/3/8.

2. D. A. Buckingham, D. J. Francis and A. M. Sargeson, Inorg. Chem. 1974, 13, 2630.

3. P. C. Ford, Inorg. Chem. 1971, 10, 2153. 4. H. H. Schmidtke and T. Schiinherr, Z. Anorg. Allg.

Chem. 1978, 443, 225. 5. D. A. House, Acta Chem. &and. 1972, 26, 2847. 6. L. Monsted and 0. Msnsted, Acta Chem. Stand.

1973, 27, 2121. 7. D. A. House, Aust. J. Chem. 1969, 22, 647. 8. L. Monsted and 0. Msnsted, Acta Chem. &and.

1974, 28, 23. 9. L. Monsted and 0. Mnmsted, Acta Chem. &and.

1974, 28, 569. 10. L. Msnsted and 0. Mamsted, Acta Chem. &and.

1974, 28, 1040. 11. T. C. Matts and P. Moore, J. Chem. Sot. Chem.

Comm 1969, 29. 12. T. C. Matts and P. Moore, J. Chem. Sot., A 1969,

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