The hydrolysis products of dichlorodiaquodiammionecobalt(III) chloride and the chromium(III) analog

J. inorg,nucl.Chem.,1971,Vol.33, pp. 3455to 3462. PergamonPress. Printedin Great Britain

THE HYDROLYSIS PRODUCTS OF DICHLORODIAQUODIAMMINECOBALT(III) CHLORIDE A N D THE CHROMIUM(III) A N A L O G M. C L A I R E C O U L D W E L L , D. A. P I C K E R I N G and D. A. H O U S E Department of Chemistry, University of Canterbury, Christchurch, New Zealand

(Received I 1 February 1971) A b s t r a c t - T h e green trans-dichloro-trans-diaquo-trans-diammine salts, [M(NHz)2(OH2)2CI~]CI (M = Co, Cr) have been prepared and are isomorphous. In aqueous acid solution, the dichloro ions hydrolyse to give the blue-violet, trans-triaquo-trans-diammine M(NHz)2(OH2)3C1 ~+ cations, which have been isolated by ion exchange chromatography and characterised in solution. The half-life for the dichlorocobalt(lll) complex in 0.6F HC104 at 25 ° is less than 1 rain, whereas the Cr(lII) complex has a half-life of 213 min under the same conditions. The final hydrolysis products are the pink, transdiammine M(NHa)2(OH2)4 a+ cations, which have been generated from solutions of their precursors by addition of Hg z+, or isolated by ion exchange chromatography and characterised in solution. The tetraaquocobalt(lll) cation is photosensitive to ordinary laboratory light. Visible absorption spectra are presented for these cations. INTRODUCTION

As PART of a continuing study of di- and tri-amine complexes of cobalt(liD [1-3] and chromium(llI)[1, 4-13], we have investigated the synthesis, structure and electronic absorption spectra of the isomorphous[M(NH3)~(OH2)2Cl2]C1 salts (M = Co, Cr) and their hydrolysis products. Only one each of the five possible geometric isomers of the M(NH3)2(OH~)2CI2 ÷ cations have been detected, and these are believed to have the all trans configuration. In aqueous acid solution, these green cations hydrolyse, apparently without isomerisation to give the transtriaquo-trans-diammine M(NH3)2(OH2)3C12+ cations. These blue-violet cations have been isolated by ion exchange chromatography and characterised in solution by their visible absorption spectra and M : N : Cl atom ratios. The final hydrolysis products are the pink trans-M(NH3)2(OH2)4 3+ ions, also characterised in solution by visible absorption spectroscopy and M: N atom ratios. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

S. H. Caldwell and D. A. House, J. inorg, nucl. Chem. 31, 811 (1969). A. R. Gainsford and D. A. House, J. inorg, nucl. Chem. 32, 688 (1970). M. C. Couldwell and D. A. House,J. inorg, nucl. Chem. JINC ms 5436. D. A. House and C. S. Garner, lnorg & Nucl. Chem. Lett. 1, 137 (1965). D. A. House and C. S. Garner, lnorg. Chem. 5, 84 (1966). E. A. V. Ebsworth, C. S. Garner, D. A. House and R. G. Hughes, lnorg. & Nucl. Chem. Lett. 3, 61 (1967). D. A. House, lnorg. & Nucl. Chem. Lett. 3, 67 (1967). D. A. House, R. G. Hughes and C. S. Garner, lnorg. Chem. 6, 1077 (1967). D. M. Tully-Smith, R. K. Kurimoto, D. A. House and C. S. Garner, lnorg. Chem. 6, 1525 (1967). R. F. Childers, Jr., K. G. Vander Zyl, Jr., D. A. House, R. G. Hughes and C. S. Garner, lnorg. Chem. 7,749 (1968). S. H. Caldwell and D. A. House, lnorg. Chem. 8, 151 (1969). D. A. House,Austral. J. Chem. 22, 647 (1969). A. D. Fowlie, D. A. House, W. T. Robinson and S. Sheat-Rumbal, J. chem. Soc. A, 803 (1970). 3455

3456

M.C.

C O U L D W E L L , D. A. P I C K E R 1 N G and D. A. H O U S E EXPERIMENTAL

General The ion exchange chromatography, analyses, reaction rate data, X-ray powder diffractometry and spectrophotometric methods used were similar to those previously described[l, 3, 11]. The preparations and analyses of the cations isolated by ion exchange chromatography were repeated at least five times to check the reproducibility of the absorption spectral parameters and the element ratios. Absorption spectral measurements made on solutions containing weighed amounts of solid were repeated at least three times.

Preparation of compounds Erdmann's salt, trans-(NH4)[Co(NH3)~(NOz)4]'HzO was prepared by the method of Schlessinger [14a]. This was converted to [Co(NHa)~(OH~)~CI2]Ci[15] using Schlessinger's method[14b] to convert Co(NHa)a(NO2)3 to [Co(NHa)3(OH~)CI2]CI. The grass green product was recrystallised from a solution in dilute HCI (0. IF) by the addition of 12F HCI. Anal. Calcd. for [Co(NH3)~(OH~)~CIdCI; NH3, 14.5. Found: NHa, 14.7. [Cr(NHa)2(OH2),Br~]Br'H~O (prepared as described previously [12]), (1.5g) was suspended in 12F HCI (10 ml) in a test-tube cooled by ice. A slow stream of Cl2 gas was bubbled through and the bromide salt slowly dissolved evolving red-brown Br2 gas. After 20-30 rain, green crystals of the product precipitated. These (0"9g) were collected by filtration, washed successively with 2-propanol and ether and air dried. Anal. Calcd. for [Cr(NHa)2(OH~)2CI~]CI: Cr, 22-7; CI, 46.6. Found: Cr, 22.4; CI, 46-8. Chromium(ill) cations The green dichlorodiaquodiamminechromium(lll) cation was generated by dissolving weighed amounts of the solid dichloro chloride in 0.6F HCIO4. The absorption spectrum was measured immediately and analysis of the solution gave a Cr: N : CI ratio of 1 : 1.95 : 2.90, in agreement with the formula [Cr(NHs)z(OH2)~CI~]Ci. Alternatively, about 60 mg of the dichloro chloride was dissolved in ca. 30 ml of 0.01F HCIO4 and the solution run through an ice-cooled cation exchange column (Zeo-Karb, 225, 14-52 mesh, H ÷ form, 10 by 1 cm) that had been pre-washed with 1F and then 0.01F HCIO4. The blue band that formed was washed with 30 ml of 0.01F HCIO4 to remove the anionic chloride and then eluted with 0.3F HCIO, into a 50-ml ice cooled flask, the first 10 ml being discarded. The absorption spectrum was measured immediately and analysis of the green effluent solution (ca. 4mF in complex) gave a Cr: ratio of I : 1.99: 2.03 in agreement with the formula Cr(NHa)2(OH2)2CI2 +. The blue-violet chlorotriaquodiamminechromium(llI) cation was obtained by dissolving ca. 100 mg of the dichioro chloride in ca. 30 ml of 0.1F H CIO4. The solution was warmed to about 50 ° for 10 min and after cooling to room temperature, was run through a water cooled ion exchange column, pre-washed with 2F and then 0.1F HCIO4. The violet band that formed was washed with 0.6F HCIO4 (50 ml) and then eluted with 1.1F HCIO4, the first 10 ml being discarded. The blue-violet effluent was collected in a 50-ml ice cooled flask and the absorption spectrum was measured immediately. Analysis of the effluent solution (ca. 3mF in complex) gave a Cr: N : CI ratio of 1 : 1.98: 1-07 agreeing with the formula Cr(NH3)~(OH~)sCi 2+. The pink tetraaquodiamminechromium(l I 1) cation was prepared in solution as previously described [12], except that the dichioro chloride was used in place of the dibromo bromide. Analysis of the effluent solution (ca. 3F in HCIO4 and 4mF in complex) gave a Cr: N atom ratio of 1 : 1.90, in reasonable agreement with the formula Cr(NH3)~(OH2), 3+. Absorption spectral parameters (Table 1) were in good agreement with those obtained previously [ 12]. Cobalt(Ill) cations The green Co(NHs)2(OHz)2Ch + cation was generated from the dichloro chloride in aqueous methanolic HCI. About 60 nag of [Co(NHs)2(OHz)sCidC! was suspended on filter paper in a funnel above an ice cooled 50-ml flask containing 10 ml of 12F H CI. A solution of ice cold 50:50 CHsOH: 3F 14. G. G. Schlessinger, Inorganic Laboratory Preparations (a,) p. 264. (b) p. 252. Chemical Publishing New York (! 962). 15. A. Werner, Z. anorg, allg. Chem. 8, 172 (1895); A. Weruer and R. Feenstra, Chem. Ber. 39, 1538 (1906).

Hydrolysis products

3457

HCI was run onto the solid and the filtrate (apart from the first 10 ml) was collected in the flask. The absorption spectrum was measured immediately 50 mi had been collected. Analysis of the solution (ca. 3mF in complex) gave a Co: N ratio of I : 2-04, as expected for the formula Co(NHs)~(OHz)2CIz+. Alternatively, about 100 mg of the dichloro chloride was suspended in 10 ml of ice cold 3F HCI. After ca. 10 min at 0°, the undissolved solid was decanted, 20 ml of ice water was added and the diluted solution (now 1F in HCI) was run down an ice cooled ion exchange column (pre-washed with 2F HCIO4 and then IF HCI). The most concentrated fraction (after ca. 10 ml) of the green effluent was collected in an ice cooled 25-ml flask containing 10 ml of 12F HCI. The visible absorption spectral parameters of this solution were identical to those obtained previously and analysis of the effluent solution (ca. 4mF in complex) gave a Co : N atom ratio of 1 : 1-99. The blue-violet Co(N Ha)~(OH2)3CIz+ cation was isolated from hydrolysed solutions of the dichloro chloride. About 80 mg of [Co(NH~)2(OH2)2CI2]CI was dissolved in ca. 30 ml of 0" IF HCIO4 and the solution was allowed to stand at room temperature for 20 rain. This solution was subjected to a similar ion exchange separation as used for the Cr(Ill) analog. Analysis of the effluent solution (ca. 4mF in complex) gave a Co : N : CI ratio of 1 : 2-04: 0-95, in acceptable agreement with the formula Co(N Ha)2 (OH2)3C!~+. The pink tetraaquodiamminecobalt(lll) cation was prepared in solution (0.6F HC104 containing Hg2+) as described previously l 12] for the Cr(l I 1) analog, except that ICo(N Ha)2(OH2)2CI2]CIreplaced the[Cr(NH3)2(OH~)2BrdBr-H~O. Spectral parameters obtained after 2'5 hr in the dark, remained constant for a further 3 hr, but exposure to laboratory light resulted in a rapid decrease in the 362 nm maxima. Analysis of the solution for NH3 and using a Co(IIl) concentration calculated from the weight complex taken, gave a Co:N ratio of I : 2.08, in agreement with the formula Co(N H3)2(OH2)4~+. RESULTS A N D D I S C U S S I O N

Absorption spectra The

visible absorption

s p e c t r a l p a r a m e t e r s ( 3 2 0 - 7 0 0 n m ) for t h e g r e e n M ( N H a ) 2 ( O H 2 ) a C I 2+ a n d p i n k M ( N H 3 ) 2 (OH2)4 a÷ c a t i o n s ( M = C o , C r ) a r e p r e s e n t e d in T a b l e 1 a n d m o l a r a b s o r b a n c y i n d e x vs. w a v e l e n g t h p l o t s a r e g i v e n in F i g s . 1 a n d 2.

M(NHa)2(OH2)2CI2 +, b l u e - v i o l e t

Kinetic data P r e l i m i n a r y d a t a o n t h e r a t e o f h y d r o l y s i s in a c i d s o l u t i o n ( 0 . 6 F H C I O 4 ) at 25 ° h a v e b e e n o b t a i n e d s p e c t r o p h o t o m e t r i c a l l y f o r t h e r e a c t i o n Cr(NHa)2(OH2)2CIz + + H 2 0 = Cr(NH3)2(OH2)3CI 2+ + C1-.

(1)

T h e r e s u l t s a r e r e p o r t e d in T a b l e 2, a l o n g w i t h d a t a f o r r e l a t e d s y s t e m s . A c o m p l e t e k i n e t i c s t u d y w a s n o t u n d e r t a k e n , as t h e d a t a a r e u s e d h e r e o n l y to s u p p o r t s t r u c t u r a l a s s i g n m e n t s . H o w e v e r , t h e s p e c t r a l s c a n s in t h e 3 2 0 - 7 0 0 n m r a n g e d u r i n g t h e h y d r o l y s i s r e a c t i o n h e l d g o o d i s o s b e s t i c p o i n t s ( o v e r t w o half-lives) at 425 n m (aM = 23"6M -1 c m - 0 , 475(8.2) a n d 575(23-6), in r e a s o n a b l e a g r e e m e n t w i t h t h o s e p r e d i c t e d f r o m F i g . 1 at 428 n m (aM = 22"8M -1 c m - i ) , 472(9.8) a n d 577(24.8). T h i s s u g g e s t s t h a t t h e h y d r o l y s i s p r o d u c t s a r e t h o s e i n d i c a t e d in (1) a n d n o o t h e r s p e c i e s a r e f o r m e d , e.g. b y i s o m e r i s a t i o n , in s p e c t r o s c o p i c a l l y d e t e c t a b l e amounts. The rate of hydrolysis for the analogous cobalt(Ill) complex was too r a p i d (t~/2 < 1 m i n at 25 °) t o m e a s u r e u s i n g t h e t e c h n i q u e s a d o p t e d f o r t h e c h r o m i u m ( l I I) c o m p l e x .

The dichloro chlorides T h e s y n t h e t i c r o u t e to[Co(NHs)~(OH2)2CI2]C1 is well d e s c r i b e d b y W e r n e r

3458

M . C . C O U L D W E L L , D. A. PICKERING and D. A. H O U S E

Table 1. Absorption maxima and minima in the 320-700 nm range of some aquo and chloroaquotrans-diamminecobalt(IIl) and chromium(Ill) complexes in aqueous HCIO4 at 20-25 °* Complex

HCIO4(F)

Cr(NHa)2(OH2hCI~+t

0.3

Cr(N Ha)2(OHz)aCI~+'~

1.1

~:

1.5

Cr(NH3)2(OH2)43+t

Amln

420 (23.9) 406 (29.0) 403 (30.2) 390 (18"9) 390 (18-9)

3'0 §

Co(N Ha)2(OH~)zCIz+~ Co(N Hah(OH2)3CI z+ t Co(NHah(OHz)4a+T

3"0 50/50 MeOH/3F HCI I. 1 0"6+Hg 2+

~l

Xmax

379 (44.3) 310 (36"4)

395 (45.2) 362 (73"2) 375 (40.4)

)~raln

)~max

490 (7-6) 475 (9.7)

453 (6"0) 450 (6"7) 490 (8.0) 465 (10.7) 444 (10-9)

562 sh (24) 564 sh (35-3)

~'max 606 (30.3) 563 (25.7) 551 (24.8) 530 (21"6) 530 (21"0) 645 (50.0) 604 (37.8) 537 (43"7) 540 (40.8)

*Number in parenthesis is the molar absorbancy index (aM, M -1 cm -1) (molar extinction coefficient), defined by A = log ( 1o/1 ) = aucd, where c is the molarity of absorbing complex, and d is the optical path length in centimeters. tThis research. ~:T. J. Williams and C. S. Garner, lnorg. Chem. 9, 2058 (1970). § Ref.[12]. tfRef. [20], tentatively assigned as the cis isomer.

and Feenstra[ 15] and the conversion from trans-(NH4)[Co(NHa)2(NO2)4].H20 (Erdmann's salt) with H2SO4/HC1 proceeds smoothly. However, the chromium (III) analog has not been well characterised [16]. We have prepared[Cr(NHz)2(OH2)~C12]C1 by displacement of the coordinated and anionic bromide from the known dibromo bromide[12] with Clz gas in 12F HCI. We note here that the corresponding[Co(NH3)~(OH2hBr2]Br has not been isolated. Addition of HBr to the dichloro chloride causes decomposition to give Co(H) and Br2. X-ray powder diffractograms of the [M(NHa)2(OH2)2CI~]CI salts, show them to be isomorphous (Fig. 3) and thus they have similar geometric configurations in the coordination sphere [17]. Both salts are readily soluble in aqueous acid solution, but only the Cr(IIl) complex has reasonable solution stability with regard to chloride release. Spectral parameters for aqueous acid solutions of the solid Cr(III) complex and for the Cr(NHa)2(OH2)2CI2÷ cation isolated by ion exchange chromatography were identical. Two symmetrical absorption bands are observed for Cr(NHa)z(OH2)2CIz+ (Fig. 1) with no obvious evidence of band splitting or broadening 16. C. S. Garner and D. A. House, In Transition Metal Chemistry (Edited by R. Carlin), Vol. 6. p. 77. Marcel Dekker, New York (1970). 17. A. F. Wells, Structurallnorganic Chemistry 2rid Edn. p. 147. Oxford University Press, (1950).

Hydrolysis products

I

40

I

I

I

3459

I

I

B

"7.

z 30 "i". TAC

BAC

_z 20 Z

t

O tla

10

dz

350

400

450

500

550

600

650

700

WAVELENGTH

Fig. 1. Visible absorption spectra of green Cr(NH3)2(OH2)zC12 + (DAC) in 0.3F HCIO4, blue-violet Cr(NH3)2(OH2)aCI 2+ ( T A C ) i n 1.1F HCIO4 and pink Cr(NHz)2(OHz)43÷ (TA) in 3F HCIO4.

usually associated with trans-dichlorochromium(III) complexes. Nevertheless, we assign a trans-dichloro-trans-diaquo-trans-diammine configuration to this complex on the basis of the green colour, mode of formation, primary hydrolysis rate, ion exchange properties and the formation of the known trans-diammine-Cr (N H3)z(OHs)43+ cation as the final hydrolysis product. The high wavelength band for most trans-dichlorochromium(Ill) complexes is in the 580-600 nm range, whereas the cis analogs have their maxima in the 528-538 nm range (Table 2). Thus the 606 nm maxima for Cr(NHa)2(OHE)2C12 + suggests a trans rather than a cis dichloro configuration. The method of formation by halide exchange from trans-[Cr(NHs)2(OH2)2Br2]Br would not be expected to cause isomerisation and the ready elution of the dichloro cation with 0.3F HCIO4 also suggests a trans configuration. (Cis complexes generally require similar volumes of more concentrated eluting agents for removal from ion exchange columns[l/). The rate of primary halide hydrolysis in acid solution is similar to that for known trans dichloro complexes (Table 2) and much slower than that expected for cis complexes. Also, the rate ratio bromide/chloride = 19 is of the expected order for Cr(llI) complexes with trans dihalo configurations. Treatment of solutions of Cr(NH3)s(OH2)2C12 + with Hg z+, generates the known [12] trans-Cr(NHa)z(OHz)4 a+ cation, as the final hydrolysis product, suggesting

3460

M . C . COULDWELL, D. A. P1CKERING and D. A. HOUSE

,°I 60

z

/,0

,<

20

350

/.00

/.50

500

550

600

WAVELENOTH rim.

Fig. 2. Visible absorption spectra of green Co(NH3)2(OH~)2CI2+ (DAC) in 50:50 MeOH:3F HCI, blue-violet Co(NHa)z(OH2)aC!z+ (TAC) in 1.1F HCIO4 and pink Co(NHah(OH2h 3+ (TA) in 0.6F HCIO4. I

T

T

-'r

T---'-

O"

I0 I

L_

2O

30 l

4O I

5O I

Co

[_ Fig. 3. X-ray powder diffraction patterns (20 = 4-55 °) of[M(NHah(OHe)2CI2]CI (M = Co, Cr). Vertical lines represent the positions and intensities (arbitary units) of the major peaks.

3461

Hydrolysis products Table 2. Rates of primary hydrolysis in acid solution at 25° of some dichloro and dibromochromium(l1I) amine complexes Complex

Medium

t~ (rain)

trans-Cr(en)2Cl~%t trans-Cr(en)2Br2+§ trans-Cr(en)(OH ~hCl~+" trans-Cr(en)(OH2)2Br2+'~'~T trans-Cr(N Hs)a(OH~)CI2+** trans-Cr(NH3)a(OHz)Br~÷** trans-Cr(NHs)~(OH2)~CI2+TT trans-Cr(NH3)2(OH~)2Br~-§§ cis- Cr(enhCl2÷~" cis-Cr(en)2Br2+§

0"IF HNO3 0.1F HNOa 0. IF HCIO 4 0.3F HCIO4 0.6F HCIO4 0.6F HCIO 4 0.6FHCIO4 0.6FHCIO4 0. IF HCIO 4 0.1F HCIO4

512 35"4 370 22.6 280 15 213~t~ 11.3 35 4.12

Br: CI ratio*

~max'~ 578

14.5 580 16.4 595 18 606 19 528 8.5

*Hydrolysis rate ratio for the dibromide relative to the dichloride. t High wavelength absorption band for the dichloride (nm). , D. J. MacDonald and C. S. Garner, J. inorg, nucl. Chem. 18, 219 (1961). §L. P. Quinn and C. S. Garner, Inorg. Chem. 3, 1348 (1964); A. M. Weiner and J. A. McLean,lnorg. Chem. 3, 1469 (1964). i~Ref.[9]. tT'~R. G. Hughes and C. S. Garner, lnorg. Chem. 7, 1988 (1968). **Ref. [11]. tt This research. Mean of three determinations, individual determinations agree within 2 per cent. §§aef.[12]. ,,ij. Selbin and J. C. Bailar, J. Am. chem. Soc. 79, 4285 (1957); 1. Bratushko and P. Nazarenko, Russ. J. inorg. Chem. 12, 1118 (1967). that the t r a n s - d i a m m i n e configuration is retained from the parent and that the thermal and H g 2+ catalysed hydrolysis reactions proceed without isomerisation. T h e analogous C o ( I I I ) cation, Co(NHa)2(OH2)2CI2 + could only be stabilised in solution by the addition of methanolic H C I (3F) to provide sufficient chloride ion concentration to prevent hydrolysis. T h e dichloro ion formed in this manner was characterised in solution by absorption spectral parameters, C o : N atom ratios and ion exchange properties typical of a + 1 charged species. T h e isomorphism of the parent chloride with the C r ( I I l ) analog is convincing evidence that the two cations have the same all-trans configuration. Nevertheless, the difference in the rate of hydrolysis is perhaps unexpected as normally analogous C r ( I l I ) and C o ( I l l ) acido amine complexes hydrolyse (aquate) with rather similar speeds [18]. T h u s the Co(NHz)2(OH2)2C12 + cation is in the class of "labile diamagnetic C o ( I I I ) c o m p l e x e s " [ 1 9 ] , along with Co(tmd)2C12+[19], Co(dpt)(OH2)Cl2 +, Co(2,3-tri)(OH2)Cl2 + [3]* and perhaps Co(NH3)z(OH2)CI2÷T. *Abbreviations used: trod= NH2(CH2hNH2, dpt = NH2(CH2hNH(CH2)3NH2, 2,3-tri= N H2(CH~)2NH(CH~)3NH2. ~'The hydrolysis products and reaction rates of this complexare under investigation. 18. M. Esparza and C. S. Garner, J. inorg, nucl. Chem. 29, 2377 (1967); ibid. 30, 1984 (1968). 19. I. R. Jonassen, R. S. Murray, D. R. Stranks andJ. K. Yandell, Proc. Xll I.C.C.C.p. 32. Sydney, Australia (1969).

3462

M.C. COULDWELL, D. A. PICKERING and D. A. HOUSE

The chlorotriaquo cations The products from the primary hydrolysis of the dichloro cations are the blueviolet M(NH3)~(OH2)zCF + cations (M = Co, Cr). Only one form of each of these has been isolated by ion exchange chromatography and characterised in solution by absorption spectral parameters and M:N:C1 atom ratios. These cations presumably have the trans-diammine-peripheral-triaquo configuration, assuming the hydrolysis reaction from the parent proceeds without isomerisation. Both the M(NH3)2(OH2)~C12+ and M(NH3)2(OH~)3CI ~÷ cations produce trans-M (NH3)~(OH2)4 a+ in aqueous acid solution containing Hg z+, with no evidence for isomerism. In the absence of Hg 2÷, hydrolysis of the monochloro cations to the tetraaquo cations is slow at room temperature and is probably photocatalysed. The tetraaquo cations The trans-Cr(NH3)2(OH2)43+ cation obtained here from Cr(NH3)2(OH2)~C12+ is identical to that described previously[12]. The Co(III) analog, generated in acidic solution from Co(NH3)2(OHz)2CI~ + with Hg 2+, showed distinct photochemical sensitivity to laboratory light. The spectral parameters for Co(NH3)2( O H 2 ) 4 3+ w e r e constant as long as the solutions were kept in the dark, but exposure to normal lighting conditions resulted in a marked reduction of intensity of the 362 mn maxima (aM decreasing from 73.2M -1 cm -1 to about 50M -~cm -1 in 0.5 hr). The positions and intensities of the other maxima and minima being virtually unchanged. This apparent photosensitivity could be due to either the increased reactivity of Co(III) complexes with respect to reduction to Co(II), as the number of nitrogen donor ligands is decreased, or photocatalysed isomerisation to the cis form [20]. Acknowledgement-We acknowledgewith thanks, the receipt of a research grant 70/3/33 from the Universityof Canterbury. 20. J. D. White,J. C. Sullivanand H. Taube,J.Am. chem. Soc. 92, 4733 (1970).