Table l-e-Effect of Varying [H +] on the Rate of Chloride Release in cis-cx-[Co\trien) (n-propylamine)CI] (CI04h and cis-cx-[Coltrien) (benzylamine)CI] (CI04h

[Ionic strength =0.3 mol drn ":']

105 kobs (s - ')55'

Indian Journal of ChemistryVol. 21A, November 1982, pp. 1070-1071

Kinetics of Acid & Base Hydrolysis ofcis-a-[Co(trien)(benzylamine)CIJ2 + &cis-a-[Co(trien)(n-propylamine)CIJ2 +

NAYAN K MOHANTYt & R K NANDA*

Department of Chemistry, Utkal University,Bhubaneswar 751004

Received 5 August 1981; re oised and accepted 26 June 1982

The rates and activation parameters of the acid and basehydrolysis reactions of the title cations have been reported. The acidhydrolysis rate constants of both the complexes are found to beindependent of [H +] in the pH range I to 3. In the base hydrolysispath the benzylamine complex reacts nearly 10 times faster than then-propylamine complex presumably due to more significant stcriceffect of the bulky benzylamine in this path.

As a part of our systematic investigation t - 3 on thereactivity of [CourienjtarnineCl]? + systems, the acidand base hydrolysis of [Cottrienltbenzylaminelt.l]? +and [Co(trien)(n-propylamine)Cl]2+ have now beenundertaken to examine the effect of. steric factors incomplexes containing non-labile amines of similar basestrengths.

[Co(trien)(benzylamine)Cl] (Cl04lz (complex-I) and[Co(trien) (n-propylamine)Cl] (Cl04h (complex-H)were prepared following the method of Bailar andClapp". The starting materials were crude cis-[Co(trien)Clz]Cl (which is believed to be a mixture ofc;s-IX-[Co(trien)Clz]CI and c;s-!1-[Co(trien)Ciz]Cl) andthe corresponding amines. The complexes wereisolated as chloride salts which were then converted torespective perchlorate salts. The purities of thecomplexes (I) and (II) was checked by elementalanalyses. Complex-l++Found: Co, 10.47: Cl, 6.32:[Co(C6N4H 18) (C6H5CHzNHz)Cl] (Cl04)2 requires

50

n-Propylamine complex

Co, 10.78; Cl, 6.5~(). Complex-H-e-Found: Co, 11.76;CI, 7.03. [Co(C6N4H 18L (CH3CHzCHzNHz)Cl]«no.i, requires Co, 11.81: Cl, 7.12'/". Both these

I exhibit absorption maxima around 530 nm. In keepingwith earlier observations 5.6 both these complexes havebeen characterised as ciS-IX.

The spectra of the complexes were recorded on aBeckman OU2 spectrophotometer in 0.1 mol dm - 3

HCI04. HCl04 (E. Merck) and NaCI04 (Reidel) wereused to adjust the acidity and ionic strength of thereaction medium respectively. 2,6-Lutidine/HCI04

buffers were prepared by mixing requisite volumes ofthe amine and HCI04. All pH measurements weremade with the help of a ECI L pH-meter model 5651.

The rates of acid and base hydrolysis of complex-Iwere followed potentiometrically by titrating theliberated chloride against 0.01 mol dm -3 AgN03. Thehydrolysis of complex-l l was followed spectrophotom-etrically at 540 nm, the decrease of absorbance of thechloro complex with time being monitored. The rateconstants reported are average of atleast duplicateruns. All calculations were made by the IBM 1130computer of the Utkal University.

The pseudo-first order rate constants werecalculated from the plots of log (V x - V,) versus time inthe case of complex-I (V, and V x represent the volumeof 0.01 mol drn":' AgN03 solution at time t and aftercompletion of the reaction respectively) and from theplots of log (0, - Ox) versus time in the case ofcomplex-II (0, and 0 x represent the absorbances attime t and after completion of the reactionrespectively).

The values of kOb' for both the complexes wereindependent of [H +] in the pH range 1 to 3. Averagevalues of the rate constants in this pH range, therefore,

.- - - ------ ..---~--

60105 kobs Is - 1 )

65 7060

Benzylamine complex

0.1 0.90±0.1 1.40 ±0.03 2.46±0.06 1.38 ±0.01 2.68 ±0.05 4.6±0.080.01 0.85±0.08 1.49 ±0.040.001 0.90±0.03 1.38 ±0.07

105 ko(S-1) 0.9±O.03 1.40 ±O.03 2.46±0.06 1.45 ±0.02 2.68 ±0.03 4.6±0.05AHt. kJ mol- 1 86.6±7.4 106±4Ast, JK - 1 mol-' -74±22 -19±11

tPresent address:Dhenkanal College, Dhenkanal 759001.

1070

Table 2-Base Hydrolysis or cis-:l-[Co(lrien) (11-

propylaminc)CI] (C10~12 and cis-:l-[Co(lricn)(bcnzylaminelCI] (("104)2

: Ionic strength =O.~ mol drn -.\:

10'kobJs II IO'lH -J 10'kohJs' I)moldOl-3

lO'lH'Jmol dm-3

I.77X0.X9130.50120224

n-Propvlumine complc»: Hel1:ylul1lill(, complexTemp =40'

419±0.02 1.9959.13 ± 0.05 1.26I S.X ± 0.1 0.611

41.75 ±0.5

2.9 ±O.I4.4X ± 0.05

X.X ± 0.1

Temp =45

6.91 ±OOS 4.46717.1 ±O.I 2.XIX32'()± 1.0 1.995'1','1.0± 7,0 1.5'1',5

1.26

Temp= 50

15.0 ± 0.3 17.7X I.X4 ± 0.0624.9±09 7.079 3.69±0.165.0 ± 1.0 5.012 4.4X ± O.O~

140.0± 10.0 3.16 9.15 ±O.I1.77'/', 15.0 ± 2.5

10-2 KOH (drrr' mol-I s-II= 1.59±0.3 (40).2.1 ±0.3 (45) and 3.8±0.6(50 ) for complex-It and IO.55± 1.0 (40 I. 14.2±0.9(4S land27.6iO.5 (50 I for cornplcx-I.LlHt (kJ mol-1)=94±3 (cornplcx-Ll] and 89± 17 (cornplcx-l ).LlS! UK -I mol 1)=94±9 (cornplcx-l l l and l)S±5.3(complcx-l).

1.77X0.X9130.50120,224

1.7780.89130.50120.2239

2.22 to.063. 75 ±O.OX5,65 ± 0.05

7.0 ± 0.29.1±0.OS

were taken to be the aquation rate constants (ko) (seeTable I). Base hydrolysis rate constants of thecomplexes collected in Table 2 were evaluated from theslopes of the plots of (koh, - ko) versus [OH-l

NOTES

The results show that the cornplex-l l aquates nearlytwice as fast as cornplex-I. Both I'lH* and I'lS*contribute towards this rate difference. However. in thebase hydrolysis path, complex-I reacts nearly 10 timesfaster than complex-H. Presumably steric effect of thebulky benzylamine in complex-I is relatively moresignificant in the base hydrolysis path, Agreementbetween the base hydrolysis rate constants ofcomplexes I and II with the corresponding chloro(amine) (enhCo(l1l) complexes is reasonable 7: (kOH for[Colen], (bcnzylaminejClj ' + = 1,02 drrr' mol- I s - t

(0'. 1=0,1 mol dm-3): and for [Cotcn), (11-

propylarnine.Cl]? + = 11.0 drn ' mol- I s - I (25 .1 = 0.1mol dm -3)), This is in accord with the structuralsimilarity between these complexes.

The authors are thankful to Dr A C Dash for helpfulsuggestions and to the UGc. New Delhi for the awardof a teacher fellowship to one of them (NKM).

ReferencesI Dash A C. Mohanty N K & Nanda R K. J Indian citelll S" •.. 54

(1977) g9.2 Mohanty N K & Nanda R K. Communicated to J inorg nucl

Chern.Mohanty N K & Nandu R K. Transition M('I C/I<'IIl. (in press).

4 Bailar J C & Clapp L B.J AIIl cll<'111S()L 67 (1945) 171.5 House D A & Garner C S.II1()I'j( CIIt'IIl. 6 (1967) 27'2.

6 (a) Marzilli L G & Buckingham D A./n()rg Chelll. 6 (1967) 1042.

(bl Buckingham D A. Forster D M. Mar7illi L G & Sargeson AM.1/1<11'1( ['hem. 9 (1970) 12.

7 House D A. Coord Cll<'m R('/', 23 (1977) 223 and data andreferences cited therein.

1071