Kinetic study of addition of nitrogen and phosphorus ...

NOTES

Kinetic study of addition of nitrogen and phosphorus donors bis(maleonitriledithiolate)cobalt(III)l

Can. J. Chem. Downloaded from www.nrcresearchpress.com by 41.203.89.245 on 06/03/13 For personal use only.

HON GEETSIANGAND C. H. LANGFORD, Department of Chemistry, Carleton University, Ottawa I , Canada Received3 October 9, 1969

Kinetics of adduct formation of the dimeric form of bis(maleonitriledithiolate)cobalt(III) with pyridine, triphenylphosphine, triphenylarsine, and 1,lO-phenanthroline are reported. Rates are first order in ligand and half order in complex dimer in acetone. This suggests addition of the fifth ligand to a n uncoordinated axial position of the square planar species as rate controlling. A (presumably) five coordinate intermediate can be detected kinetically on the way to six coordination in the reaction with 1,lO-phenanthrioline and its rearrangement to an octahedral six coordinate chelate is slow. All these results may be interpreted in terms of the significant delocalized n bonding in the square planar dithiolene complexes. Canadian Journal of Chemistry, 48,2776 (1970)

Results and Discussion Introduction The dimer-to-adduct reactions were followed The recent literature is full of accounts of from small spectral changes which correspond to syntheses, physical properties, and electronic "relaxatioo conditions" and are effectively structure of square planar metal complexes containing sulfur donor ligands in which the sulfur linearized. Therefore, the order of reaction with atoms are part of unsaturated organic systems (1). respect to complex could not be determined from However. kinetic and mechanistic asDects have these experiments at fixed metal complex conreceived little attention. It has been reiorted that centration. Table 1 collects, therefore, reaction Co(MNT),- (MNT = C,N,S,,or maleo- half times as a function of ligand concentration nitriledithiolate) forms adducts with ligands such when ligand is in excess. The rate constants in as pyridine, triphenylphosphine, and-triphenyl- Table 2 where initial complex concentration was arsine which are five coordinate and adducts with varied show that the reaction is in fact halforder ligands such as 1,lO-phenanthroline which are six with respect to complex. The rate law may be written as shown in eq. [ I ] coordinate (2). This note reports the kinetics of such adduct formation and discusses the mech- [I ] d [Co(MNT),L]/dt = anism. It should be noted here that the studies ~,[(CO(MNT),),~]+[I-] were carried out in the essentially non-coordinating solvent acetone where the species Co- where k, is the specific rate constant in M-* s-l. This is consistent with the mechanism shown in (MNT),- has been shown to be dimeric (1). eq. PI. kd Experimental [2] (CO(MNT),),~- C 2Co(MNT),(fast, The rates were measured using a Durrum-Gibson stopped flow spectrometer. Equilibrium quotients were evaluated spectrophotometrically using a Cary 14 instrument. The complex Bu4NCo(MNT), was prepared after the recipe in ref. 3. Other chemicals were of reagent grade and were used without additional purification. 'We thank the National Research Council of Canada for financial support 'Alfred P. Sloan Foundation Research Fellow, 19681970. Author to whom correspondence should be addressed. 3Revision received May 19, 1970.

equilibrium)

k Co(MNT),- + Co(MNT),L-

(slow)

At fixed excess ligand concentration, the pseudo half order rate constants calculable from the half times in Table 1 are equal to kd3k [L]. The specific rate constants are these values divided by the ligand concentrations. The reaction with o-phenanthroline is unique. In addition to the fast reaction described-in Table 1, there was a slower process observable in the

NOTES

TABLE 1 Half times for the formation of various adducts of CO(MNT)~-at 25" in acetone solvent* --


Ligands (L)

Wavelength (md

Concentration (M)

ti 6 )

Pyridine

Triphenylarsine

'Concentration o f Co(MNT),- = 0.206 M. ?There are two resolvable steps in the stopped flow records for this reaction. The faster one is assumed to refer to the formation o f a five-coordinate species which precedes formation o f the final octahedralcomplex. Dataon thefast processappear here for comparison with systems going only to five coordinate species. Data on the slower process appear in Fig. 1.

L

TABLE 2 Specific rate constants of the adduct formation of Co(MNT),- at 25 "C in acetone solvent -

[Co(MNT)2I2(MX 104)

-

Specific rate [Pyridine] constants ( M X 104) ( ~ - 1 S-1 1 ~x lo2)

stopped flow traces. The final product corresponds to the (Co(MNT),o-phen)- six coordinate species shown to be octahedral by crystallography (4). We suggest that the slow step be assigned to conversion of a 'quare pyramidal five coordinate intermediate (analogous to the adducts formed with (C,H,),P, (C,H,),As, and pyridine) into the octahedral species. high concentiation of o-phenanthroline, the formation of the five coordinate s~eciesbecomes fast and a limiting process is observed in which formation of the six coordination species becomes observably first order, since it is no more than isomerization of the five coordinate complex to the six coordinate one of the same stoichiometry. Pseduo first order rate constants (calculated from the slow step in the traces) are plotted as a function

0

- 0

-

0.01

0.05 [phenIlM)

FIG. 1. The rate constant (pseudo first order) for adduct formation with k in s - l as a function of the molar concentration of phen. T = 25". These data apply to the slower process described in the text.

of the (excess), o-~henanthrolineconcentration in Fig. 1. The expected limiting behavior is clear. A remark is warranted by the relationship between rate and equilibrium constants for five coordinate adduct formation collected in Table 3. As the attacking ligand is varied, the formation rate parallels the formation constant. This skggests that the transition state is similar to \~~

A

CANADIAN JOURNAL OF

2778

product with respect to the role of the attacking ligand. This is a little surprising since the ligand is attacking an "uncoordinated" position. The


TABLE 3 Specific rate constants and equilibrium quotients of adduct formation

Ligand

Equilibrium auotient

Specific rate constant

Triphenylarsine Pyridine Triphenylphosphine

CHEMISTRY. VOL.

48, 1970

only obvious explanation for strong product like binding in the transition state is that the transition state lies well along the reaction coordinate because the square planar four coordinate complex is resistant to alteration. Such resistance is consistent with the often suggested (') n delocalization over the planar metal ligand system. 1. J. H. MCCLEVERTY. Prog. Inorg. Chem. 10, 49

(1968). 2. C. H. LANGFORD, E. BILLIGS. I. SHUPACK, and H. B. GRAY. J. Amer. Chem. Soc. 86,2958 (1964). 3. A. DAVISON and R. H. HOLM. Inorg. Synth. 10, 15 11 967) \---..,.

4. G. P. KHARE,C. G. PIERPONT, and R. EISENBERG. Chem. Commun. 1692 (1968).

The decomposition of aqueous dithionite 111. Stabilization of dithionite by cations W. J. LEM'AND M. WAYMAN Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto Received February 10, 1970

At the same conditions of pH (3.5 to 5.0), temperature (23 "C), and concentration (1 to 10 x M), ZnSZO4is more stable in aqueous solution than Na2SZ04,and the rate of decomposition is much less sensitive to [H+]. At pH 4.5 and 5.0, the decomposition of the Zn salt is essentially linear, whereas the Na2S20, decomposes slowly at first, during an induction period, and then rapidly by an autocatalytic reaction. The rate of decomposition of dilute aqueous Na2S204at pH about 4 is slowed greatly by the addition of small quantities of In3+, CdZ+,or ZnZ+.The effect of the stabilizing ions is attributed to anti-catalytic action. Canadian Journal of Chemistry, 48, 2778 (1970)

Introduction ZnS204is used industrially as a reducing agent or for bleaching, especially by the pulp and paper industry. It is unstable in aqueous solution, especially under acidic conditions, but the kinetics and mechanism of its decomposition have not been investigated. Qualitatively it is known to be more stable than Na2S,04 under similar conditions of p H (1). Following our studies of the kinetics and mechanism of the decomposition of Na2S20, (2, 3), aqueous ZnS204 was studied in the same manner. The dependence of its rate of decomposition on p H was determined. Methods of stabilizing dithionite by cations such as Zn2+ were tried since, if successful, they could be of commercial significance.

Experiments and Results

(a) Decomposition of ZnS204 Several concentrations of ZnS204 from 0.8 to M were studied, some at fixed p H 8.0 x values in the same way as aqueous buffered Na2S204 described in Part 1 (2), and some in unbuffered solution, with p H measurement (3). The continuous polarographic method previously described (4) was used to determine the concentration of ZnS204 against time continuously; the experimental arrangements are shown in Fig. 2 of ref. 2 and Fig. 1 of ref. 3. The ZnS204 used was the best commercial grade obtainable, but was only 80 % pure, the impurities being mainly the sulfate and sulfite, with about 0.3 % of each of thiosulfate and sulfide. Sodium acetate - acetic acid buffers were used, where indicated, to provide constant p H values of 3.5 'Present address: Imperial Oil Enterprises Ltd., to 5.0. The work was carried out at 23 "C. Sarnia, Ontario. The decomposition of Na2S204 in aqueous