2318 Inorganic Chemistry, Vol. 15, No. 9, I976
Notes
Table I. Mossbauer Data and Proportions of E d 1 and Ed1' in Eu,Mo04
_-__
Percentage EU'I
Percentage EU'II
Material
S(EuIT)f mm s-'
Exptl
Theor
6(Eu1I1)? mm s-'
Eu0.9MoO4 Eu0.7MoO4 E'JO.C.,MOO~ "Gd0~,Eu,,,Mo04"
-11.7 -12.1
73i.2 15 i. 2 0 i 0.1 55 f. 0.8
77.8 14.3 0 See text
+0.6 +0.6 +0.7 +0.9
-12.0
Exptl 27 85
*1 i.
2
100 f. 0.1
45
2
00.7
Theor -______ 22.2 85.7 100 See text
Chemical isomer shifts 6 are quoted relative to EuF, as zero.
present as MoV, the detailed formulation being
Gd"'o.ijEu"o.~jEu'"o.~oM~~o.~~M~~~o,~~O~. Acknowledgment. We thank the U.K. Science Research Council for financial support and Instituto de Alta Cultura (Portugal) for a grant (to F.V.). The work at Polytechnic was sponsored by the U S . Army Research Office (Durham, N.C.), Grant No. DA-ARO-D-31-124-72-G-7, and the Joint Services Electronic Program under Contract No. F44620-69-C-0047.
2.96
Registry No. Eu,Mo04,52322-41-5; Gdo.sEuo.5Mo04,59753-15-0.
References and Notes
,
(..
(1) (a) University of Leeds; (b) Polytechnic Institute of New York. (2) E. Banks and M. Tiemiroff, Znorg. Chem., 13, 2715 (1974). (3) R. G . Johnston and G . T. McCarthy, J . Znorg. Nucl. Chem., 37, 1923 ( 1975). (4) C. S. Dimbylow, I. J. McColm, C. M.P. Barton, N. N. Greenwood, and G. E. Turner, J . Solid State Chem., 10, 128 (1974). ( 5 ) T. C. Gibb, R. Greatrex, N. N. Greenwood, D. C. Puxley, and K. G. Snowdon, J . Solid State Chem., 11,,,17 (1974). (6) N. N. Greenwood and T. C. Gibb, "Mossbauer Spectroscopy", Chapman and Hall, London, 1971.
1
-
. ,.
j
0.36
Contribution from the Department of Inorganic and Structural Chemistry, The University, Leeds LS2 9JT, England
Evidence for a Dissociative Mechanism in the Reaction of Glycine with Cr(NH3)5H203+. Ionic Strength Contributions (as a 1:l Electrolyte) and Ion-Pairing(K1p) Ability of the Glycine Zwitterion 0.92
T. Ramasami, R. S. Taylor, and A . G . Sykes* -2n
-10
r
tin
+2n
Figure 1.
Receiued February 24, I976
nances on the assumption that both species have the same Mossbauer recoil-free fraction. The theoretical values were calculated according to the cation vacancy defect model with no contribution from Mo". Since saturation effects will tend to decrease the intensity of stronger peaks,6 the concentration of EuI1 in Euo 9Mo04 will be slightly greater than the uncorrected experimental value of 73% and the concentration of EulI1 in Euo 7Mo04 will be slightly greater than 85%. The close agreement of these values with those calculated on the basis of the cation vacancy model rules out the possibility of any significant reduction of molybdenum to Mo" and the presence of [MoV04I3-ions in these phases since this would substantially reduce the concentration of Eu" required for charge balance. In the case of "Gdo 5Euo 5Mo04", the two-phase mixture of limiting solid solutions was studied because no samples of the single-phase materials remained from previous work. Table I shows that 45% of all the europium is EulI1. The phase Gdo ssEuo 1jMo04 contained predominantly [MoV04I3-ions, implying that both gadolinium and europium are in the 3+ state; this accounts for one-third of the EulI1 resonance area. The second phase (Gdo 15CUO 85Mo04) contains the other two-thirds of the EuIrr (Le., EuII'o 30); charge balance then requires that part of the molybdenum in this phase is also
There is at present no general agreement as to the mechanism of substitution of H2O in aquopentaamminechromium(III), C ~ ( N H ~ ) ~ H Z O and ~ + ,the ' - ~reaction with glycine is of interest in further evaluating the substitution behavior of this complex. Features of the recently studied reaction of oxalate with Cr(NH3)5H2O3+are the initial replacement of H 2 0 5 at a rate comparable to that of water exchange,2 followed by the chelation of the oxalate with displacement of ammonia.6 Experimental Section
AIC60149T
Materials. The complex [Cr(NH3)5H20] (c104)3 was prepared as previously de~cribed.~.'Glycine (BDH Analar) was used without further purification. Triply distilled and CO2-free water was used in malang up solutions. Lithium perchlorate was prepared from HC104 (Analar 72%) and Li2CO3 (reagent grade) and recrystallized. Kinetics. Preliminary experiments on the reaction of Cr(NH3)sH203+ (5.8 X lo-) M) with glycine (5.8 X lo-* M), [H+] = (3.2-16) X M, I = 1.00 M, 50 "C, indicated nonretention of isosbestic points and therefore the formation of more than one product. Ion-exchange separation of reactant solutions (for details see ref 6), over periods in which there was up to 50% consumption of Cr(NH3)jHz03+,indicated two products, Cr(NH3) j(02CCH2NH3)3+ and Cr(NH3)4(H20)(02CCH2NH3)3+, which have previously been characterized.6 The kinetics were monitored at the 506-nm isosbestic point for the two products (e 39.8 M-' cm-I). Concentration ranges
Inorganic Chemistry, Vo1. 15, No. 9,1976 2319
Notes Table I. Rate Constants, kobsd, for the Anation Reaction of Glycine with Cr(NH,),H,O'' at h 506 nm, I = 1.00 M (LiCIO,)
6.0-
1031Cr-
40.0
0.50
4 .O 3.5 4.0 3.O 14.0 4.0 4.0 3.5 3.5 16.0 4.0 3.5 3.0 3.5 16.0 4.0 3.5 4.0 3.0 4.5 4.0 3.1) 3.0 4.0 4.0 3.5 12.0 4.0 3.5 4.0 4.0 4.0 4.0 3.5 3 .O 4.0 3 .O 3.5 3.O
1.26
3.16
0.50
47.5
1.26
3.16
55.0
0.50
1.26
3.16
0.15 0.30 0.45 0.60 0.60 0.15 0.30 0.45 0.60 0.60 0.15 0.30 0.45 0.60 0.60 0.15 0.30 0.45 0.60 0.15 0.30 0.45 0.60 0.15 0.30 0.45 0.60 0.15 0.30 0.45 0.60 0.15 0.30 0.45 0.60 0.15 0.30 0.45 0.60
0.30 0.55 0.76 0.99 0.99 0.23 0.45 0.61 0.86 0.88 0.17 0.32 0.48 0.63 0.62 0.75 1.34 1.85 2.35 0.58 1.15 1.70 2.06 0.45 0.87 1.32 1.60 1.89 3.75 5.08 6.15 1.61 2.78 4.62 5.27 1.17 2.20 3.12 3.93
[Glyl, ( M
Figure 1. Dependence of pseudo-first-order rate constants, k o b d , on [ G l y ] for ~ the reaction of Cr(NH,),H,O'+ with glycine at temperatures as indicated and [H'] = 3.16 x M (A), 1.26 X lo-' M (*),and 0.50 X lo-' M (.);I= 1.00 M (LiClO,).
I
I
covered were total glycine, [ G l y ] ~= 0.15-0.60 M, and free [H+] = (0.5-3.2) X M. The total perchloric acid concentration [H+]T required in making up solutions was calculated from (I), where Ka
is the acid dissociation constant for glycine, (2). The free [H+] was 'NH,CH,CO,H
Ka
*+NH,CH,CO,-
+ H'
ii
/&
Figure 2. Linear dependence of kobsd-' on [ G l y ] ~ - 'for the reaction of Cr(NH,),H,O3+ with glycine at 47.5 'C, I = 1.00 M (LiClO,), and [H'] = 3.16 x lo-, M (A), 1.26 X lo-' M (e), and 0.50 x 10-3 M (m).
(2)
measured on a Radiometer pH meter 4 calibrated with M HC104 at I = 1.00 M (LiC104). Plots of log ( A , - At) against time, where A , was calculated from known spectra, were linear to 40-50% completion, and from the slopes (X2.303) pseudo-first-order rate constants k&d were obtained which were independent of complex concentration. The ionic strength was adjusted to 1.00 M using LiC104. When NaC104 was used, rate constants differed by 20-30'3'0. It was not clear at first whether the presence of the zwitterion, +NH3CH2C02-, should be taken into account or not in ionic strength calculations. From an extensive series of runs in which it was assumed that +NH3CH2C02- (0.07-0.52 M) did not contribute to I , the kinetic M at 30 "C, I = 1.00 interpretation gave a value of Ka = 2.8 X M (LiC104). Potentiometric measurements indicated a higher value consistent with the literature determination of 3.55 X M at 25 O C , I = 1.00 M (NaC104) Since the lower Ka was unacceptable, a different approach was tried in which it was assumed that the zwitterion behaves as a 1:l electrolyte.
*
Results and Discussion Preliminary studies were consistent with a reaction sequence, (3) and (4), where subsequent reactions of C r ( N H 3 ) 4 ( H 2 0 ) ( 0 2 C C H 2 N H 3 ) 3 + occur at >50% conversion of Cr-
Table 11. Data Obtained from Kinetics of the Reaction of Glycine with Cr(NH,),H,O3+, I = 1.00 M (LiC10,)
40.0 47.5 50.0 Cr(NH,),H,O'+
2.31 i 0.06 5.73 k 0.23 16.2 ? 0.9
0.38 t 0.06 0.54 i 0.11 0.66 t 0.15
3 . 1 2 t 0.13 3.91 i 0.31 3.04 i 0.28
+ +NH,CH,CO;
kobsd
-Cr(NH,),(0,CCH,NH3)3+
+ H,O
(3)
H+
Cr(NH3),(0,CCH,NH,)3+ -CI(NH,),(H,O)(O,CCH,NH,)~+ + NH,' (4) (NH3)5H203+. At the 506-nm isosbestic point reaction 3 only is monitored. The dependence of kobsd on [H+] and [ G l y ] ~ , Table I and Figure 1, is consistent with an ion-pairing mechanism, (5) and (6). It was assumed that, because of the udavorable charge factor, there is no significant ion pairing of 'NH3CH2C02H with Cr(NH3)5H203+ and that this
2320 Inorganic Chemistry, Vol. 15, No. 9, 1976
Notes
Table 111. Details of Ion-Pairing Constants (KIP) and Rate Constants (/can) for Replacement of H,O in Cr(NH,),H,03' (after Ion-Pair Formation) at 50 "C, I = 1.00M (LiClO,)
+NH,CH,CO,HC,O, c,O,zH*O
7.82 6.45 29.1
0.5Sa 1.16 4.5
a From data a t 40,47.5, and 5 5 'C. at other temperatures.
14.2= 5.86 6.46 13.7b
Registry No. [Cr(NH3)5H20](C104)3, 32700-25-7; glycine, 56-40-6; C ~ ( N H ~ ) ~ ( O Z C C H ~ N 59765-77-4. H~)~+,
This work 5
References and Notes
5 2
Extrapolated from data
Cr(NH,),H,O3+ + 'NH,CH,CO,KIP Cr(NH,),H,03';0,CCH,NH,' Cr(NH,) ,H,03+;0,CCH,NH,' + H,O
% Cr(NH,),(O,CCH,
NH3I3+
(6 )
(7) can be rearranged to give (8). Plots of kobsd-l against [Gly]~-' - [H']+Ka kobsd kanKaKIP
1 [GlylT
N. V. Duffy and J. E. Earley, J . A m . Chem. Soc., 89, 272 (1967). T. W. Swaddle and D. R. Stranks, J . Am. Chem. Soc., 94,8357 (1972). T. W. Swaddle, Coord Chem. Reo., 14, 217 (1974). Discussion as to the mechanism of substitution of chromium(lI1)--amine complexes is to be found in D. A. House, Inorg. Nucl. Chem. Lett., 12, 259 (1976), and references therein. 0. Nor and A. G . Sykes, J . Chem. Soc., Dalton Trans., 1232 (1973). T. Ramasami, R. K. Wharton, and A. G. Sykes, Inorg. Chem., 14, 359 ( 1975).
species is nonreactive. Accordingly (7) can be derived and
--1
Cr(NH3)5H203+ at pH -5.0 (20 "C) we find that Cr(Gly), is obtained. Acknowledgment. T.R. is grateful to the Indian Government for the award of a National Merit Scholarship.
+- 1
kan
M. Mori, Inorg. Synth., 5 , 131 (1957). R. P. Martin and R. A. Paris, Bull. Soc. Chim. Fr., 570 (1963). The second acid dissociation constant has also been determined and is 1.6 X M under the same conditions. R. H. Moore and R. K. Zeigler, 1.0s Alamos Report LA 2367 (1959) and Addenda. C. H. Langford, Inorg. Chem., 4, 265 (1965); A. Haim, ibid.. 9, 426 (1970). J. H. Espenson, Inorg. Chem., 8, 1554 (1969); D. Thusius, ibid., 10, 1106 (1971). T. Ramsami and A. G . Sykes, J . Chem. Sac., Chem. Commun., 378 (1976); Inorg. Chem., in press. See, e.g., values at 45-50 "C of D. F. C. Morris arid S. D. Hanimond, Electrochim. Acta, 13, 545 (1968), and H. S. Gates and E. L. King, J . A m . Chem. SOC.,80, 5011 (1958), and values at 25 "C listed by H. Diebler, Z . Phys. Chem. (Frankfurt am Main), 68, 64 (1969). H. Ley and K. Ficken, Eer. Dtsch. Chem. Ges., 45. 377 (1912). B. J. Trzebiatowska. L. Pajdowski, and T. Starosta, Roczn. Chem., 35, 433, 445 (1961). A. Earnshaa and J. Lewis, J . Chem. Soc., 396 (1961). J. T. Veal, W. E. Hatfield, D. Y . Jeter, J. C. Hempel, and D. J. Hodgson, Inorg. Chem., 12, 342 (1973).
at constant [H+] should therefore be linear with a common intercept for all [H+]. This is the case as illustrated in Figure 2. The slopes give a first-order dependence on [H+] as required by (8). From a least-squares fit9 of rate constants kobsd to ( 7 ) a t 40, 47.5, and 55 OC, respectively, kanKIP and KIPwere obtained, Table 11. The Ka term was allowed to float yielding the values shown in Table 11, which are in good agreement with those determined potentiometrically, e.g., 3.42 Contribution from the Michael Faraday Laboratories, X M at 47.5 OC. Activation parameters for the composite Department of Chemistry, Northern Illinois University, term kanK1pfor formation of the glycinato complex are A P DeKalb, Illinois 601 15 = 25.3 f 0.9 kcal mol-' and AS* = 5.6 f 2.9 cal K-l mol-l. Thermodynamic parameters for K I Pare less precise, AH = Fluoro-Containing Complexes of Chromium(II1). 5.1 f 4.7 kcal mol-' and A S = 14.5 f 14.6 cal K-' mol-'. 7. Isolation and Some Reactions of the Reactions of Cr(NH3)5H203+ in which K I Phas been obcis-Fluoroaquobis(ethylenediamine)chromium(III) Cation' tained, thus allowing kan for anation within the ion pair to be evaluated, are listed in Table 111. Values of kan are seen to Joe W. Vaughn* and Alan M. Yeoman2 lie within a narrow range (5-14) X s d at 50 OC and are comparable to the rate constant for H2O exchange on CrReceived March 22. 1976 AIC60230W (NH3)5H203+.2 This observation that rate constants kan for 1-, 2-, and zero charged reactants as well as the glycine The cis-fluoroaquobis(ethylenediamine)chromium(IH) zwitterion are comparable is indicative of Cr-OH2 bond cation was first detected in solution by Fehrmann and Garner3 breaking being the dominant factor. It has previously been during an investigation of the perchloric acid hydrolysis of the demonstrated that substitution reactions of Co(NH3)5H203+ cis-difluorobis(ethylenediamine)chromium(III) ion. In 1968 occur by a dissociative mechanism.1° For Cr(Hz0)63+ Vaughn, Stvan, and Magnuson4 prepared the ion in solution however there is a strong case for an associative p r o ~ e s s . ~ , ~ ~by utilizing the Ag+-induced aquation of cis-chlorofluoroThe effect of different ligand environments on the mechanism bis(ethylenediamine)chromium(III). More recently the ion of substitution and other results are discussed elsewhere.12 was again detected and characterized in solution by Wirth and There are two additional points which emerge from this Lin~k.~ study with regard to the behavior of the glycine zwitterion, Since this particular cation would appear to be a good +NH3CH2C02-. First of all, the zwitterion behaves as a 1:l starting material for the rapid preparation of a number of cis electrolyte as far as ionic strength calculations are concerned. FX complexes, this investigation was undertaken to develop Second, values of the ion-pairing constant KIP are as expected a reliable method for the synthesis of solid salts of this cation for a 3+,1- interaction.13 in quantity. In addition some reactions of the cis-fluoroaquo There is no evidence for chelation of glycine in the present complex were to be studied. study. For reaction times corresponding to >50% completion, Experimental Section product analyses indicate that there is entry of a second glycine Caution! Perchlorate salts of metal complexes with reducing ligands and formation of diamine and triamine complexes. It is known such as amines are potentially explosive and care should be exercised that the reaction of glycine with some Cr(II1) complexes at when handling these materials. pH -7 yields the chelated tris(g1ycinato) complex Cr(Gly)3 Preparation of Starting Material. cis-Difluorobis(ethy1enedialong with the now well-characterized binuclear complex amine)chromium(III) tetrafluoro(ethy1enediamine)chromate(111)-I-water was prepared by the reaction of 42.0 g (0.24 mol) of C r 2 ( G l y ) 4 ( ~ O H ) 2 . ~ ~By- ~ reacting ~ excess glycine with
