J. Am. Chem. SOC.1985, 107, 369-376
in the absence of water (see Figure 4) and under CO. W e will report on the second mechanism together with some anomalies noted in the above studies at a later date.49
transalkylation in anhydrous alcoholic media. However, in aqueous media the reaction does proceed but not in the absence of benzalaniline or benzaldehyde. Et3N PhCH,NHPh no reaction (34)
+
Bu3N + PhCHO
-
Bu,NCH2Ph
+ PrCHO
369
Acknowledgment. W e thank Prof. H. Kaesz for graciously sending us a copy of his manuscript before publication, Prof. C. P. Casey for several enlightening comments on the proposed mechanism, and Dr. Ripudaman Malhotra for many valuable discussions. The major portion of this work was generously supported by the Army Research Office through Contract DAAG29-81-K-0086. A portion of this work was supported through N S F Chemical Engineering Grant 82-19541.
(35)
Fe(CO)5 does not catalyze C for H exchange on aliphatic amines.49 Consequently, in (35) benzaldehyde must activate the catalyst and/or the amine to promote transalkylation perhaps by electron transfer. W e have independently noted that we can promote transalkylation and reaction (35) by using ruthenium catalysts and water with mononuclear ruthenium catalysts.49 Deuteration and product selectivity studies reveal an additional and different transalkylation mechanism that can be promoted in the presence of H,O. This mechanism appears to be quite different from the mechanism described here, which works best
Registry No. R U , ( C O ) ~15243-33-1; ~, Fe(CO)5,13463-40-6;Me,N, 75-50-3; Et,N, 121-44-8; Pr,N, 102-69-2; N,N,N’,N’-tetramethylethylenediamine, 110-18-9; N,N-dimethylpiperazine, 106-58-1; N,N,N’,N’-tetraethylethylenediamine, 150-77-6;N,N”’,N”,N”-pentaethyldiethylenetriamine, 24426-21-9; N,N,N’,N”,N”’,N”’-hexaethyltriethylenetetramine, 24426-32-2; perethyltetraethylenepentamine, 9378307-4.
~~
(49) Blum, Y.; Wilson, R. B.; Laine, R. M.; Shvo, Y., unpublished results
Aqueous Solution Chemistry of p-Pyrazinepentaammineruthenium(11,111) Pentacyanoferrate( 11,111): Formation, Redox Reactions, and Intervalence Properties’ Andrew Yeh and Albert Haim* Contribution f r o m the Department of Chemistry, State University of New York, Stony Brook, New York 11794-3400. Received July 3, 1984
Abstract: Several pyrazine (pz) bridged binuclear complexes of the general formula (NC)5Mp~M’(NH3)5n were prepared in aqueous solution via the substitution reactions M(CN)50H2e M’(NH,),pz? + (NC)5Mp~M’(NH3)5”, where M = Fe(IJ,IJI) or Co(I1,III) and M’ = Ru(I1,III) or Rh(II1). All the complexes containing Fe(I1) and/or Ru(I1) are intensely colored substances, displaying a metal-to-ligand charge-transfer band in the 50G600-nm region. Rate constants for the formation and dissociation of the binuclear complexes with M = Fe(I1) were measured. Selected redox reactions of two of the binuclear complexes, namely, (NC)5FepzRu(NH3)5n(V) ( n = 1-, 0), were studied in some detail. Cyclic voltammetry of V (n = 1-) shows that its oxidation proceeds in two one-electron steps at 0.72 and 0.49 V , respectively. Chemical oxidation of V ( n = 1-) with peroxydisulfate (second-order reaction with rate constant (3.79 k 0.04) X lo3 M-’ s-’ at 25 OC, pH 4.28, and ionic strength 0.10 M) yields V (n = 0). The latter complex exhibits its metal-to-ligand charge-transfer band at 590 nm with molar absorbance -lo4 M-’ cm-’ and an intervalence band (in D20)at 1650 nm with molar absorbance 1.55 X lo3 M-’ cm-’ and half-width 0.43 Fm-’. Spectroscopic, electrochemical, and kinetic data on V (n = 0) are interpreted on the basis of a trapped valence formulation containing the localized oxidation states Fe(I1) and Ru(1IJ). The stability of the mixed valence state V (n = 0) with respect to electronic isomerization ( V with n = 0 but with localized valences Fe(II1) and Ru(I1)) and with respect to disproportionation to form the isovalent states V (n = I-) and V (n = 1+) is discussed in some detail.
+
The feature that dominates the chemistry of the pentaamineruthenium(I1) moiety is its ability to enter into a-bonding interactions with appropriate ligands, e.g., dinitrogen,* pyridine^,^ and nitrile^.^ One of the consequences of the a-donating ability of R u ( N H ~ ) is ~ ~the + increased basicity2 of the remote nitrogen atom in I. This led to the syntheses5 of the binuclear complexes
n ON*+
Ru(NH315N
W
I
n u
11. Since then a great deal of work has been devoted to this system, in particular to the mixed valence ion with n = 5.6 The nitrogen heterocyclic chemistry3 of R u ( N H ~ ) has ~ ~ been + duplicated7 with the isoelectronic moiety Fe(CN)53-. In particular, the basicity of the remote nitrogen in I11 was noted,7 and the syntheses of the binuclear complexes IV were reported.’
nr
(NH3)sRuN O N R u ( N H 3 ) 5
n = 4-6
I1
(’) the This work was by Grants 79709253 and 8203887 from National Science Foundation. (2) Harrison, D. F.; Weissberger, E.; Taube, H. Science (Washington, D . C.) 1968, 159, 320. (3) Ford, P. C.; Rudd, D. F. P.; Gaunder, R. G., Taube, H. J . A m . Chem.
SOC.1968, 90, 1187. (4) Clarke, R. E.; Ford, P. C. Inorg. Chem. 1970, 9, 495. (5) Creutz, C.; Taube, H. J . A m . Chem. SOC.1969, 91, 3988.
III
IV
In the work referenced above, the primary concerns have been synthetic and structuraL6 In particular, the electronic structures Of the mixed valence ions have been the subject of much work.6 In view of our long standing interest in the dynamic solution (6) Creutz, C. Prog. Inorg. Chem. 1983, 30, 1 . (7) Toma, H. E.; Malin, J. M. Inorg. Chem. 1973, 12, 1039
0002-7863/85/ 1507-0369$01.50/0 0 1985 American Chemical Society
370 J. Am. Chem. Soc.. Vol. 107, No. 2, 1985 behavior of heterobinuclear complexes? we began a mechanistic study centered on complexes V. In the present paper we report
r'\
Yeh and H a i m Table I. Absorption Spectra of Iron(I1) and Ruthenium(l1) Pyrazine (pz) and 4,4'-Bipyridine Complexes (buv) xma,
n
(NCI5FeN O N R U ( N H ~ ) n~ = -1.0, t 1
Lf V
studies on the formation, dissociation, and some redox reactions of complexes V, with emphasis being placed on the characterization of the mixed valence molecule V with n = 0. Analogues of V where Fe is replaced by Co or R u by R h have also been prepared and characterized. Similar studies, but using 4,4'-bipyridine as a bridging ligand, have also been carried out. In some instances, the compounds are too insoluble to permit solution studies. The solid-state work in this class of compounds was reported earlier.I0 Experimental Section Materials. Sodium amminepentacyanoferrate(I1) trihydrate and sodium amminepentacyanoferrate(II1) monohydrate were prepared by following standard procedures." The former compound was purified as described earlier.I2 The preparation of solutions containing -2 X IO4 M Fe(CN),0H,2- followed the literature method.I3 The syntheses of mononuclear ruthenium and rhodium complexes were as reported earlier.I0 Solutions of (NC),Fe"pzRu"(NH3)< were obtained by mixing a solution of Fe(CN),0H23-( < I X M) with a solution containing a 2-fold excess of Ru(NH,),~z~+. The resulting solution was passed through a column of Sephadex CM 25 cation-exchange resin. All the operations were carried out at I O O C and in the absence of light and oxygen. Solutionsof (NC)5Co"'pzRu"(NH3)5 were obtained by reaction M, prepared by aquation of (-8 h) of CO(CN),OH,~-(-2 X C O ( C N ) ~ C I ) -with ~ ~ )an equimolar amount of R U ( N H J , ~ Zat~ +56 OC under argon. The purifications of the water, argon, and lithium perchlorate were described previo~sly.'~All other materials were of reagent grade and used as received. Kinetic Measurements. Rates of formation of the binuclear complexes (NC),Fe"LRu"(NH,),- (L = pyrazine or 4,4'-bipyridine) and (NC),Fe"pzRh1I1(NH,), were measured by mixing fresh!v prepared solutions of Fe(CN)50H23-with an excess of RU"(NH,),L*' or Rh(NH3),pz3+in a Durrum D-1I O stopped-flow instrument interfaced to an IBM 1800 computer. The measurements were carried out by following the absorbance increase near or at the maximum wavelength for the binuclear complex. Rates of oxidation of the ruthenium(I1) complexes were measured by mixing the desired complex with an excess of peroxydisulfate in the stopped-flowapparatus. The measurements were carried out by following the absorbance decrease at the maximum wavelength for the ruthenium complex. Rates of dissociation of the binuclear complexes were measured by adding an excess of pyridine to a solution of the complex and following the slow, subsequent absorbance decrease at the maximum wavelength for the complex. The measurements were carried out in a Cary 17 or 1 18 recording spectrophotometer equipped with a thermostated cell compartment modified as described previously.16 All the kinetic measurement were carried out at 25 O C and ionic strength 0.10 M (lithium perchlorate). Observed rate constants were calculated by least-squares fitting of In ( A , - A , ) to time. A , and A , are the absorbances at time r and at the completion of the reaction. Electrochemical Measurements. A Princeton Applied Research Corp. Model 170 electrochemistry system was employed in the cyclic voltammetry measurements. Cyclic voltammograms were obtained at sweep rates varying from 20 to 200 mV/s, using platinum wires as counterelectrode and working electrode and a saturated calomel electrode as the reference. Solutions were kept at 25 O C in a PAR cell Type 9300 and were (1-5) X lo4 M in the electroactive species and 0.10 M in lithium perchlorate. (8) Felix, F.; Ludi, A. Inorg. Chem. 1978, 17, 1782. (9) Haim, A. Prog. Inorg. Chem. 1983, 30, 273. (10) Yeh, A.; Haim, A,; Tanner, M.; Ludi, A. Inorg. Chim. Acta 1979, 33, 31. (1 1) Brauer, G.'Handbook of Preparative Inorganic Chemistry", 2nd ed.; Academic Press: New York, 1965; Vol. 11, pp 1511-1512. (12) Jwo, J. J.; Haim, A. J . Am. Chem. SOC.1976, 98, 1172. (13) James, A. D.; Murray, R. S.; Higginson, W. C. E. J . Chem. Soc., Dalton Trans. 1974, 1273. (14) Haim, A. Inorg. Chem. 1982, 21, 2887. (15) Szecsy, A. P.; Haim, A. J . Am. Chem. SOC.1981, 103, 1679. (16) Miralles, A. J.; Armstrong, R. E.; Haim, A. J. Am. Chem. SOC.1977, 99, 1416.
log
t
complex PZ boy PZ bpv ref Fe*'(CN),L3452 437 3.70 3.75 a Fe"(CN),LH2-625 -505 b, c Ru"(NH3),L2+ 474 482 4.26 4.08 d, e Ru"(NH3),LH3+ 528 563 4.11 4.14 d, e (NC),FeT1LRu"(NH,),522 497 4.36 4.26 this work 505' 4 this work (NC),F~"LRU"'(NH~)~ -590 5 15' 4.039 this work (NC)5Fe11LRh"1(NH3)5 576g (NC),CO"'LRU"(NH,)~ 520 5321 4.24 this work 'Reference 7. Johnson, C. R., Shepherd, R. E. Inorg. Chem. 1983, 22, 1117. CGaswick,D. E.; Haim, A. J . A m . Chem. Sor. 1974, 95, 7845. "Reference 20. eFleischer, E. B.; Lavallee, D. K. J . A m . Chem. SOC.1972, 94, 2599. /Solid-state spectrum, ref 10. gCompare with , , A, = 572 nm and log t = 3.97, ref 27. Results and Discussion At the outset it is necessary to state the conditions chosen to handle the solutions utilized in the present work. Cyanoferrate(11) and ammineruthenium(1II) complexes with nitrogen heterocycles have different p H regions of stability. Below pH 4, the iron complexes begin to protonate and are susceptible to oxidation by dioxygen. Above p H 6, the ruthenium(II1) complexes generate ruthenium( 11) via poorly understood disproportionation reactions. Many if not all of the complexes are photosensitive. Therefore, as a matter of routine, solutions were handled under an atmosphere of argon, in subdued light and in the pH range 4-6. Absorption Spectra and the Electronic Structure of ( N C ) , F ~ " ~ Z R U ' ~ ~ ( N HAll , ) ~the . ruthenium(I1) and iron(I1) complexes with nitrogen heterocycles utilized in the present work exhibit characteristic MLCT bands in the visible region with molar absorbances of several thousands inverse molar inverse centimeters. The bands are sensitive to substituent effects3%'and disappear upon oxidation. In Table I we list the spectral features of interest in the present work. It will be seen that the addition of an electron withdrawing substituent to the remote nitrogen of the mononuclear pyrazine or 4,4'-bipyridine complexes results in shifts of the MLCT bands toward lower energies, the effect being considerably more pronounced for the iron(1I) than for the ruthenium(I1) complexes.'" Except for complex V ( n = 0) and for the analogue of V ( n = 0) where pyrazine is replaced by 4,4'-bipyridine, the oxidation states of the metal centers in the binuclear complexes listed in Table I can be assigned with confidence. Since oxidation states other than +3 for the metal centers in the pentaamminerhodium and pentacyanocobaltate moieties are thermodynamically unstable, the oxidation states of iron and ruthenium in (NC) Fe"pzRh"'( NH3) and in (NC) ,Co"'pzRu"( NH,) are as indicated. There is uncertainty in the assignment of the oxidation states of iron and ruthenium for complex V ( n = 0), e.g., a delocalized situation or trapped valences such as Fe(I1)-Ru(II1) or Fe(I1I)-Ru(1I) could obtain. In trying to distinguish between the alternative formulations, it is instructive to consider first the shifts in the MLCT bands that occur in systems with well-defined oxidation states. The 452-nm band of Fe"(CN),pz,- is shifted to 576 nm upon coordination of the exposed nitrogen to Rh"'(NH3)53+.Similarly, the 474-nm band of R U " ( N H ~ ) ~ P Z is ~ + shifted to 520 nm when the remote nitrogen is bound to Co"'(CN),Z-. Taking the values 576 nm and 520 nm as representing Fe"pz and Ru"pz M L C T bands shifted by coordination of the remote nitrogen to a non-a-bonding, electropositive + 3 metal center, the observed value of -590 nm for complex V ( n = 0) is taken to indicate the presence of Fe(I1) and Ru(II1) oxidation states. The somewhat larger shift for Ru(II1) as compared to Rh(I1I) is consistent with the r-acceptor properties of the former.17 The infrared spectral measurements of solid samples" of (NC)5Fe11pzRu"'(NH,), in the cyanide stretching and symmetric ammonia deformation regions also confirm the above assignment
,
(17) Elias, J. H.; Drago, R. Inorg. Chem. 1972, 11, 415
p - Pyrazinepentaammineruthenium
J . A m . Chem. Soc., Vol. 107, No. 2, 1985 371
Pentacyanoferrate
Chart 1. Molecular Orbital Diagram for Ru(NH,),pz"
and ( N H , ) , R ~ ~ z R u ( N H , ) , ~ ' / /
0-.\ 0
-
0 /
,,
x z , y z - h O b ' ,b2 -1 'a XY 2'.
\
';-
bl
=*
..
b3u I
/
=*
i
b2 a2
I
.
\ \
\
\
\
\ \
\
bl
\
\ 'II
\
\
( I * *
- RU/'I
Table 11. Reduction Potentials of Iron(II1) and Ruthenium(II1)
Table 111. Reduction Potentials of R u " ' ( N H ~ ) ~ Land Ru"'(EDTA)L
Complexes"
Complexes"
E1/7, V couple PZ F-,11'/11(CNS)L2-/30.55,b 0.66' Rulll/ll (NH3)SL3+/2+ 0.52, 0.49e ( N C) 5Fe'1'/1'LRu11'(NH3)51+~o 0.7 2 (NC)5Fe"LRu"'I''(NH3)so/10.49 (NC)5Fe"'i''LRh"'(NH3)51'/o 0.71, 0.71' (NC) S ~ o l l l ~ ~ ~ l(NH3),'+lo l l / l l 0.64
bPY 0.46,d 0.51' 0.34; 0.33c 0.64gsh 0.319 0.52'
"At 25 O C , 0.10 M ionic strength (LiClO,), and pH 4.28 (acetate buffer). Separation between anodic and cathodic peak potentials = 60-80 mV. bFrom ref 7. 'In 1 M KC1, ref 19. dPhillips, J.; Haim, A. Inorg. Chem. 1980, 19, 1616. 'In 1 M NaCI; Lim, H. S.; Barclay, D. J.; Anson, F. C . Inorg. Chem. 1972, 11, 1460. /Phillips, J.; Haim, A. Inorg. Chem. 1980, 19, 76. gAt pH 6.16 (phosphate buffer). " N o cathodic wave observed; value estimated from E , , AE,, with AEp = 60 mV.
+
Ru"'(NH3),Lb
L
Ru"'(EDTA)LC
EO 0.52 0.87 0.69 0.71 0.67 0.67
L pz pzCH3+ pzRh11'(NH,),3+ pzRh"'(EDTA)pzRu"'(EDTA)PZRU"(EDTA)~-
EO 0.24 0.50 0.37 0.28 0.32 0.18
0.56 0.74 0.35 0.49 OAt 25 OC and ionic strength 0.10 or 0.20 M. bFrom ref 20 or present work. CCreutz, C.; Kroger, P.; Matsubara, T.; Netzel, T. L.; Sutin, N. J . Am. Chem. SOC.1979, 101, 5442.
Additional comparisons, relevant to the electronic structure of complex V (n = 0), relate to its reduction potential which, in the trapped valence formulation given above, corresponds to reduction of Ru(II1) to Ru(I1). The pertinent data is presented in Table 111. It will be seen that, in general, addition of a Lewis acid to the remote N of Ru"'(NH3),pz3+ or Ru"'(EDTA)pz- results in an increase in the reduction potential of the complex. The lowering of the ligand a * orbital by the electron withdrawing substituent is accompanied by stabilization of the +2 oxidation state via d a-to-ligand a * bonding.22 It will be seen that this trend applies to all the substituents except for Fe11(CN),3-and R u " ( N H , ) ~ ~ + on Ru"'(NH3),pz3+ and for Ru"(EDTA)~-on Ru"'(EDTA)pz-. Coordination of the remote pyrazine nitrogen in Ru"'(NH3),pz3+ ~ ~ + in a decrease in the reto Fe11(CN),3-or R U " ( N H ~ ) results duction potential, the effect being larger for ruthenium(I1) than for iron(I1). Similarly, coordination of the remote N in Ru"'(EDTA)pz- to Ru"(EDTA)- decreases the potential.23 These substituents are acting as Lewis acids and remove electron density through the CT framework (just like all the other substituents in Table 111) but return electron density via the a framework. Since the Ru"(NH,),~+ moiety is a stronger a base than Fe11(CN),3-,7 the decrease in reduction potential is more pronounced for the former than for the latter. Note that the order is opposite to that predicted on the basis of simple electrostatic charge effects." An alternate, but equivalent way of viewing this effect was discussed ( 18) It is noteworthy that the MLCT bands of ( N C ) , F ~ " ~ Z R ~ ' ~ ( N H ~ ) ~by - Sutton and Taube2' in their enumeration of the factors that occur at virtually the same energy (522 and 520 and (NC)SCo11'pzRu'1(NH3)5 account for the stability of the mixed valence state nm, respectively). Since the orbitals utilized in a bonding by iron(I1) are 3d (NH3)5Ru11'pzRu11(NH3)55+ compared t o the corresponding whereas those utilized by ruthenium(I1) are 4d and, in addition, in view of isovalent states. They recognized that in making the strong a-accepting ability of the cyanide ligands, it is expected that the (NH3),R~11pzR~11(NH3)~+ from two Ru11(NH3)?+and pyrazine, competition between Fe(I1) and Ru(I1) for a bonding to the pyrazine bridge
of oxidation states and moreover serve to rule out the delocalized formulation.ls Entirely analogous arguments can be brought forth to support the assignment of +2 and +3 oxidation states to iron and ruthenium, respectively, in (NC)SFe11bpyRu111(NH3)5. Additional evidence in favor of a localized formulation featuring Fe(I1) and Ru(II1) in complex V (n = 0) comes from a consideration of the reduction potentials of the various couples. The relevant values are listed in Table 11. It will be seen that the reduction potentials of (NC)5Fe111pzRh111(NH3)5+ l 9 and of ( N C ) , C O ~ ~ ~ ~ ~ R U " 'are ( N 0.71 H , ) ~and 0.64 V, respectively. Since the reductions of Co(II1) or Rh(II1) require much more negative potentials, the observed values are taken to be a measure of the oxidizing abilities of the Fe"'(CN),pz3- and Ru"'(NH3),pz3+ moieties with the remote pyrazine nitrogen coordinated to a tripositive electron withdrawing tripositive metal center, Rh"'(NH3),3+or CO"'(CN),~-,respectively. The observed value for 0.72 V, the one-electron reduction of (NC),F~"'~ZRU~~'(NH,),+, is almost identical with the value for the Fe(II1)-Rh(II1) complex where reduction occurs at the Fe(II1) center. The comparison indicates t h a t the electroactive metal center in (NC),FelllpzRu"'(NH,),+ is Fe(II1) as well, and, therefore, we infer that complex V (n = 0) features trapped Fe(I1) and Ru(II1) vaIences.20.21
will be strongly in favor of Ru(I1). Under these circumstances, the pentacyanoferrate moiety acts just as a u acid, in the same way as the non-abonding pentacyanmbaltate moiety. Thus, although for complex V ( n = -1). where the molecular orbital treatment presented by Lauher (Lauher, J . Inorg. Chim. Acto 1980, 39, 119) would be applicable, the localized formulation represents a good approximation. (19) Moore, K. J.; Lee, L.; Mabbott, G . A,; Petersen, J. D. Inorg. Chem. 1983, 22, 1108.
(20) Creutz, C.; Taube, H. J . Am. Chem. SOC.1973, 95, 1086. (21) Sutton, J. E.; Taube, H. Inorg. Chem. 1981, 20, 3125. (22) Zwickel, A. M.; Creutz, C. Inorg. Chem. 1971, 10, 2395. (23) Evidently we are not dealing with a mere charge effect" since positive or negative substituents increase and decrease the reduction potential of the RU"I(NH,),~+center.
Yeh and Haim
372 J . Am. Chem. SOC.,Vol. 107, No. 2, 1985 Table IV. Oxidation of Ru(I1) and Fe(I1) Complexes by Peroxydisulfate at 25 O C , pH 4.28 (acetate buffer), and Ionic Strength 0.10 M (LiC104) complex k , M-' s? (3.05 f 0.06) X lo3" RU~*(NH~)~~Z~+ (NC),CO"'~ZRU"(NH~)~ (1.40 0.06) X lozb (3.79 0.14) x 103~ (NC),Fe"pzRu"(NH,),-1.5 (NC),Fe"pzRh"'(NH,), F~"(CN)QZ~-2.5 [Ru"] a [Ru"] = 1.35 X lo-' M, [ S 2 0 2 - ] = (1.67-5.25) X lo4 M. M. c [ R ~ l l=] 1.80 X = 1.77 X lo-, M, [S20s2-]= (2.36-7.10) X 10-5 M, [s2oS2-]= (1.47-3.66) x 10-4 M.
*
of oxidation are undoubtedly the Ru(I1) and Fe(I1) centers, respectively, since higher oxidation states of Co(1II) and Rh(II1) are not readily accessible, if at all. For the binuclear complex (NC),Fe"pzRu"(NH,),-, however, the question arises as to whether the Fe(I1) or the Ru(I1) center (or both, in competition with each other) is oxidized. In other words, is isomer VI or isomer VI1 (or an isomeric mixture) of complex V ( n = 0) formed as the primary reaction product? In this context it must be noted that ( N C ) , F ~ " ' ~ Z R U ~ ~ ( N H , )(NC),Fe"pzRu"'(NH,), , VI VI1
the discussion presented above strongly suggested that VI1 is the thermodynamically stable isomer. Therefore, the question raised the addition of the second Ru"(NH3)?+ does not result in as much above can be rephrased as follows. Is VI1 the kinetic (as well as a-backbonding stabilization as the first, and they suggested that thermodynamic) product, or is VI formed first and then, in a subsequent intramolecular electron transfer, VI1 is produced? In there is an inherent electronic instability in the isovalent 11, I1 order to answer this question, the following criterion is proposed. complex. To be sure, this instability makes (NH3),Ru"pzRu"The rate constants for oxidation of Fe"(CN),pz3- and of the ( N H 3 ) 5 4 + a better reducing agent and, conversely, (NH3)5RuT11pzRu"(NH3):+a poorer oxidant than if R U ( N H ~ ) , ~ + (NC)5Fe11pzRh111(NH3),model compound are taken to be characteristic of oxidation of Fe(I1) to Fe(II1) in these types of was not a a base. It is also instructive to make the comparison compounds. It will be seen that the constants fall in the range on the basis of the molecular orbital diagram in Chart I.I8 Although in discussing redox potentials it is necessary to consider 1-3 M-' SKI. Similarly, the rate constants for oxidation of Ru"(NH,),pz2+ and of the (NC)5Co"'pzRu"(NH3)s model the M O diagrams for reduced and oxidized forms of the couple, compound are indicative of oxidation of Ru(I1) to Ru(III), the it is noteworthy that for the parent compound Ru"(NH3),pz2+ range of rate constants covered being 102-103 M-' SKI. On this and for the complexes where the substituent is acting solely as basis the observed value 3.79 X l o 3 M-I s-' for a Lewis acid, the diagram on the left is applicable, and it will be (NC),Fe"pzRu"(NH,),- is taken to indicate oxidation of Ru(") seen that the electron removed from Ru(I1) comes from a nonthe diagram bonding orbital. For (NH3),Ru11pzRu11(NH3),4+ to Ru(II1),and, therefore, the kinetic, as well as thermodynamic, product is isomer VII. Additional support for the suggestion that on the right is obtained, and it is noteworthy that the electron in the oxidation of (NC),Fe"pzRu"(NH,),- the electron is removed comes from an antibonding orbital. transferred from an orbital primarily ruthenium in character comes Oxidation of Ruthenium(I1) and Iron(I1) Pyrazine Complexes. from a comparison of the rate constants for oxidation of Ru"Peroxydisulfate was used as the oxidant in all oxidation experi(NH3),pz*+, (NC)5C0111pzRu11(NH3),,and (NC),Fel'pzRu"ments because of its stability at pH 5 and because of the possibility (NH3),-. For outer-sphere oxidation of a series of related comof detecting two-electron reactions. However, in all cases, the plexes by the same reagent, variation in rates are explicable on reaction stoichiometries corresponded to one-electron reactions, the basis of electrostatic, thermodynamic, and self-exchange eq 1-3. The 2:l stoichiometry in eq 1 and 2 is, to be sure, not considerations. Taking ion pair formation constants as a measure 2(NC),Fe"pzRh"'(NH3), of the electrostatic factors and the Marcus relationship to estimate S,OSz- = ~ ( N C ) , F ~ " ' ~ Z R ~ " ' ( N H ~2S042) ~ + (1) the thermodynamic contribution, we calculate that the reactivity ratio for the complexes in the same order as given above is ~(NC),CO~~~~ZRU"(NH~), 1:0.045:0.5 to be compared with the observed values 1:0.046:1.24. Sz082-= ~ ( N C ) , C O ~ ~ ~ ~ ~ R ~+I2S042~ ~ ~ ( (2) N H ~ ) ,Considering + the approximations involved in estimating ion pair formation constants, the agreement between experimental and ~ ( N C ) , F ~ " ~ Z R U ~ ~ ( N HSZOs2,),- = calculated reactivity ratios is taken to be excellent. Since elec2(NC),Fe"pzRu"'(NH3), 2SOd2- (3) trostatic and thermodynamic factors are sufficient to account for the reactivity differences, we infer that the rate constants for surprising but very noteworthy for eq 3. As discussed below, the self-exchange in the three complexes are equal. The equality of product of eq 3, complex V ( n = 0), does not undergo further the rate constants of self-exchange for Ru(NH3),pz3+12+and for oxidation but a decomposition reaction. (NC)5C011'pzRu(NH3),+/ois not unexpected since it has been The reactions of the Ru(I1) complexes follow mixed secondshown24that self-exchange rate constants for Ru(NH3),L3+l2+ order kinetics-first order in peroxydisulfate and first order in (L = substituted pyridine) are rather insensitive to the identity Ru(I1)-quite accurately. Second-order rate constants defined of L. This may be rationalized by recognizing that the electron by eq 4 and 5 are listed in column 2 of Table IV. The oxidations transferred from Ru(I1) is in a nonbonding orbital" (b,) which k MI1 S2OS2- MI1' S042- SO4- ( M = Ru or Fe) is metal in character (4dJ and somewhat unaffected by the detailed nature of the pyridine ligand. It is noteworthy that the (4) equality of self-exchange rates appears to extend to -d[M"]/dt = 2k[M11][Sz082-] ( M = Ru or Fe) (5) ( N C ) , F ~ " ~ Z R U ( N H , ) ~In ~ / this - . complex, the electron that is transferred is in an antibonding orbital (b2g)which is mostly the of the Fe(I1) complexes were found to be quite slow, and the xzl xz2 combinationI8 of iron and ruthenium orbitals. The last-squares fits of In ( A ,- A,) vs. time were not as good as those of the rate of self-exchange is taken to indicate that there equality for the Ru(I1) complexes. The deviations in the fits are probably is little mixing of the iron orbital and that the electron that is caused by competition of dissociation with oxidation. For example, transferred is in an orbital with predominantly ruthenium charwith [FeI1(CN),pz3-] = 3.77 X lo-, M, [Sz082-]= 7.38 X acter. This suggestion is consistent with the spectroscopic eviM, and at 25 O C p = 0.10 M, pH 4.28, the observed rate constant dence.I8 s-l, compared with 4.2 for disappearance of Fe(I1) is -2 X Rates of Formation and Dissociation of Pentacyanoferrate(I1) X s-l for dissociation of Fe(CN),pz3- into Fe(CN),0H23Binuclear Complexes. Rate constants for the forward and reverse and pyrazine.' Approximate value of the second-order rate reactions in eq 6 (M = Ru(I1)or Rh(Irr)and L = pz or bpy) were constants defined by eq 4 and 5 are listed in column 2 of Table measured spectrophotometrically. The forward reactions with IV. M = Ru(") were followed at 550 nm to avoid the broad absorption For the mononuclear complexes listed in Table IV, the oxidation reactions refer to bringing up the oxidation state of the central metal ion from +2 to +3. For the binuclear complexes (24) Brown, G. M.; Krentzien, H. J.; Abe, M.; Taube, H . Inorg. Chem. 1979, 18, 3374. ( N C ) , C O ~ ~ ~ ~ Z R U " (and N H (, )N~C ) , F ~ " ~ Z R ~ " ' ( N H ,the ) ~ ,sites
+
+
+
+
+
-
+
+
+
+
J. Am. Chem.SOC.,Vol. 107,No. 2, 1985 373
p-Pyrazinepentaammineruthenium Pentacyanoferrate Fe11(CN),0H23-+
Table V. Kinetics of Formation and Dissociation of k
M(NH3),Lnf & (NC)5Fe11LM(NH3)5("3)++ H 2 0 (6)
Pentacyanoferrate(I1) Binuclear Complexes"
kd
by the Ru(NH3),L2+ reactants at the maxima of the binuclear complexes. Since Rh(NH3),pz3+has no absorption in the visible, the reaction was followed at 576 nm, the maximum for the product. The reactions were found to be first order in Fe(CN)50H,3- and first order in M(NHJ5Ln+. The corresponding second-order rate constants are listed in column 2 of Table V. For the measurements of the reverse reaction, pyridine was used as a scavenger.' The stability constant of Fe(CN),py3- is very high,' and therefore in the presence of excess pyridine, the reverse of reaction 6 proceeds to completion and the absorbance measurements yield first-order rate constants (independent of the pyridine concentration) identified as kd. The quantitative formation of Fe(CN),py3- and M(NH3),L"+ in the dissociation experiments was verified by spectrophotometric examination of the product solutions. Values of kd are listed in column 3 of Table v. The results obtained conform to and provide additional support for the mechanistic patterns discerned previo~sly.',~~-~' Undoubtedly, the replacement of water in Fe(CN)50H23-by various ligands proceeds by a dissociative mechanism, with values of kf for neutral ligands falling in the range 200-400 M-' s-I.~, As the positive charge of the incoming ligand increases, values of kf also increase and have values of -500, -3 X IO3, and -6 X lo3 for 1, + 2 , and +3 charges, r e s p e ~ t i v e l y . ' ~ * ' ~ * ~ ~ ~ ~ ~ Formation, Dissociation, and Decomposition of (NC)SFe11pzRu111(NH3)5. Included in Table V are values of kr and kd for the reaction between Fe(CN),0H23- and Ru(NH3)5p~3+ These . values were not obtained, as was the case for the other entries in the table, by direct measurements of the forward and reverse reactions. When solutions of Fe(CN)50H23and Ru(NH3),pz3+ are mixed, the 474-nm absorption of Ru(NH3),pz2+develops very rapidly. This suggests that the first reaction that takes place between Ru(NH3),pz3+(- lo4 M) and Fe(CN)50H23-(- lo-, M) is the rapid, outer-sphere electrontransfer reaction, eq 7. The equilibrium and rate constants for
Fe"(CN)50H23-+ L"+
+ Ru(NH3),pz3++ Fe(CN),OH?-
+ Ru(NH3),pz2+ (7)
eq 7 are 157 (calculated from Eo values of 0.3929and 0.52 V for Fe(CN),0H22-/3- and R u ( N H , ) , ~ z ~ + / respectively) ~+, and 1.5 X lo7 M-' s-I (estimated from the values 1.9 X 1O4 and 1.1 X IO5 M-' SKIfor the self-exchange rate constants for iron30 and ruthenium24complexes, respectively, utilizing the Marcus cross relation correctedI6 for electrostatic effects). Evidently, the inner-sphere reaction given by eq 8 with an estimated rate constant of 6 X I O3 M-' S-' (by analogy with the corresponding reactions of tripositive complexes3') cannot compete with the outer-sphere reaction given by eq 7. Following the rapid equilibration ac-
-
Fe(CN),0H23- + Ru(NH3),pz3+e ( N C ) , F ~ ~ Z R U ( N+ HH ~ )2~0 (8) cording to eq 7, subsequent slow changes are observed. The 474-nm absorption of Ru(NH3),pz2+shows a shift toward longer wavelengths, and, depending on the initial Ru(NH3),pz3+and Fe(CN),0H23-concentrations, maxima in the 540-570-nm region develop after 10 min. Evidently, following the very fast outer-sphere reaction, the binuclear complex (NC)5Fe11pzRu111(NH3), is formed. Since equilibration according to eq 7 is faster than reaction according to eq 8, in principle it is possible to start with either Fe(CN),0H23--Ru(NH3)5pz3+or Fe(CN),0H2Z--Ru-
-
(25) Toma, H. E.; Malin, J. M. Inorg. Chem. 1973, 12, 2080. (26) Szecsy, A . P.; Miller, S. S.; Haim, A. Inorg. Chim. Acta 1978, 28, 189. (27) Pfenning, K. J.; Lee, J.; Wohlers, H. D.; Petersen, J. D. Inorg. Chem. 1982, 21, 2477. (28) Jwo, J.-J.; Gaus, P. L.; Haim, A. J . Am. Chem. SOC.1979, 101,6189. (29) Toma, H.; Creutz, C. Inorg. Chem. 1977, 16, 545. (30) Szecsy, A. P.; Haim, A. J . Am. Chem. Soc. 1982, 104, 3063. (31) Gaswick, D. G.; Haim, A. J . Am. Chem. Soc. 1974, 96,7845.
(NC)5Fe11L("-3)+ + H20
10-3kf, L
M-1
I 04kd,
s-I
s-I
106K, M-1
b
7.2 f 0.1 5.1 f 0.3 R U " ( N H ~ ) ~ ~ Z ~3.7 + "f 0.1 0.64 f 0.20 Ru11(NH3)5bpy2+e 1.6 f 0.1 24.7 f 4.0 Rh11'(NH3)Spz3+df 8.8 f 0.3 1 1 . 1 f 0.1 7.9 f 0 16 f l l h R u ~ ~ ' ( N H , ) , ~ z ~ 5.5 + f 0.5 29 f 209 'At 25 OC, ionic strength 0.10 M (LiCIO,), [Fe(CN)50H23-]= (1-3) X lo-' M, [L] = (1-3) X IO-, M, [Fe(CN),L] = (2-5) X IO-' M. [Pyridine] = 0.05-0.20 M. 'Calculated from kf/kd., dpH 6.06, [ascorbic acid] = 1.0 X M. epH 8.03, [ascorbic acid] = 1.0 X IO-, M. 'Compare with kf = 2.6 X IO3 M-' a nd k, = 7.4 X s-l at ionic strength 0.50 M, ref 27. gCalculated from K/kf. *Indirect estimate; see text.
(NH3),pz2+mixtures in order to measure the rate constant for eq 8. In practice we found it more convenient to start with the latter mixtures (using an excess of iron) and we followed the absorbance increase at 560 nm. Under these circumstances, the mechanism of formation of (NC),FepzRu(NH3), proceeds via the reverse of eq 7 followed by eq 8, and the rate constant kf for eq 8 was obtained from the slope of Y ( t )vs. time, eq 9. [Fe-
+
Fe(CN),0H23-
kd
Y(t)= -kr[Fe(CN)50H22-]ot/2K + C
(9)
(CN),0H22-]o is the initial concentraiton of Fe(CN),0H22(present in at least 10-fold excess over Ru(NH3),pzz+), kf and K are the rate and equilibrium constants for eq 8 and 7, respectively, and Y(t)is defined in eq Six experiments were Y ( t ) = In 0,- 1) - 1 / 0 , - 1)
y = (1 x =
+
x)'/2
(4K/[Fe(CN),0H22-]0)([Ru(NH3),pz2+]0 + [V(n = 011,) (10)
carried out with [Fe(CN),0H2*-l0 = (4.62-7.07) X M, [Ru(NH3)Spz2+]o= (3.23-7.13) X M, t = 25 OC, p = 0.10 M (LiC104), and pH 4.28 (acetate). From the slopes of eq 9 (plots were linear for at least 80% reaction) and K = 158, values of kf were found to be in the range (4.8-6.0) X lo3 M-' s-I. The average value (5.5 f 0.5) X IO3 M-' s-l falls nicely near the range of rate constants for the reactions of Fe(CN),OH?- with other tripositive pyrazine complexes, such as Rh(NH3),pz3+(k, = 8.8 X IO3 M-I s-I) and Co(NH3),pz3+ (kf = 7.0 X IO3 M-l s - I ? ~ 1. The rate constant kd for dissociation of V ( n = 0) could not be measured by the pyridine scavenging method. When pyridine is added to V (n = 0) prepared by reaction of Fe(CN),OH?- with Ru(NH3),pz2+,the 590-nm absorption of V (n = 0) disappears with a rate constant of (4.4 f 0.3) X SKI(average of two experiments at 25 OC, p = 0.10 M, [pyridine] = 0.10 M, [Ru(NH3),pz2+] = (4-7) X IO-, M, and [Fe(CN),0H23-] = (4-7) X M). However, the observed rate constant cannot be assigned to the reverse of reaction 8 because spectrophotometric examination of the product solutions revealed the presence of both Ru(NH3),py2+and Ru(NH3),pz2+. For example, when V ( n = 0) was prepared by reaction of 4.01 X lo4 M Fe(CN),OH?- with Z ' +then treated with pyridine, the 3.96 X lo-, M R U ( N H ~ ) ~ ~and product solution was found to contain 2.63 X lo-, M Ru(NH3),py2+and 1.37 X IO-, M Ru(NH3),pz2+. Evidently, any Ru(II1) complex present initially is reduced by an unspecified base-dependent process. Perhaps in the unbuffered pyridine medium, disproportionation of Ru(II1) to Ru(I1) and Ru(1V) is obtained and is followed by rapid loss of pyrazine from Ru(IV) and eventual reduction to Ru(I1). (32) Yeh, A. Ph.D. Thesis, State University of New York, Stony Brook, 1978. (33) Malin, J. M.; Ryan, D. A,; O'Halloran, T. V . J Am. Chem. Soc. 1978, 100, 2097.
374 J . Am. Chem. SOC.,Vol. 107, No. 2, 1985
Yeh and Haim
An estimate of the value of kd was obtained from a study of and reinforces the interpretation of the kinetic measurements of the decomposition of V ( n = 0 ) in the absence of excess monothe decomposition of V ( n = 0 ) . nuclear iron or ruthenium complexes. As indicated above, solutions Intervalence Absorption and Intramolecular Electron Transfer of V ( n = 0 ) can be prepared cleanly by oxidation of V ( n = -1) Mixed valence binuclear complexes in (NC)SFe11pzRu11'(NH3)5. (purified by ion exchange) with 1 equiv of peroxydisulfate. Such are characterized by a metal-to-metal charge-transfer absorption solutions are initially blue and display a maximum at 590 nm. (intervalence band) which provides information about the extent However, this absorption shifts with time, and, after about 20 min, of coupling between the metal centers and the rate of electron a fairly stable solution is obtained which exhibits a maximum at transfer from one metal center to the other.6 Freshly prepared -490 nm and a shoulder at s570 nm. We suggest that the solutions of V ( n = 0 ) in D 2 0 (obtained by oxidation of 1 X observed spectral changes of (NC),FepzRu(NH,), are initiated IO4 M solution of V ( n = -1) with Sz0,2-)exhibit an intervalence by its dissociation via the reverse of eq 8, and this is followed by band at 1650 nm (6.06 X IO3 cm-I) with molar absorbance 1.55 X lo3 M-' cm-' and bandwidth 0.43 pm-I. the fast outer-sphere redox reaction according to eq 7, until equilibrium between V ( n = 0 ) , Fe(CN)=,OH?-, R U ( N H ~ ) ~ P Z ~ + , Theoretical treatments6 of intervalence absorption provide useful Fe(CN),0H23-, and Ru(NH3),pz3+ is reached. This proposed relationships for the bandwidth v l j 2 ,which in the high temperature limit is given by eq 11, and for the oscillator strengthf, eq 12. reaction sequence is supported by three experimental pieces of information. First, the spectra of solutions obtained by mixing = (2310 ijmax)llz (1 1) equimolar solutions of Fe(CN),0HZ3-and Ru(NH3),pz3+ are, after about 15 min, essentially identical with those obtained after f = 4.6 x 10-9e,a,Aslj2 (12) the decomposition of (NC),FepzRu(NH3), has proceeded to equilibrium. Both sets of equilibrium solutions display the The oscillator strength can also be expressed by eq 13, which when characteristic absorption maximum of Ru(NH3),pz2+ near 490 combined with eq 12 yields eq 14. 3,,, and emaxare the wavenm as well as a shoulder at 570 nm assigned to V ( n = 0). Second, f = 1.085 X 10-5ijma,(ad)2 (13) when the equilibrium solutions are treated with ascorbic acid, (NC),FepzRu(NH3),- is produced in more than 90% yield. This number and molar absorbance, respectively, of the intervalence observation indicates that in the sequence of reactions that lead absorption, and d is the distance between the metal centers. a' to the equilibrium mixture, at no time is the Ru-pz bond broken.34 cy2 = 4.24 x 1 0 - ~ ~ ~ ~ ~ ~ ~ , ~ ~ (14) / i 4 , , ~ ~ d ~ Additional confirmation bf this inference comes from the observation of quantitative production of equimolar amounts of is a measure of the delocalization in the ground state and is related Fe(CN),py3- and Ru(NH3),pz2' when the equilibrium product to the electronic coupling HA, between the metal centers by eq solutions from the decomposition of V ( n = 0 ) are treated with 15. Values of Aslj2,a2,and HAB calculated from eq 11, 14, and pyridine and ascorbic acid. By measuring the decrease in the 590-nmabsorption of (NC)=,FepzRu(NH,),, we obtained a nice HAB = s m a x a (15) fit to an exponential decay with a rate constant of (1.6 f 0.1) 15 for the intervalence band in V ( n = 0 ) are 0.37 pm-', 9.8 X X s-' at 25 OC, pH 4.0, and p = 0.10 M (average of two and 600 cm-I, respectively. The small magnitude of the latter measurements). In the interpretation suggested above, this rate two values indicates that there is little ( < I % ) electron delocaliconstant represents a measure of the rate constant for approach zation in V ( n = 0). Additional evidence for this conclusion comes s-'.~' Support to equilibrium, and we estimate that kd 1 X from a comparison between observed and calculated values of the for the interpretation and the reliability of the estimated value bandwidth, an important diagnostic to assess the extent of deloof kd comes from a comparison of the equilibrium constant for calization. The observed bandwidth (0.43 pm-l) being larger than = eq 8 calculated from the ratio k f / k d = 5.5 X 103/l.0 X that calculated from eq 11, it is inferred6 that V ( n = 0 ) is a 5.5 X lo6 M-' with the value estimated from the following cycle.29 valence-trapped species. The inferences drawn from the interE l o ,E 2 0 , and K'have been measured in the present work (0.52, valence absorption further corroborate those reached above based 0.49 V, Table 11, 5.1 X IO6 M-I, Table V) and therefore K = 1.6 on MLCT spectroscopic and electrochemical considerations, X IO7 M-I. The agreement between this value and the value 5.5 namely that V ( n = 0 ) is to be formulated as a trapped valence 3t species featuring localized Fe(I1) and Ru(II1) oxidation states. Fe(CN)50H:' t Ru(NH3)spz (NC)5FepzRu(NH3)5 t H 2 0 It is noteworthy that, except for the Creutz and Taube ion, (NH3),RupzRu(NH3)55+,all other reported ruthenium pyra1.2 zine-bridged mixed valence binuclear complexes6belong to Robin and Day's class 11; e.g., a trapped valence formulation represents Fe(CNI50H:C Ru(NH3I5pz2+ x ' (NC)gFepzRu(NH&- t HzO the most adequate description of the electronic structure of these compounds. In this respect, complex V ( n = 0 ) conforms to this X IO6 M-' estimated above is not spectacular. However, it must generalization, but there is one unusual observation for V ( n = be recognized that errors of 0.01 V in the values of E l 0 and EzO 0). It has been noted6 that asymmetric mixed valence binuclear change the calculated value of K by a factor of 2, and, thus, complexes display the intervalence band at energies higher than considering the approximations and experimental errors, the the average of the energies of the parent symmetric complexes. agreement between the two values of K is considered acceptable However, for the asymmetric complex V ( n = 0 ) the opposite situation prevails, e.g., its intervalence band occurs at 6.06 X IO3 cm-' compared to 8.30 X lo3 and 6.40 X lo3 cm-' for (34) If the Ru-pz bond had been broken in the decomposition process, (NC)5FepzRu(NH3)5- would not have been produced rapidly and quantita(NC)=,F~~ZF~(CN)=,~and ( N H R U ~ Z R U ( N H ~ ) ~Now, ~+.~~~.~~ the relationship ij",b,, > (nFaX +3ijma,)/2 'h is applicable under the tively. Note that substitution of HZO in Ru(NH,)~OH?+ by pz or Fe(CN),pz3- is a relatively slow process. Moreover, the reaction of Fe(CN)5pz3assumption that the inner-shell and solvent reorganization energies with Ru(NHJ50H?+ produces, in part, a cyanide-bridged binuclear complex of each moiety in the asymmetric complex are equal to the values (Ludi, A,, private communication, 1976). No such complex was observed in our system. in the corresponding symmetric complexes, e.g., each mononuclear (35) The differential equations corresponding to the scheme reverse of center is not affected by the presence of the other. Although this reaction 8 followed by eq 7 were solved utilizing the Run e Kutta approxiassumption may be applicable to the inner-shell contribution, it k, = 2 X lo7 mation and values of kd = 1 X IOw3 s-l, kf = 5 X lo3 M-?S?, might not be correct for the solvent contribution in the case of M-' s-l, k-, = 1 X lo5 M-' s'l , and [VI, = 2 X M. Absorbance vs. time
-
-
?.I
at 570 nm was calculated from the concentrations and known molar absorbances, and the absorbance values were fitted to the equation A, = A, + ( A , - A,) exp (-kf) by a nonlinear least-squares program. The exponential dependence was strictly obeyed for more than five half-lives, and the resulting values of k was 1.58 X IO-? SKI. Since the conditions in the simulation duplicate quite closely the experimental conditions, we infer that the observed value (1.6 f 0.1) X s-' corresponds to a k , value of - I X 10-3sc'.
~~
(36) It may be argued that this comparison is not applicable to V since one moiety belongs to class I1 and the other (probably) to class 111. However, based on the dependence of B, upon the distance between the metal centers, we estimate that a hypothetical class I1 Creutz and Taube ion would exhibit the intervalence band at -8 X I O 3 cm-].
J. Am. Chem.SOC.,Vol. 107,No. 2, 1985 375
h-Pyrazinepentaammineruthenium Pentacyanoferrate
data are rather insensitive to the identity of the metal centers and a complex such as V ( n = 0) where the two moieties have equal their coordination sphere (ketfor (EDTA)Ru"~zRu~~'(NH~),+/~,, and opposite charges. Undoubtedly, the extent of solvation of for (NC)SFe"pzRu111(NH3)s+ 4), whereas rate constants for (NC),FepzRu(NH,), is considerably less than that of the same systems calculated from thermal reactions measured (NC),FepzFe(CN),,- or ( N H , ) , R U ~ Z R U ( N H ~ ) , ~If+the . deexperimentally display a higher discrimination (ket for creased solvation of V ( n = 0) compared to the parent symmetrical (EDTA) Ru"pzCo"'( NH,) 5 + / k,, for (N C) Fe"pzCo"'( N H ,) ,, complexes also implies a decrease in solvent reorganization en400). Second, the reactivity ratio of Ru(NH3),pz3+ vs. Coergy,,' then the relationship < (ijYax+ pkaX)/2would be (NH3),pz3+calculated from intervalence data for Ru and from obeyed, provided that the difference in energy between the two thermal data for Co is 5 X lolo, whereas the ratio is 5 X lo5 when electronic isomers of the asymmetric complex is small (for the both Ru and Co are compared in thermal reactions.16 As noted reaction VI1 F= VI, the free energy difference is 0.07 V; see below). before,37values of k,, estimated from intervalence bands may be Another example that brings out the importance of the signs of overestimated. A similar discrepancy is found when comparing the electric charges in determining the position of, , ,? is found the thermally measured and spectroscopically calculated rate in (NH,),RU"~ZRU~~~(EDTA)+, p,, for the latter is 9.80 X lo3 constants for intramolecular electron transfer in the ion pair cm-' compared to 1.03 X lo4 and 6.40 X lo3 cm-' for Fe(CN)6" and Ru(NHJ5,+, lo3 and lo6 s-', respectively.16,a ( E D T A ) R u ~ z R u ( E D T A ) ~and - (NH3)SRupzRu(NH3)sS+.3*~39 All these comparisons are not encouraging regarding the validity When the value of vmax for the asymmetric complex is corrected of calculated rate constants for electron transfer from the propfor the endoergonicity of the reaction (NH,),Ru1'pzRu"'(EDTA)+ erties of intervalence bands, and clearly additional work is called (NH3),Ru111pzRu1r(EDTA)f (-0.3 V), we obtain 7.4 X lo3 for in this area. cm-I compared to the average 8.4 X lo3 cm-I for the corresponding symmetric complexes. As was the case for V (n = 0), the opposite Stability of the Mixed Valence Complex (NC),Fe"pzRu"'(NH,),. The electron-transfer isomerization reaction (eq 18) that charges in the asymmetric complex (NH3)5RupzRu(EDTA)+ produces the unstable electronic isomer of V (n = 0) is, as shown result in considerably less solvation than in the parent symmetric above, unfavorable by 0.08 V. It is important to elucidate the complexes (EDTA) RupzRu( EDTA) 3- and (NH,) ,RupzRufactors that stabilize one or the other electronic isomer, and, for (NH3),,+. We suggest that the decrease in solvation is accomthis purpose, it is instructive to consider the following quasipanied by a decrease in solvent reorganization, and therefore the value for omaxfor the asymmetric complex is, after correction for thermodynamic cycle. Since the two electronic isomers differ endoergonicity,higher than the average of the values for the parent Fe%N)5pz3+ R;?NH3kpz 3 t Fe111(NC)sp?- -t R:1(NH3)5p~ 2t symmetric complexes. By utilization of the experimental results and eq 16-17 where
-
jeaX
-
-
-
g
Eth
= [Eo,,/ [4(&, - Ed1 - HAB]
(16)
= exp(-Eth/R7? (17) Ethand E,, are the thermal and optical electron transfer barriers, E, and k,, are the energy change and rate constant for intramolecular electron transfer according to eq 18, and vet is an electron
ka
(NC)=,F~"~ZRU~~'(NH,)=, (NC),Fe*"pzRu"(NH3),
(18)
hopping frequency taken to be 5 X 10l2s - ' , ~ we calcualte Eth= 3.12 kcal and k,, = 2.7 X 1O'O s-'. The value of E, was obtained from the following cycle: E'' was measured in the present work
-
(NC)sFe11piRu"x(NH3)5
Eo
( N C )5 Fe'I'pz
Ru" ( N H 3
)5
Id
P 1.
ket
I1
I11
(NC)5Fe pzRu (NH3)5
E?
I11 + pzRu (NH3I5
In
(-0.72 V) and E,' was taken as 0.64 V, the value for reduction of (NC)5Co"'pzRu1r1(NH3)~ to the corresponding ruthenium(I1) complex. It is instructive to compare the value of k,, calculated above for reaction 18 with the calculated value for the reaction of the analogous asymmetric complex (EDTA)Ru"~zRu"'(NH3),+ (1 X 10" S - ' ) ~ and with the measured values for the corresponding reactions of the cobalt(II1) complexes (NC),Fe"pzCo"'(NH3), (5.5 X lo-, s-')~ and (EDTA)Ru"pzCo"'(NH,), (21 s - ' ) . ~ In making the comparisons, the quantities of interest are the reactivity ratios of Ru"(EDTA)*and Fe11(CN)53-toward the common oxidants Ru"'(NH,),pz3+ and Co"'(NH3),pz3+, 4 and -4 X lo2, respectively, and the reactivity ratios of Ru(NH,),pz3+ and C O ( N H , ) ~ ~toward Z ~ + the -5 X lo9 and common reductants R u ( E D T A ) ~ Fe(CN)53-, -~~~ - 5 X lo", respectively. These reactivity ratios bring out two notable features. First, as noted previously by Creutz6 for other systems, the rate constants calculated from intervalence absorption (37) It must be noted that the theoretical equations utilized to calculate the solvent reorganization energy are independent of the charge: Cannon, R. D.Adu. Inorg. Chem. Radiochem. 1978, 21, 179. (38) Footnote c of Table 111. (39) Oliveira, L. A . A.; Haim, A. J . Am. Chem. Sot. 1982, 104, 3363.
In
XI
pzRu (NH3)5
in the charges of the respective moieties (3+ and 3- and 2+ and 2-), the cycle includes as two of the steps the electrostatic contributions of disassembling (E,') or assembling (E4') the stable or unstable binuclear isomers into or from the parent mononuclear complexes. These values were estimated from the relevant equations for ion pair formation constantsi6 by taking radii of 3.6 X and 4.1 X cm for cation and anion, respectively (these are the radii of the spheres of equal volume). The ion pair constants yield the values E,' = -0.12 V and E4' = 0.06 V. The value of E,' (from Table 11) is -0.03 V, and the value of El0 estimated above is -0.08 V to be compared with the value -0.09 V calculated from E2' E,' E4'. The excellent agreement between the value of Eloestimated from redox potentials and the value of E,' calculated from the cycle justifies the validity of the approximations used in the proposed cycle. According to the cycle the value of E,' is determined by two factors. First, an "intrinsic" relative stability of Fe(III)/Fe(II) vs. Ru(III)/Ru(II) is measured by E,', the difference in oxidation potentials between the mononuclear units. This factor favors Ru(II1) and Fe(I1) over Ru(I1) and Fe(II1) by 0.03 V. Second, an electrostatic factor is measured by E,' + E4', the difference in the stability of the two isomeric binuclear complexes associated with the electrostatic attraction between the two pairs of moieties. This term also favors the Ru(II1)-Fe(I1) pair over the Ru(I1)-Fe(II1) pair, this time by 0.06 V. It will be seen from the above analysis that the increased stability of the binuclear complex containing Ru(II1) and Fe(I1) vs. the one containing Ru(I1) and Fe(II1) as compared to the corresponding mononuclear units is entirely rationalized on the basis of the more favorable electrostatics of the Ru(II1)-Fe(I1) combination (+3, -3) vs. the Ru(1I)-Fe(II1) combination (+2, -2). An alternate way of assessing the increased stability of (NC)sFe"pzRu11'(NH,)5 vs. (NC)5Fe111pzRu11(NH3), as compared to the mononuclear complexes is to determine the increased stability of Fe(I1) attending coordination of the remote N in Fe1'(CN),pz3- by R u ~ ~ ' ( N H , ) ,and ~ + the iccreased stability of Ru(I1) attending coordination of the remote N in Ru"(NH3),pz2+
+
(NC15Fe
(NC),Fe
+
(40) Curtis, J. C.; Meyer, T. J. Inorg. Chem. 1982, 21, 1562.
Yeh and Haim
376 J . Am. Chem. SOC.,Vol. 107, No. 2, 1985 Scheme I 2FeZ1(CN)SOH:-
+ 2Ru1"(NH3)5pz
3+
by Fe111(CN)53-. The stabilization amounts to 0.17 V in the former case ( E O for (NC)5Fe11'~"pzRu"1(NH3)1+~o is 0.72 V vs. 0.55 V for Fe111/'1(CN)5pz2-/3-)and to 0.12 V in the latter (Eofor (NC)5C0111pzRu111~11(NH3)5'+~o is 0.64 V vs. 0.52 V for R U " ' / " ( N H ~ ) ~ ~ Z ~ +According / ~ + ) . to this analysis, the increased stability of the binuclear complex containing Fe(I1)-Ru(II1) vs. the one with Fe(II1)-Ru(I1) compared to the corresponding mononuclear units is 0.05 V, in remarkably good agreement with the value 0.06 V estimated from the electrostatic terms (E,'
+
Finally, we inquire into the stability of the mixed valence compound with respect to the isovalent states, eq 19. The 2(NC)5Fe11pzRu111(NH3)5 + (NC) sFellpzRull(NH3)5- + (NC) 5Fe111pzRu"1(NH3)5+ ( 19) equilibrium constant for eq 19 calculated from the appropriate and therefore the reduction potentials (Table 11) is 1.3 X additional stability of V (n = 0) as compared to V (n = -1) and V ( n = +1) is 2.6 kcal/mol. An excellent analysis of the factors and we make that contribute to the stability has been presented,'"~~~ use of this analysis in the present system. The stability associated with electronic delocalization (a2vmax) is 0.17 kcal/mol. Since the system is best described as possessing trapped valences, it is not surprising that the resonance stabilization energy is quite small. The electrostatic factor, in contrast, is quite sizeable. The mixed valence state (2 mol) contains +3 and -3 moieties (for an electrostatic attraction of 3 X 3 X 2 = 18) whereas the isovalent states contain +2,-3 and +3,-2 moieties (for an electrostatic attraction of 2 X 3 + 3 X 2 = 12). The electrostatic contribution was calculated from the appropriate ion pair formation constants and is 0.9 kcal/mol. It is noteworthy that the electrostatic contribution of V (n = 0), a complex made up of moieties of opposite electric charge, is considerably larger than for binuclear complexes of similar size but having moieties with charges of the same ~ i g n . ~ ' , ~ ' The third factor, again a very important one, has to do with d a K * back-bonding. In the isovalent 11, I1 state, the metal centers acting as T bases compete for the K acidity of the bridging ligand. In contrast, in the mixed valence 11, I11 state, the inductive effect (41) Richardson, D. E.; Taube, H. J . Am. Chem. SOC.1983, 105, 40. (42) For a +3, +2 mixed valence state the electrostatic term is 3 X 2 X 2 = 12 compared to 3 X 3 + 2 X 2 = 13 for the isovalent states.
of the M(II1) metal center coordinated to the pyrazine increases its P acidity and thus stabilizes the M(I1)-pzM(II1) interaction.2' The very large shift in the MLCT band of Fe(CN)5pz3- upon coordination of the remote N to Ru(NH,),~" (452 to 590 nm) suggests that the enhanced P acidity of pyrazine contributes appreciably to the stability of the mixed valence ion, perhaps the full 1.4 kcal/mol not accounted for by electron delocalization and the electrostatic factor. From a purely thermodynamic viewpoint, the stability of V (n = 0) compared to the isovalent states can be assessed by considering Scheme I. Values of AGIO, AG,', AC30, and AG40 can be calculated from equilibrium constants and reduction potentials listed in Tables I1 and V. The value of AGso can be obtained from the following cycle. Values of AG,', AG70, and AG90, calculated from E o values given in Table 11, Fe11(CN)50H,3'
FelI1(CN),OH
+
% (NC)5Fe11pzRu'1(NH3)5
Ru'~(NH~)~Pz'+
:+
5 (NC)5Fe'11pzRu111(NH3)5+
RuI'I(NH~)~~z~'
are -8.99, -1 1.99, and 27.90 kcal, respectively. AG40 (from K in Table V) is -9.12 kcal. Therefore, AGSo = -2.20 kcal (Keq = 41 M-')."3 Values of AGIO, AG20, and AG30 are 5.29, -19.59, and -3.00 kcal, respectively. Evidently, the most important factor in determining the stability of the mixed valence ion as compared to the isovalent states resides in the contrasting affinities of the Fe111(CN)52-and Fe11(CN)53-moieties for the remote N in R U " ' ( N H , ) ~ ~ Z ~The + . strong affinity exhibited by Fe11(CN)53(9.12 kcal, 5.1 X lo6 M-I) is, to be sure, a consequenceof its strong back-bonding ability. In contrast, the decreased u basicity of the pyrazine N caused by the inductive effect of the R U " ~ ( N H ~ ) ~ ~ + center and the absence of P basicity for Fe111(CN)52-combine to give a low stability to the Fe111(CN)53--pyrazinebond.
Acknowledgment. One of the authors (A. H.) benefited from many illuminating discussions with Prof. Joseph Lauher and with Dr. Urs Furholz. (43) A similar calculation of the equilibrium constant at 25 OC for Fe"'(CN)50H22- Rh(NH3)*pz3+=+(NC)5Fe"1pzRh'''(NHj)5C + H2O yields 31 M-' ( p = 0.10 M) to be compared with the value 6.7 M-' ( p = 0.50 M) reported previo~sly.'~
+
