6874
of the lower component of resonance 3, i.e., the syn proton in the (+) configuration. Therefore, inversion of configuration occurs with syn-anti interchange, and a a-bonded intermediate ( 1 4 ) is implied for the first epimerization process of the anti-Ac complex. Similarly, spin saturation labeling experiments and observation of coalescences allow us to conclude that a 1-h a-bonded intermediate is primarily responsible for epimerization of the syn-Ac complex. Furthermore, isomerization appears to occur predominantly via a 3-h a-bonded intermediate. Acknowledgment. We wish to acknowledge the financial support of the Connecticut Research Commission and the Petroleum Research Fund administered by the American Chemical Society. We wish t o thank the National Science Foundation for Grant GP-6938 which allowed the purchase of the Varian HA-100 spectrometer. J. W. Faller, M. E. Thomsen Department of Chemistry, Yale University New Haven, Connecticut 06520 Received June 23, 1969
Formation and Reactions of [(NH3)6RuN202+]
Sir: When NzO is added to an aqueous solution of (NH3)5RuOHz2+, produced by reduction of (NH3)jRuC12+ by Cr2+, Pt-H2, or Zn(Hg), a new absorption band develops having a maximum at 238 nm (see Figure l). It increases in intensity when the concentration of N 2 0 increases, disappears when NzO is removed by passing argon through the solution, and is restored when NzO is again added. The rate at which the absorption grows can conveniently be measured spectrophotometrically. Plots of In ( A , - A , ) us. t are found to be strictly linear up to at least 9 5 z completion of reaction. In Figure 2 the values of kobsdas they are obtained from the plots are shown as a function of NzO concentration. The data of Figure 2 show that kobsd is given by the k2[Nz0]. The general observations derelation kl scribed above and the kinetic data can be interpreted on the basis that N 2 0 associates reversibly with (NH3)5RuOHz2+.
+
+
kr
+
(NH~)~RuNzO'+ HzO
(NH~)~RuOH~'+ NzO kr
(1)
On the basis of this interpretation, the rate of approach to equilibrium at constant concentration of N 2 0 should follow pseudo-first-order behavior, and, moreover, the first-order specific rate kobsd should be given by k, kf[NzO](assuming, that is, that the rate laws of the forward and reverse reactions are as indicated in eq 1). Identifying kl (= 1.35 X sec-l) with k, and kz (=9.5 X M-' sec-1) with kf, K1 for reaction 1 at 6.8" and p = 0.023 (CI- as anion) is calculated as 7.0 (using NzO(aq) rather than NzO(g) as the standard state for this reactant). It should be noted that the k, term makes by far the greater contribution to kobsd,and thus the value of k, is rather well defined by the data. The coefficient kf is obtained from the slope of the line in Figure 2 and is, therefore, less well defined. The value of kf suggested by the above data is, however, confirmed
+
Journal of the American Chemical Society
1 91:24 /
by an independent measurement which will now be described. When Cr2+ is present in a solution containing both ( N H ~ ) ~ R U O Hand Z ~ NzO, + the reaction
+ (NH&RuOHzZ+ + NzO + 2H+ =
2CrZ+
2Cr3+
+ (NH&RuNz2+ + 2H20
(2)
takes place quantitatively. The kinetics of reaction 2 are of particular interest. Though Cr2+ is consumed, the reaction rate does not depend on [Cr2+],and the rate law for the reaction is given by dr(NH3)5RuNz2f1 = k[(NH3)5RuOH22+][N20] (3) dt Good kinetic data were obtained following the growth of the nitrogen complex spectrophotometrically at 22 1 nm. In determining the rate law, NzO was in excess, but the initial concentration covered the range 8.38 X 10-3-4.52 X M for the series.' Chromous ion M ; the covered the range 1.2 X lo-* to 1.2 X initial concentration of Ru(I1) was fixed at 6 X 10-5 M. The coefficient k at 6.8" is found to be 10.1 X 10-3 M-1 sec-'. The form of rate law 3 and the excellent agreement of k with kf shows that the rate of formation of the NzO complex is rate determining for reaction 2 and this, in turn, suggests that the NzO complex is reduced virtually as rapidly as it is formed. The stoichiometry for reaction 2 was checked covering the range [Cr2+]/[Ru(II)] from 1.2 to 10.0 by using Fe3+ to quench the reaction, and then developing the color of Fez+ with o-phen.2 Under our conditions, the oxidation by Fe3+ of the nitrogen complex was slow compared to that of Cr2+or (NH3)5RuOH2z+. By waiting 1 hr after the Fe3+was added, the nitrogen complex was destroyed quantitatively, and the amount of nitrogen liberated was determined by gas chromatography using this reaction. Within the limits of accuracy of the methods, f5 the agreement with requirements of eq 2 was exact. The first method was also used to follow the rate of reaction, again in satisfactory agreement with the rates determined spectrophotometrically. The kinetic measurements were repeated at 20.1 '. At the higher temperature, k, contributes proportionately even more to kobsdthan at the lower, and, as before, rather accurate value of this parameter can be obtained. The value of kf was determined by measuring the rate of reaction 2 at the higher temperature. sec-I The values of k, and kf obtained, 9.1 f .5 X M-I sec-', respectively ( p = 0.02 with and 4.51 X Cl-), combine to yield Kl at 20.1 O as 5.0. The affinity of (NH3)jRuOHz2+for NzO was also determined by a vacuum-line method, comparing the partial pressure of N 2 0 over a solution containing (NH3)5RuOHz2+with that registered in a blank experiment identical in every respect, but with sodium chloride replacing the ruthenium complex. k was measured as 8.3 at 24" and 8.6 at 22'. Because the pressure changes recorded are very small, and because
z,
(1) The concentration of N20 in solution was determined using the value of Henry's law constant as obtained from "International Critical Tables," Vol. 111, McGraw-Hill Book Co., Inc., New York, N. Y., 1928, p 259.
(2) T. J. Meyer and H. Taube, Inorg. Chem., 7, 2369 (1968); G. Charlot, "Colorimetric Determination of Elements,'' Elsevier Publishing Co., New York, N. Y., 1964, p 274. (3) The authors wish to express their appreciation to Mr. P. R. Jones for his assistance with the vacuum-line work.
November 19, 1969
6875 -1
I
I
I
13 O coo3
wotc,-
3I 0 0
4 0P0
I
I
1.o
2c,
I 30
I
'to
[NzOIeoln X 10' M.
h (nm)
Figure 2. The rate constant governing the approach to equilibFigure 1 . (A) Uv spectrum of (NH3)5RuOHzz+.(B) (NH&rium in the reaction of (NHa)aRuOHz'+ with NaO as a function of RuNZO2+in equilibrium with (NH3)sR~OH22+. [ ( N H ~ ) ~ R u O H ~ ~ +[N20]: ] [NzO]~>> [ ( N H ~ ) ~ R U O H ~temperature ~+]~; = 6.8". = 3.29 X M, [HCl] = 1 X M , [NzO],, = 2.8 X 10-2 M , = 6.12 X M. p = 0.023 with C1[(NH~)~RuOH~'+] p = 0.012with C1-. Using the value of Kl = 5.0 at 20.1 the extinction coefficient of the nitrous oxide complex may be calculated M-l cm-lat 238 nm. as 1.7 i 0.2 X O,
tion of the complex, K? is calculated as 5.5 X IO4. Equilibrium with respect to the bridged nitrogen comthere is some net reduction of NzO during the manipulaplex was not attained in these solutions. Other experitions, the value of Kl obtained by this direct method is ments allowing ( N H & R U N ~ to ~ +reach complete equinot considered to be as accurate as that recorded above. librium under nitrogen at 1 atm have led t o a value of The measurement does serve, however, to lend credence -5 X IO4 for K4 at 25" ( p = 0.1 with chloride) and to the interpretation of the data which was advanced. indicate that the quotient [(NH&RUNZRU(NH~)O-~+]/ That the variations of Kl with temperature is small ([(NH3)5RuNz2+][(NH~)~R~OHz2+]) at equilibrium is of was confirmed by measuring the absorbance of a soluIO4. the order of tion containing NzO and (NH&,RuOHZ2+at fixed conThe slow reduction of NzO by ( N H 3 ) 5 R ~ O H z 2 + centration. The ratio of absorbance at 238 nm at 6.8" has already been referred to. In a solution with to that at 20.1 O was observed to be 1.39, to be compared M and [NzO] = 0.028 [(NH3)5RuOH22+]= 3.3 X to a ratio of 1.4 determined from the values of k M, no significant loss of reducing power is observed above. within 0.5 hr. But on the time scale of days, (NH& Using the values of the equilibrium constant and its RUN^^+, [(NH3)5Ru]zNz4+,' ( N H & R u O H ~ ~ + , and temperature coefficient, AF", AH", and AS' for reaction (NH3)5RuC12+ (chloride medium) are formed. In con1 are calculated as - 0.9 kcal/mole, - 4.2 kcal/mole, and tact with amalgamated Zn or platinum wire, the reduc- 11 eu, respectively. Combined with the value of AF" tion of N 2 0 by (NH&RuOHz2+ is greatly accelerated. (25') for reaction 4 determined as described below, In contrast to ( N H & R u O H ~ ~ +the , reducing agent (NHa)jRuOHz'+ Nz = (NHa)sRUNz'+ Hz0 (4) Cr 2+ reacts extremely rapidly with (NH&RuN20 2+. By taking account of the fact that the rate of reaction AFo = -6.5 kcal 2 is independent of [Cr2+]even at the lowest concentra(NHa)sRUNz02++ N2 = (NH~)SRUNZ'+ + Nz0 ( 5 ) tion used, a lower limit for the rate of reaction Cr2+ that for reaction 5 can be calculated as - 5.6 kcal. This with the species (NH3)jRuN202+of IO2 M-I sec-' is value is significant in showing that the affinity of NzO calculated. The reaction of Cr2+with NzO is very much for 0 is diminished when NzO coordinates to (NH& slower, requiring a time period of the order of days for RuOHz2+. This outcome was not expected, it having half-reaction.* Preliminary measurements (Cr2+ at seemed reasonable to conjecture that the back-bonding 0.01 M , NzO at 0.028 M), assuming a second-order reinteraction between NP and Ru(I1) would increase the M-' sec-'. action, show the specific rate to be negative charge on the terminal nitrogen and thus Product studies on reaction 2 using ion-exchange techstrengthen the bond to a Lewis acid such as atomic niques to identify Cr(II1) species show that Cr3+ and oxygen. CrCI2+ are the dominant products. Thus a one-elecThe equilibrium constant for reaction 4 was detertron, rather than a two-electron reduction of the comined by measuring the rate of loss of nitrogen from ordinated N 2 0 , is indicated. Because, as shown by ( N H & R u N ~ ~in + solution, using a stream of Ar t o AF" for reaction 5, the N-0 bond is weakened when sweep out Nz as it is formed. Under these conditions Nz0 associates with (NH&RuOHZZ+, the increase in both (NH&RuOHZ2+ and [ ( N H & R U N ~ R U ( N H ~ ) ~ ~reactivity +] toward Cr2+ of the coordinated oxide is not are produced. These were determined spectrophotounexpected. It is possible also that Cr2+ and Ru(I1) metrically, the first by developing its color with isointeract on NPO in concert, and that the increase in nicotinamide4 and the second using the absorption reactivity is partly attributable to this. This point maximum at 262 nm.j Combining the specific rate cannot be settled, however, without further experifor the decomposition of the nitrogen complex thus mentation. determined, -1.3 X IO+ sec-l at 25", with the value measured by Itzkovitch and Page6 for the rate of forma(6) I. J. Itzkovitch and J. A. Page, Can.J . Chem., 46,2743 (1968).
+
+
(4) R. G. Gaunder, Ph.D. Thesis, Stanford University, Stanford, Calif., June 1969. (5) D. F. Harrison, E. Weissberger, and H. Taube, Science, 159, 320 (1968).
(7) Molecular nitrogen complexes of pentaammineruthenium(I1) have been prepared by reducing (NHa)aRuCl*+in the presence of N20: A. A. Diamantis and G. J. Sparrow, Chem. Commun., 23,469 (1969). (8) R. G. S. Banks, R. J. Henderson, and J. M. Pratt, J . Chem. Soc., A, 2886 (1968); W. Traube and W. Passerge, Ber., 49, 1692 (1916).
Communications to the Editor
6876
The value of the second-order specific rate for formation of the NzO complex may be compared with the formation rates of other (NH3)5R~"L2+complexes at ~ + , CO, pyridine, 25". For L = Nz, ( N H ~ ) ~ R u N zN20, and isonicotinamide, k = 7.1 X 10-2,6 4.2 X 10-2,6 7.21 X 12 X 10-2,911.8 X 10-z,lo and 6 X M-l sec-l." Acknowledgment. Financial support by the National Institutes of Health, Grant No. G M 13638-03, and the National Science Foundation, Grant No. G P 5322, is gratefully acknowledged.
The compounds appear to have strong interactions with solvents, which affect their appearance markedly. The unsolvated Yb compound is, for example, pink. It is insoluble in ammonia, but becomes orange. Removal of excess ammonia at atmospheric pressure yields a compound which is a very intense blue. Removal of ammonia under vacuum yields the original compounds. Yb(Cot) is insoluble in hydrocarbons and ethers but dissolves in more basic solvents, such as pyridine and dimethylformamide. The solutions are deep red and the solids in contact with them are nearly black. These solvents may be removed under vacuum. (9) D. F. Harrison, Ph.D. Thesis, Stanford University, Stanford, Both compounds are stable to 500" under vacuum. Calif., Jan 1969. Neither sublimes at 500" and 10 p. (10) A. R. Allen, R. Hintze, and P. C. Ford, to be submitted for publication. There is no marked difference between the epr spectra (11) J. N. Armor, unpublished result. of unsolvated Eu(Cot) and the compound solvated with NH3. This means only that there has been no remarkJohn N. Armor, H. Taube able change in the electrical asymmetry of the environDepartment of Chemistry, Stanford Unicersity ment of the Eu2+in the two cases. Stanford, California 94305 The compounds were prepared by the dropwise addiReceiced August 23, 1969 tion of cyclooctatetraene to ytterbium or europium metal dissolved in anhydrous ammonia. The reactions were carried out under a purified nitrogen atmosphere. Synthesis of Cyclooctatetraenyleuropium and In a typical reaction, 0.01 mole of cyclooctatetraene Cyclooctatetraenyl ytterbium was added slowly to 0.01 mole of the metal, yielding an orange or light green precipitate for ytterbium or euSir: ropium, respectively. After stirring for 2 hr, the reThe preparation of bis(cyclooctatetraeny1)uraniummaining ammonia was removed and a bright blue or (IV), which has received the trivial name "uranocene," light green product recovered. Upon heating to 200" has renewed interest in new types of compounds conat mm, a pink (Yb) or orange (Eu) product was obtaining cyclooctatetraene. Aside from uranocene, comtained. Carbon and hydrogen analyses were slightly pounds of formula M(Cot)z, with M = V, Ti, have been off, but consistent with a 1 : 1 complex. Anal. Calcd: known for some time. Several compounds of formula C, 34.7; H, 2.9. Found: C, 30.5; H, 3.0. Metal M(Cot) are also known,2 as is one mixed cyclopentaanalysis, carried out in the same manner as that for dienyl-cyclooctatetraenyl compound, C ~ ( c p ) ( C o t ) , ~ ytterbium cycl~pentadienide,~ gave excellent agreement and many Cot-carbonyl compounds. We report here for a 1: 1 complex. Anal. Calcd for Yb(Cot): Yb, the preparations and some properties of cyclooctatetra62.4. Found: Yb, 62.1. Analysis of the pink Yb(Cot) ene compounds of Yb and Eu. We believe these to be indicated that less than 0.5 % nitrogen was present. All the first reported cyclooctatetraene complexes of rare solvents used were purified by refluxing over calcium earth elements. hydride or by contact with potassium mirrors. ElecIt is well known that solutions of ytterbium and tron paramagnetic resonance measurements were pereuropium in liquid ammonia may be used to prepare formed on a Varian V-4502-15 X-band spectrometer, the respective cyclopentadienides. The cyclooctaand magnetic susceptibilities were determined with a tetraene derivatives of these elements were successfully simple Gouy balance using HgCo(NCS)4 as a standard. synthesized using similar techniques. Because of the Acknowledgment. The partial support of this almost explosive air oxidation of both compounds, all work by National Science Foundation Grant No. 7881 studies were carried out under anaerobic conditions. is gratefully acknowledged. Also, in contrast to U(Cot),, both compounds undergo immediate hydrolysis in the presence of water. ( 5 ) J. M. Birmingham and G. Wilkinson, J . Am. Chem. SOC.,78, 42 (1956). One would expect to obtain compounds of the type Robert G . Hayes, Joseph L. Thomas M(Cot) from Yb and Eu, since these elements both have Department of Chemistry, Unicersity of Notre Dame stable +2 oxidation states. This expectation is fulNotre Dame, Indiana filled. Analysis of the Yb compound substantiates the Receiced August 4 , 1969 formulation Yb(Cot). The Yb compound is, furthermore, diamagnetic. The Eu compound has an epr spectrum visible at 77"K, which is consistent only Fluxional Behavior of (Tvihuptocycloheptatrieny1)with Eu2+. The epr spectrum of the powdered com(pentuhuptocyclopentadieny1)monocarbonyliron pound consists of a strong resonance 4 kG wide, geakto-peak, centered on g = 2.00. Sir: (1) A. Streitwieser, Jr., and U. Muller-Westerhoff, J . Am. Chem. SOC., 90, 7364 (1968). (2) H. Brei1 and G. Wilke, Angew. Chem. Intern. Ed. Engl., 5 , 898 (1966). (3) A. Nakamura and N. Hagihara, Bull. Chem. SOC.Jap., 33, 425 ( 1960). (4) E. 0. Fischer and H. Fischer, J . Organometal. Chem., 3, 181 (1965).
Journal of the American Chemical Society
/ 91:24 /
We recently reported the preparation of a substance which was assigned the structure of a 7-monohuptocycloheptatrieneiron complex. Further examination of this substance and a comparison of its nmr spectrum (1) D. Ciappenelli and M. Rosenblum, J . Am. Chem. Soc., 91, 3673 ( 1969).
Nocember 19, 1969
