Inorganic Chemistry, Vol. 13, No. 5, 1974 1239
Reduction of Cr(OH2)&H212+by Cr(I1) Table 111. Summary of Spectral Data for the Three Oxopentahalouranate(V) Species ~~
~~
~~~~~
Electronic transitions obsd, cm-'
Best-fit parameters, cm-'
'uoci, 'UOBr, 'UOF.
t
A
8
1850 1770 1750
4630 3810 4070
6540 3110 2150
r6(r8)
2380 1555 1490
Table IV. Epr Data for Uranium(V) Complexes Complex UF,UOF, 'uc1,UOCl, ,UBI,UOBr, '-
(g)
peak-to-peak signal width, G
a
0.58 1.12 1.09 1.21 1.24
a -8000
--
1200 1400 85 0 1600
a Values of the average g tensor for polycrystalline MUF, (M = Li, Na, Cs) at 77°K have been reported' as-0.768, -0.748, and -0.709, respectively.
pounds and that the spin-orbit coupling constants for the compounds decrease in the order F- > Cl- > Br-. We suggest that the symmetry of UOXs2- should not be characterized as strongly axially distorted (note that the tetragonal distortion parameter 7 = 700 cm-' is adequate to account for the electronic spectral bands) in the same way that NpOz2+and other trans-dioxoactinyl species are. For example, the first excited level in S f ' RbNp02(N03)3 is probably below 1000 cm-'.
r,(r8) 6792 5050 4880
rdr,') 6837 6161 6080
r6(r8')
r,(r,')
r6(r6)
11,448 8,584 8,163
12,420 10,616 10,460
18.660 16,835 16,194
This low-lying level apparently shortens the longitudinal relaxation time Tl sufficiently to require very low temperatures for epr signal observation of the N ~ 0 2 ~species. ' Although one might expect relatively shorter relaxation times (due to the r8 splitting into higher and lower levels) and larger g anisotropies in the oxo species relative to the hexahalo species, these factors do not appear to be sufficient to eliminate (but only greatly broaden) room-temperature epr signals, as has been suggested by Lewis, Hecht, and Eastman.6 Their theoretical predictions in this regard simply are not borne out by the experimental facts illustrated in Figures 5 and 6. Acknowledgment. The authors are grateful for a grant from the National Science Foundation (GP-30661x1) (to J. S.) which supported this research. Registry No. (Et,N),UOF,, 30917-64-7; (Et,N),UOBr,, 3091762-5 ;UCl,-, 44491-58-9; UOCl, '-, 4 1677-65-0;UBI,', 44491 -06-7; (C, H,),NF, 665-46-3 ; (C, H,),NUCl,, 17 14 1-96-7; UCl, .TCAC, 20574-41-8.
Contribution from the Department of Chemistry, University of the Pacific, Stoekton, California 95204
Stoichiometry and Kinetics of the Reduction of Cr(OH2),CH,12+ by Chromium(I1)' RONALD S. NOHR and LARRY 0. SPREER*
Received August 24,1973 AIC3063 1 1 The stoichiometry of the reaction between Cr(I1) and Cr(OH1)5CH21Z+ was found to be 2Cr" 2H+ CrCH,12+= CrI'+-t 2Cr3+ + CH,. The rate law for the disappearance of Cr(I1) was (-1/2d[Cr2+]/dt =k,[Crz+][CrCH,12+];at 25" k , = 2.9 0.2 X lo-' 1. mol-' sec-' with AH* = 7.7 kcal/mol, and A S * =-39 eu. The rate law for the appearance of CH, was found to be djCH,] /dt = k,[I] [H+],where I is a long-lived organochromium intermediate; k , at 25" was 5.2 0.3 X 1. mol-' sec". Evidence is presented that I was CrCH3'+, and a possible mechanism is proposed.
+
Introduction Chromium(I1) has been widely used as a reducing agent for a variety of organic compounds: and organochromium species have been proposed as intermediates in some of these react i o n ~ . ~Castro '~ and Kray5 report that some polyhalomethanes are reduced to methane by excess chromium(II), and they suggest "chromium-complexed carbenes" as intermediates. We find that methane is also produced when iodomethylpentaaquochromium(II1) ion, CrCH212+,is allowed to react with chromium(I1). This paper presents the results of our study of this reaction, and a mechanism is proposed. Experimental Section Reagents. Reagent grade diiodomethane (MCB) was used without (1) Presented in part at the 165th National Meeting of the American Chemical Society, Dallas, Texas, April 1973. (2) J. R. Hanson and E. Premuzic, Angew. Chem., Znt. Ed. Engl., 7 , 2 4 7 (1968). (3) K. D. Kopple, J . Amer. Chem. SOC.,84,1586 (1962). (4) J . K. Kochi and P. E. Mocadlo, J. Amer. Chem. SOC.,8 8 , 4 0 9 4 (1966). (5) C. E. Castro and W. C. Kray, Jr., J. Amer. Chem. SOC.,88, 4447 (1966).
+
*
further purification, as was reagent grade perchloric acid. NaClO, was prepared bv the neutralization of Na,CO, (Baker Analyzed Reagent) with HClO,. Stock solutions of NaClO, were analyzed by passing an aliquot through a cationexchange column in the H+form and titrating the liberated H+ with standard base. Cr(I1) solutions were prepared by dissolving electrolytic grade Cr metal (-99.999% pure, Apache Chemicals) in deoxygenated perchloric acid solutions. All solutions were prepared using H,O which had been deionized and then distilled using an all-glass still (Corning Model 1-a). Analysis. Chromium(II1) or total chromium content was determined using H,O, in base as an oxidant and measuring chromium as chromate ion (CrO,'-, 4815)." Cr(I1) was measured by determining the decrease in absorbance of acidic chromium(V1) solutions (X 348 nm) after reaction with Cr(II).' Carbon present in gaseous compounds was measured by absorption of CO, on an Ascarite tube after catalytic (CuO) combustion at 930". (Control experiments were run to ensure that complete combustion occurred under these conditions.) Gravimetric AgI analyses were made for I-. Cr(OH,),CH, I,+. Iodomethylpentaaquochromium(II1)ion, (CCH,I'+), was prepared, after the method of Anet,' by adding Cr(l1) solutions to CH,I,. The reactants were stirred for a 2-hr period (6) G . W. Haupt, J. Res. Nat. Bur. Stand., 48,414 (1952). (7) R. V. James and E. L . King,Inorg. Chem., 9 , 1301 (1970). (8) F. A. L. Anet, Can. J. Chem., 3 7 , 5 8 (1959).
1240 Inorganic Chemistry, Vol. 13, NO. 5, 1974
Ronald S. Nohr and Larry 0. Spreer
REACTION VESSEL
Figure 1. Apparatus for measuring small changes in gas volumes.
and then separated on a Dowex 50W-X8 (Bio-Rad Laboratories) ionexchange column. The brownish-orange 2+ ion was removed by NaC1O4-HC10,, mixtures (I = 1.OO M ) . The average of several independent analyses gave a Cr:C:I ratio of 1.00:0.98:0.99. Spectral measurements made on freshly prepared solutions with a C a y Model 14 spectrophotometer gave the following molar absorbancy indices for the maxima in the spectrum (hmaXnm;E mol-' cm-'): 524, 34.2; 396,322; 296, 3430; 264,3350.' CrCH,12' differs from most species having a chromium-carbon bond in that it does not undergo a rapid reaction with oxygen."
Kinetic Measurements The kinetics of the reaction between Cr(I1) and CrCH212+ were followed by a titrimetric procedure. Preequilibrated solutions of Cr(I1) and CrCH212+were mixed in a jacketed buret and temperatures were maintained by passing water from a Lauda K4-R constant temperature circulator through the jacket. (Temperature control was estimated at L0.08" .) Measured aliquots from the reaction buret were added to measured, deareated portions of acidic Cr(V1) solutions. The decrease in absorbance of the Cr(V1) solutions was used to calculate the concentration of unreacted Cr(I1) in the reaction solution. Due to complications caused by the reduction of Cr(V1) by I-,12 it was necessary to add Ag+ (-0.00015 M ) to the Cr(V1) solution in order to precipitate the I- as AgI. Under the conditions of the experiment, the solubility of Ag2Cr04 was not exceeded. CrCH212+has an appreciable absorbance at 348 nm, the wavelength at which the Cr(V1) was measured; therefore, the quenched aliquot was passed through a cation-exchange column to remove any unreacted CrCH212+. The ion-exchange column also acted as an effective filter for the AgI precipitate. The absorbancies of the Cr(V1) solutions were measured on a Gilford-modified Beckman DU spectrophotometer (accuracy > +1% in absorbance). Blank experiments were run which demonstrated the accuracy and reproducibility of the technique. Kinetic measurements were also made on the production of gases during the course of the reaction. The volume of gases produced under the concentrations and conditions of the experiments was small ( ca. 20. For chromocene, molecular orbital calculations" also predict a jA (uti3) ground state, and for this an orbital contribution to the magnetic moment is predicted. Thus one (8) See paragraph at end of paper regarding supplementary material. (9) T. S. Piper and R. L. Carlin, J. Chem. Phys., 33, 1208 (1960). (10) E. 0. Fischer and K. Ulm, Chem. Ber., 95, 692 (1962). ( 1 1) E. M. Shustorovich and M. E. Dyatkina, Dokl. Akud. Nuuk SSSR, 131, 113 (1960). (12) D. W. Clack, unpublished calculations (private communication).
