677
NOTES
investigation of a ternary mixture of 0.2 mole fraction each of nitrobenzene and o-nitrotoluene in n-dodecane a t various frequencies and at 30' gives a very good Debye circle on the Cole-Cole plot.* The relaxation time, r, corresponding to the absorption peak for this mixture is 1.9 X see., which is close to 1.7 X see. for the mixture of 0.2 mole fraction nitrobenzene in n-dodecane at the same temperature. The Debye circle for the ternary mixture may not be surprising as it could be the result of a simple overlap of the absorption curves of single component mixtures with very close relaxation times. I n the case of the binary mixtures of chlorobenzene and bromobenzene, it is interesting to note that a change in the concentration from 60% by volume of bromobenzene to 40% does not significantly change the height or the position at which the low-temperature peak occurs, but the second peak does. Also interesting is the fact that the loss factor-temperature curve of pure bromobenzene has a rather peculiar shape. An approach was made to analyze the data in terms of the relaxation times associated With the peaks. In the case of the ternary mixture involving 0.35 mole fraction of the nitro compounds in Figure 1, for example, the relaxation time corresponding to the low-temperature peak is about 20% higher than the r-values of the individual polar components in the same solvent. For the binary mixtures of chlorobenzene and bromobenzene, the relaxation times corresponding to either of the two peaks lie between the values of the individual components and tend toward the value of the relaxation time of the higher concentration component. In comparing the mixtures involving two polar components with that of the corresponding single components, it is perhaps more appropriate to compare the ratios of the relaxation times, r, to the respective viscosities, T, instead of just the 7-values. This was done for the case of the 0.35 mole fraction mixtures of nitro compounds shown in Figures 1 (a) and (b). The viscosities at the various temperatures were measured using an Ostwald-Fenske viscomsec.eter. The r/q value of 0.95 in units of poise-I for the low-temperature peak of the composite mixture compares very well with the values 0.95 and 0.93 computed at the same temperature for the single component mixtures of nitrobenzene and o-nitrotoluene, respectively. The corresponding values relating to the high-temperature peak of the composite mixture, on the other hand, are 1.37 for the ternary mixtures and 0.89 and 0.91, respectively, for the single component mixtures of nitrobenzene and o-nitrotoluene. to indicate at least in The above results the case Of mixtures studied in the present investigation, the relaxation mechanism in dipolar liquid mixtures
may not be significantly different from that of single components. Acknowledgments. The authors wish to thank Dr. T. J. Bhattacharyya for some of the calculations. The viscosity measurements were done by T. V. Gopalan, Research Assistant on the project. (8) K.S. Cole and R. H. Cole, J. Chem. Phys., 9, 341 (1941).
The Mechanism of Photochromism in Metal Carbonyl Solutions
by G. R. Dobson Department of C h k t r y , Univmsity of Georgia, Athens, Georgia (Received September IO, 1964)
Solutions of the group VI-B metal carbonyls, M(C0)c (M = Cr, Mo, W), in many organic solvents become yellow when exposed to a strong ultraviolet source; upon removal of the source the solutions may again become colorless. El-Sayed,l in discussing the color changes observed upon irradiation of Mo(C0)B in 1:1 ether-isopentane mixtures both at 77%. and at room temperature, suggested that the photochromic behavior was characteristic of the species MO(CO)~rather than of the reversible formation of a weak complex between the metal carbonyl and the solvent. Among the supporting arguments advanced by El-Sayed was that for strong charge donors (D), e.g., those which coordinately bond to the carbonyl through a lone pair on nitrogen, the production of the color is irreversible. The following evidence is presented in support of the alternative explanation that the observed photochromism is the result of ultraviolet-induced complex formation between the ether and the hexacarbonyl. A. It has been reported that ethers may form coordination complexes with the group VI-B metal carbonyls. Although spectral evidence only has been presented in support of complex formation in most cases, e.g., With ethyl ether2 and tetrahydrofuran,a the complex [bis-(2-methoxyethyl) ether]M~(CO)~,~ in which the ether functions as a tridentate ligand, has been characterized. (1) M. A. El-Sayed, J. Phys. Chem., 68, 433 (1964). (2) I. W. Stolz, G. R. Dobson, and R. K. Sheline, Inwg. Chem., 2 ,
323 (1963). (3) W. Strohmeier and I(.Gerlach, Chem. Ber., 94,398 (1961). (4) R,p. M. wernerand T.H. cofie1d, Chenz. I&. ( ~ ~ ~936d ~ ~ ) , (1960).
Volume 69,Number 2 February 1966
678
NOTES
B. The persistence of the yellow color is dependent upon the presence of ether, but not isopentane. Solutions of Mo(CO)8 in ethyl ether, when exposed to a strong ultraviolet source in a sealed cell at room temperature, remain yellow for several minutes (determined spectrophotometrically) after the completion of the irradiation. Though Mo (C0)risopentane solutions appear yellow during irradiation, no color can be detected upon the removal of the ultraviolet ~ o u r c e . ~ C. Spectral evidence strongly supports complex formation. (1) The infrared spectrum in the G O stretching region (2100-1700 cm.-l) for the photolysis product in an ether-isopentane mixture is that expected if a M(C0)6D derivative of C4, symmetry were produced, while the low-temperature spectrum reported (in saturated hydrocarbon solvents) is for Mo(CO)~~ consistent with trigonal bipyramidal molecular symmetry (D3b) analogous to the reported structure of Fe(C0)5.7J& (2) The electronic spectrum of irradiated Mo(c0)~ in ether-isopentane is quite similar to the spectra reported for M(CO)6D complexes in which D is a ligand coordinately bonded through nitrogens (Table I). The essential features of the reported spectra are independent of the metal and the donor. It is reasonable to expect that such a spectrum is characteristic of D+M(CO)5 complexes regardless of the identity of the coordinating atom; the electronic spectra of different r-arene derivatives of the group VI-B metal carbonyls, for example, are quite si~nilar.~
M(C0)5
+D
M(C0)D (yellow)
+
M(C0)5 CO +M(CO)B (very rapidly) is involved, and that the process can be reversible, ie., photochromic, only when the complexing ability of the solvent is extremely poor. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. (5) At 77OK. the color imparted to irradiated M(C0)esaturated hydrocarbon glasses may persist for longer periods of time. (6) I. W. Stole, G. R. Dobson, and R. K. Sheline, J. Am. Chem. SOC., 85, 1013 (1963). (7) A. W. Hansen, Acta Cryst., 15, 930 (1962). (74 NOTEADDED IN PROOF.The CO stretching force constants calculated from the secular equations of F. A. Cotton and C. 5. Kraihanzel (J.Am. Chem. Soc., 84, 4432 (1962)) for a postulated Mo(C0): species of C4v symmetry obtained in a 1:4 methylcyclohexane-Lsopentane glass at 77OK.B0arekl = 15.25 and kz = 16.31 mdynes .&.-I (hi = 0.33 mdyne A.-I), compared to 16.52 mdynes .&.-I for Mo(C0)e. The low values for the former suggest that the species may be a weak complex formed between Mo(C0)s and an extremely poor donor, perhaps the nitrogen under which the experiments were carried out, rather than M o ( C O ) ~ itself. (8) W. Strohmeier and K. Gerlach, Z . physik. Chem. (Frankfurt), 27, 439 (1961). (9) R.Ercoli and A. Mangini, Ric. Sci., 28, 2136 (1958). (10) W. Strohmeier and D. von Hobe, Chem. Ber., 94,2031 (1961).
Radiolysis of Cyanogen-Cyclohexane Mixtures Table I: Comparison of Electronic Spectra for M(C0)aD Complexes*with that of Irradiated Mo( CO)e-EtherIsopentane Solution1
Nuclear Sciences Division, Engineering Experiment Station, (GeorgiaInstitute of Technology, Atlanta, Georgia Received June 10,1964)
D
M
Pyridine Piperidine Quinoline Piperidine Piperidine
Cr
3900
Cr Cr
4200
2420 2500
4130 3970 4050
2510 2470
4095
2469
Mo W
Irradiated hlo( C0)petherisopentane
-AmsX,
.A-
...
It seems most probable, therefore, that a t room temperature and where colorpersists for a time after the completion of an ultraviolet expomre, complex formation according to the mechanism b t proposed by StrohmeierlO
+
M(C0)e hv +M(C0)6* ----t M(C0)s (colorless) (yellow) The Journal of Physical Chemistry
by J. A. Knight, R. A. Stokes, and David Bowen
+ CO
The radiation chemistry of cyclohexane has been the subject of a number of investigations.l-12 This is a report on the irradiation of mixtures of cyanogen and (1) H. A. Dewhurst, J . Phys. Chem., 63, 813 (1959). (2) T. D.Nevitt and L. P. Remsberg, ibid., 64, 969 (1960). (3) G.R. Freeman, J . Chem. Phys., 33, 71 (1960). (4) P. J. Dyne and J. A. Stone, Can. J . Chem., 39, 2381 (1961). (5) P.J. Dyne and W. M. Jenkinson, ibid., 39, 2163 (1961). (6) R. R. Williams, Jr., and W. H. Hamill, Radiation Res., 1, 158 (1954). (7) R. H. Schuler, J . Phys. Chem., 61, 1472 (1957). (8) G. Meshitsuka and M. Burton, Radiation Res., 10, 499 (1959). (9) L. J. Forrestal and W. H. Hamill, J . Am. Chem. SOC.,83, 1535 (1961).
