PHOTOCHEMISTRY OF AQUEOUS Cr(CN)6a-
3827
Table 111: Decay Rates of Transient Species Produced in the Flash Photolysis of Aqueous Solutions of 10-2 M P202*Ions, pH 10.5
System
nm
Transient speoies
Argon-saturated
550
A
2k/€ = 4.5
NzO-saturated NzO-saturated
580 610
A A
2kla = 2 . 6 X lo6 2k/e = 3 . 0 X lo5
Air-saturated Air-saturated Air-saturated
550 530 440
A A B
Air-saturated
440a
B
Air-s atur ated
260
2k/e = 3 . 0 X lo6 2k/r = 3.2 X 105 k = 3 . 3 rt 0 . 3 x 103 see-' k = 3 . 0 rt 0 . 3 X LO3 sec-l 2kis = 5 . 3 x 105
x,
a
In
10-3 M
0 2
-
Decay rateb
x
Comparison with Pulse-Radiolysis Xtudy of Phosphate loris. The pulse radiolysis of water and aqueous tions produces the reactive species eaq-, H, and OH, in addition to H2 and HzOz. H2O -+ eaq-, H, OH, HP,HzOz
106
Pz074-. b Deviation in decay rate good to
f20773.
~ 4 4 nm 0 (B),, , ,A 250 nm ( 0 2 - ) , and A,, -260 nm (C), are produced; see Figure 7 and Table 111. Species A decays by second-order process with 2 k / e = 3.0 X lo6a t 500 nm. Species B decays by a first-order process with IC = 3.3 X lo3 sec-'. The 0 2 - radical produced appears to decay by a second-order process, with IC7 = 4.8 X lo* M-l sec-'.
, , ,A
It is these species which react with the solutes present in solution. The pulse radiolysis of aqueous solutions of HzP04-,H P O P , and P2074-ions produces transient species similar to those observed in flash photolysis, and these have been shown14 to result mainly from the reaction of OH radicals with the oxyphosphate anions. Thus transient species with, , ,A %500 nm due to HzPod- and/or HPOd-. radicals are produced on radiolysis of mono- and dibasic phosphates. On pulse radiolysis of P ~ 0 7 ~ions, - species A with A,n, ~ 5 8 nm 0 is observed and appears to be produced by reaction of OH radicals with P207*- ions. Similarly, species B with, , ,A 440 nm is formed in the pulse radiolysis of PzO,~-ions in the presence of oxygen, but not in argon or N2O-saturated solutions. Further work is in progress to elucidate the nature of the species B and C produced in these systems. (14) E. D. Black and E. Hayon, to be published.
Photochemistry of Aqueous Cr(CN),3by A. Chiang and A. W. Adamson Department of Chemistry, University of Southern California, Los Angeles, California 90007
(Received M a y 7 , 1968)
Aqueous Cr(CN)2- undergoes photoaquation when irradiated at either of the first two ligand field bands, at 377 and 307 mp, with nearly wavelength- and temperature-independent quantum yields. The yield for cyanide production is 0.17 at 25', but analysis of the product spectra indicates that the dominant immediate product is Cr(CN)4(HzO)z-, as the result of an efficient secondary photolysis of the primary product, Cr(CN)s(H20)a-. The primary quantum yield is then 0.09 at 25'. If added cyanide is present, photostationary states may be reached, with photoaquation balanced by thermal anation. The results are discussed in relation to the photochemistry of Cr(II1) ammines. In particular, the relatively small quantum yield found for Cr(CN)Ba-is not expected in terms of the position of cyanide ligand in the spectrochemical series. The discrepancy is very likely related to the bonding character of cyanide as a ligand.
Introduction Aqueous C r ( C N ) P has been known for some time to
chemistry of this complex have been reported; one difficulty has been that C ~ ( C N ) S ( H ~ Oand ) ~ - the fur-
be photolyzed by visible and near-ultraviolet light to give free cyanide ion;' the process is one of photoaquation.2 Photoexchange with added cyanide ion occurs, presumably through the reverse anation reactiona3 However, no detailed studies of the photo-
(1) R. Schwarz and K. Tede, Chem. Ber., 6OB, 69 (1927). (2) L. Moggi, F. Bolletta, V. Balzani, and F. Scandola, J . Inorg. Nucl. Chem., 28, 2589 (1966). (3) A. G.MacDiarmid and N. F. Hall, J . Amer. Chem. SOC.,75,6204 (1953): 76, 4222 (1954).
Volume 78.Number 11 October 1968
3828 ther possible aquation products had not been characterized. Recently, however, some spectra for the series Cr(CiY)a(HzO)a3-ahave been published4 so that some guide to the spectrophotometric analysis of irradiated solutions is now available. An interesting additional point is that the acid-catalyzed thermal aquation rate of Cr(CS)6(H20)-2is sufficiently faster than that of Cr(CN)e3- that the first observed product is Cr(CN)&(HzO)z- (taken to be the cis i ~ o m e r ) . ~ There is a fairly extensive literature on the photochemistry of Cr(II1) complexes in general, which has So far, however, all reported inbeen vestigations have been with essentially u-bonding ligands (ammines, oxalates, etc.), and it was of interest to us to determine whether certain patterns of behavior for such complexes would carry over to a highly T bonded species such as Cr(CX)e3-. Thus the series 3 3 -, Cr (XCS)8 - 1 Cr (urea)6 3 -, Cr (HZO)6’ +, Cr (Cz04) Cr(xH3)e3+,and C ~ ( e n ) ~which ~ + , is in the order of increasing ligand field strength, photoaquates with quantum yields which are largely wavelength independent and which increase steadily from 0.1 to 0.5 (see ref 10). The expectation from this observation is then that Cr(CX)a3- should show a wavelength-independent photoaquation yield greater than 0.5; cyanide lies higher than ethylenediamine in the spectrochemical series. Also, the ligand field strength effect and other observations provided a rationalization for some empirically determined photolysis rules for Cr(I1I) complexes,ll on the basis of which some further predictions can be made. Thus the first photolysis product, Cr(CS)b(H20)2-, should further photoaquate, in highquantum yield, to Cr(CS)4(H20),-. To the extent that photoaquations are stereospecific, the diaquo species should be the trans isomer, which should then be stable toward further photoaquation. However, should cis isomer be present, either because it was in fact the photolysis product, or because of a thermal isomerization, then further stages of photoaquation should occur. Since the thermal anation reactions are moderately fast,s continued photolysis should lead eventually to a photostationary state. It will be seen that some hut not all of the above expectations are obeyed.
Experimental Section Materials. &Cr(C?rT)a was prepared according to a literature procedure12 and recrystallized twice from water; its spectrum agreed well with that previously reported (e.g., ref 13). The actinometer compound, KCr(n”3)z(NCS)4, was obtained from the ammonium salt (Reineclte’s salt) by recrystallization from aqueous potassium nitrate. Other chemicals used were of reagent grade. Apparatus and Pyocedures. The light source of the 370-mp Was a AH6 lamp, with that wavelength region isolated by The Journal of Physical Chemistrg
A. CHIANGAND A. W. ADAMSON means of a glass filter having a broad window (40-mp half-width) centered at 360 mp, The 305-mp irradiations were carried out by means of a Hanovia medium-pressure mercury lamp, using an interference filter whose window was at 305 mp (5-mp half-width). The two wavelength regions correspond to the first two ligand field bands of Cr(CN)e3- (377 and 307 mp). The 370-mp irradiations were done in a 2-cm path length Pyrex cell, equipped with a water jacket through which thermostated water was circulated. The available intensity of 305-mp light was much lower, and hence its heating effect, so it was sufficient to irradiate solutions in l-cm path length quartz spectrophotometer cells of the cylindrical type. In both cases, the light beam was sufficiently collimated and the solution absorption such that all incident light was contained by the irradiated solution. The absorbed light intensity was determined by reineckate actinometry,14 in the case of the 370-mp irradiations, and by ferrioxalate actinometry, in the case of those at 305 mp (e.g., ref 15). Actinometric measurements were usually made before and after a photolysis run, and in general, often enough to verify that no important change in lamp output had occurred during the experiment. Where only the sequence of spectral changes was wanted, the irradiation was carried out directly on a solution contained in a 5-cm quartz spectrophotometer cell. Such spectra were measured on a Cary 14 spectrophotometer. Where, however, released cyanide analyses were also performed, spectral changes were usually monitored at selected wavelengths by transferring a sample to a spectrophotometer ceil and making the measurement with a Beclcman nfodel DU instrument. Solutions buffered to a stated pH contained ca. 0.25 M mixtures of KH2P04 and K2HPO5. A modified Liebig titration gave good results for free cyanide determination. Solutions were made 0.3 M in ammonia and 0.02 M in potassium iodide and titrated with aqueous silver nitrate to the first permanent cloudiness. The silver salts of the various Cr(II1) cyano complexes are insoluble, but evidently (4) R. Krishnamurthy, W. B. Schaap, and J. R. Perumareddi, Inorg. Chem., 6 , 1338 (1967). (5) W. B. Schaap and R. Krishnamurthy, private communication. (6) F. Basolo and R. G. Pearson, “Mechanisms of Inorganic Reaotions,” 2nd ed, John Wiley and Sons, Inc., New Yorlr, N. Y., 1967. (7) E. L. Wehry, Quart. Res. (London), 21, 213 (1967). (8) J. Szyohlinslry, Wiad. Chem., 16, 607 (1962). (9) A. W. Adamson, Coord. Chem. Rev., 3, 169 (1968). (10) A. W. Adamson, W.L. Waltz, E. Zinato, D. W.Watts, P. D. Fleischauer, and R. D. Lindholm, Chem. Res., in press. (11) A. W. Adamson, J . Phys. Chem., 71, 798 (1967). (12) J. F. Bigelow, Inorg. Syn., 2, 203 (1946). (13) R. Krishnamurthy and W. B. Schaap, Inorg. Chem., 2, 605 (1963). (14) E. E. Wegner and A. W. Adamson, J . Amer. Chem. SOC.,88, 394 (1966). (15) J. Lee and H.H. Seliger, J . Chem. Phgs., 40, 519 (1964); C. G. Hatchard and C.A. Parker, Proc. R O ~~ o. c .A235,518 , (1956).
PHOTOCHEMISTRY OF AQUEOUS Cr(CN)63-
3829
I2O
t
Spectra of Cyano Chromium
(lU) C o m p l e x e s
Time, rnin.
Figure 1. Photolysis of aqueous Cr(CN),P- a t 370 mp. Concentration of complex: 0.0204 M , p H 6.85 (phosphate buffer), 25'. Absorbed light intensity: 7.2 X loe4 einstein/l. min.
not as much so as silver iodide; no induced cyanide release was observed. Also, added known amounts of free cyanide were accurately reported by the procedure.
Results Quantum Yield for Cyanide Release. The results for a typical run at 370 mp are shown in Figure 1; the cyanide release was linear with time, so that the system appeared to be simple in behavior. The general set of quantum yield data is summarized in Table I. A small temperature dependence is indicated at 370 mp, and, within experimental error, there was no wavelength dependence. Table I : Quantum Yields for Cyanide Release Cr(CN)sa-, M
Wavelength, mw
Temp, OC
pHa
2.04 1.97 2.00 1.94 2.11 5.23
370 370 370 370 370 305
25 25 25 15 15 25
6.75 6.85 6.75 6.85 6.85 6.85
a
Phosphate buffer present.
Quantum yield
0.15 0.16 0.16 0.11 0.10 0.Hb
' Two runs combined.
Identification of Photolysis Products. The above results would normally be interpreted as giving quantum yields for the reaction Cr(CN)G3-
+ HzO h', Cr(CN)5(H20)2-+ CN-
I I/\
4ot
(1)
but examination of the spectra of the photolyzed solutions showed that the principal product during the first states of photolysis was probably Cr(CN)r(H20)z-. The spectra for several aquocyano chromium (111) complexes are displayed in Figure 2 (from ref 4), while the spectral changes that actually occur during photol-
01
'
PO0
I
350
I
400
450
500
ml
Figure 2. Spectra of Cr(CN),(H20)bS-a complexes. From ref 4.
ysis at 370 mp and pH 6.85 are shown in Figure 3. First, the presence of three well-defined isosbestic points in Figure 3 indicates that essentially only one photolysis product was present. However, not only do these isosbestic points not correspond to those expected were Cr(CN)6(Hz0)2- the product, but, as a clear discrepancy, the optical density at 377 mp decreases rather than increases during irradiation. Concurrent cyanide titrations and optical density measurements at 377,410, and 430 mp allowed, in combination with the data of Figure 3, a calculation of the complete spectrum of this single photolysis product. This last is included in Figure 3, and it corresponds fairly closely to the reported spectrum for the tetracyano complex. On this basis, the primary quantum yields are given by the values of Table I divided by 2. The failure of the literature and our derived spectrum for Cr(CN)4(H20)z-to agree exactly could be due to experimental error, but additional possibilities exist. First, the isomeric nature of our product is not known and may not be the same as that from the thermal kinetic studies (assumed to be cis4). Second, the detailed spectra of the various aquocyano species are pH dependent. Thus, irradiation of an unbuffered solution of C r ( C N ) P leads to a somewhat different sequence of spectra, showing no isobestic points; a similar observation (of lack of isosbestic points) was made by Moggi, et aL2 Photostationary States. Prolonged irradiation at 370 ml.c of solutions buffered at pH 6.85 led to progressive Volume 72, Number 11
October 1968
A. CHIANG AND A. W. ADAMSON
3830
Phololyiii Of C r I C N l g 3 25'C, 377mp pH 6 8 5 I Initio1 2 20 min. 3 50 nun.
IO0
0.0 330
4
400
600
X,mp
0
I
I
50
100
Time of Radiolion, min.
Figure 4. Spectral sequence on long-term photolysis of aqueous Cr(CN)s3-at 25', p H 6.85 (phosphate buffer), 370-mp radiation. Right-hand graph shows the optical density changes at selected wavelengths. From ref 9.
A,
mp
Figure 3. Spectral sequence on photolysis of aqueous Cr(CN)sa- a t 370 mp. Curves 1-7 for 0-, lo-, 2 5 , 42-, 60-, 90-, and 120-min irradiations a t p H 6.90 (phosphate buffer) a t 25'; other conditions roughly comparable to Figure 1. Solid circles: points established by separate run with concurrent cyanide and spectrophotometric determinations; slope of optical density change us. cyanide release when applied t o curve 5 identified it as corresponding t o 337, conversion to tetracyano complex. Open circles: points for 337, conversion calculated according to the product spectrum given by the dashed line. Dotted line: spectrum for Cr(CN)4(H20)2-from ref 4.
spectral changes as shown in Figure 4. The spectrum terminal for this experiment corresponds to CI-(CN)~(H20)3as the principal product present. If the initial solution was made 0.07 M in cyanide ion, the continued irradiation at 370 mp led eventually to a photostationary state, as shown in Figure 5 . This state corresponds to a mixture of the tri- and the tetracyano complexes, and, in the dark, thermal anation returned the system to more nearly pure tetracyano complex. Re-irradiation gave essentially the same photostationary state as before. Another indication that thermal anation reactions occurred was that if added thiocyanate ion was also present, continued irradiation of aqueous CI-(CN)~~produced solutions whose spectra indicate the presence of coordinated thiocyanate.
Discussion The most important discrepancy between our reThe Journal of Physical Chemistry
Figure 5. Spectral sequence on long-term photolysis of Cr(CN)sa- in 0.07 M cyanide at 25", natural pH, 370-mp radiation. Right-hand graph shows the optical density change a t selected wavelengths; the arrows indicate the the direction of change in the dark, after terminating the irradiation.
sults and the predictions made in the Introduction is that the primary quantum yield is only about 0.09, or much lower than expected in terms of the correlation between quantum yield and position in the spectrochemical series. It seems likely that the special behavior of @r(CS)B3- as compared to nitrogen and oxygen complexes is related to the high degree of A bonding probably present in the former. Detailed explanation is less easy to contrive, however. The excited state which is the immediate precursor to chemical reaction has alternatively been supposed to be the first doublet state (2E,),16or the lowest-lying (16) See H. L. Schllifer, J . Phys. Chem., 69,2201 (1965).
PHOTOCHEMISTRY OF AQUEOUS Cr(CN)P quartet state (4T2e).11It was in terms of the latter intermediate that the correlation between ligand field strength and quantum yield could be explained-as a consequence of u antibonding character in the reactive excited state. That is, it appeared that the position of a ligand in the spectrochemical series gave not only the degree of ligand field stabilization in the ground state of Cr(II1) complexes but also the degree of bond weakening in the excited state. Pursuing this last emphasis, the first spin-allowed transition for a cyano complex probably involves the promotion of an electron from a n-bonding orbital to either a metal u antibonding or a cyanide n antibonding one.4,10,13,17 The extreme position of cyanide in the spectrochemical series may thus be more a reflection of r-bonding ability than of bond weakening in the excited state. Since quantum yields reflect the competition between chemical reaction and radiationless deactivation, it might be that it is the latter which is relatively fast for the cyano complex, as compared to the ammine family. Lifetimes for radiationless deactivation should be a function of the degree of excited-state distortion,Il and an alternative supposition is that such distortion is less in a highly n-bonded system. If, on the other hand, it is supposed that reaction occurs from the 2Eg state, produced by intersystem crossing, then it seems less easy to explain why a rbonded complex should be less photosensitive than a largely a-bonded one. The 2Eg state involves only spin pairing within the a-nonbonding (or, alternatively, the n-bonding) set of d orbitals, so distortion or bond weakening effects should not be present. The appearance of Cr(CN)4(H20)2-as the first observed product seems best explained on the basis that
3831 the immediate product, Cr(CN)b(Hz0)2-, shows the expected high photosensitivity for cyano complexes, and is subject to an efficient secondary photolysis. A quantum yield above 0.5 for its photoaquation, coupled with an extinction coefficient reasonably above that for Cr(CN)s3- in the region of 370 mp, would account for the situation. An alternative and not disproven possibility is that simultaneous aquation of two cyano groups occurs in the primary step. Such diaquations have not previously been reported, however, and seem improbable, nor is it likely that a fast thermal aquation of Cr(CN)6(H20)2- occurs; the required maximum half-life of about 1 min would be in orders of magnitude disparity with the very slow aquation rates for the other members of the series.6 Finally, the prediction that if Cr(CN)s(H20)2-undergoes secondary photolysis the product should be trans-Cr(CN)4(HzO)z- appears not t o be borne out. Although no definite isomer assignment is possible, the product spectrum is quite similar to that assigned t o the cis isomer; again, however, the basis for this last assignment is on indirect kinetic evidence. The predicted relative insensitivity of the photoproduced Cr(CN)4(HzO)2-to further photolysis is to a degree confirmed by the persistence of isosbestic points up to at least 50% conversion of the hexacyano complex to the tetracyano one (Figure 3). Acknowledgment. The investigations have been supported in part by Contract AT(l1-1)-113 between the University of Southern California and the U. S. Atomic Energy Commission. (17) Note J. J. Alexander and H. B. Gray, Coord. Chem. Rev., 2, 29 (1967).
Volume ‘7.9, Number 11 October 1968
