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Photochemical reactions of some Co(III) and Cr(III) complexes
Photochemical reactions of some
Co(III)
and Cr(lII) complexes J~RTHUR W. ADAMSON AND ALFRED H. SPORER D e p a r t m e n t of Chemistry, University of Southern California, Los Angeles - USA
ab*_mmLt-y: Studies have been made of the p h o t e e h e ~ s t r y of aqueous C,o(NHt) I + 8,
Co(NH~ X + s ( X
ffi SCN. CZ.Z). Co (CN), . y - - s ( X ffi CN. Ct. ~.. ~. Co (C.O,)s--s.
Cr(NHs)sSCN + 2, ~ N H ~ s (SeN)t-..- , and Cr (CsO4)s--S. Quantum yields were ebtailmd for ~ redox decompositions, aqtmtion, or raeemlzatimt I m m m u s that oeemed, mmally for two wave lengths, one in the ligand field Imnd region, and one in the electron t m m f e r band reg/on of the absot3~km spectra. In addition, qttsnttml yields were e b t a i n ~ tin' the photochemically induced oxidation of iodide ion in the Iresence of Co(NHs) I + a a n d Co(NHs),C! + 2. The m u l t s are interpreted to indicate that the ebmh~,A n a t m of the excited state is qualitatively independent of the wove lemght of the light used. In the other band, the net emmequence of l i l ~ t ~)sorptkm depmlds upon wh/eh of ~ peu/ble emwses |s taken subsquent to the fm~mation of the ptqnm~y exelted state.
The present investigation represents the initial effort of a program to develop a more quantitative understanding of the photochemistry of complex ions, and of its relationship to the thermal reactions of such ions and to the current interpretations of their absorption spectra. Linhard et al [1], observing that light of 366 mtt led to production of It and Ns from the corresponding acido-pentaminecobaltic complexes, concluded that the primary act involved an electron transfer from ligand to metal, in accord with the general interpretation of the near ultra-violet, high intensity. bands as being electron transfer in type. On the other hand, the low intensity bands, usually in the visible, observed with most first row transition metal complexes are interpreted in terms of transitions between 3d levels whose degeneracy has been partially removed by the ligand field [2,3] Thus, for an octahedral field, the d~, d=, and d,, orbitals, whose electron density is directed away from the ligand positions, are lowered in energy relative to the d., and d...~ orbitals, whose electron density is directed toward ligand positions. The absorption of light in the ~[igand field band is thus thought to involve promotion of an electron from a T,, to an E~, 3d orbital, and should lead to a repulsion between the ligand and the metal ion. Qualitatively, then, it might be expected that absorption in the region of an electron transfer band should lead to a redox process, and absor14
210
A R T H U R W. A D A M S O N , A L F R E D H. S P O R E R
ption itt the region of a ligand field band, to a displacement of the ligand and hence to a substitution process. One of the purposes of the presente investigation has been to test this conclusion. Experimental. - The various compounds were prepared by standard procedures, which cannot be described here, for lack of space. For the irradiations, a water cooled, high pressure General Electric A H - 6 mercury arc was used, and wave lengths were selected by means of suitable BairdAtomic interference or colored glass filters. Intensities were measured by means of a bolometer. Absorption spectra were obtained by means of a Cary recording spectrophotometer, and radioactivity measurements by a G.M. or scintillation counter, as appropriate. Referring to Tables I and I I, the rates of reaction were obtained for systems 1, 2, and 3 by a colorimetric test for S C N - ; in system 1, the amounts of aquo-complex and of Co(I I) formed were determined by the use of Co6°.labeled complex and fractional precipitation procedures. In system 7, Cl ~ labelled complex was used, and aquation determined by the amount of AgCP 6 precipitatable from the solution. The reaction was followed spectrophotometrically in systems 5,6,8 and 10; Figure 1 shows the sequence of absorption spectra for system 6, and figure 2, the first order rate plots for the four systems. Decomposition was followed in system 4 spectrophotometrically, and by CoCzO, precipitation, using labelled complex; racemization was followed by the loss in optical rotation, after correction for decomposition. The rate of 12 production was determined-in systems 11, 12, and 13 by back-titration of an excess of standard S z O s - added to the irradiated solution. Complete first order plots were obtained in those cases where the products did not absorb strongly; otherwise only initial rates were used, so that accurate quantum yield could be obtained. Where decomposition occurred, acetic or hydrochloric acid was added in sufficient amount to maintain a slight acidity. Results and discussion. - The results are summarized in Tables I
and II; for the convenience of the reader, the absorption maxima and corresponding extinction coefficients are given for each compound. The first systems studied, as marked by the sequence numbers in the tables, tended to bear out the initial hypothesis, namely that absorption in the region of an electron transfer type band should l~ad to a homolytic breaking of the metal-ligand bond and hence to redox decompositions, while absorption in the region of a ligand field band should lead to an ionic splitting and hence to substitution reactions. Thus, in the photochemical decomposition of Co(NHs)~SCN+z, if aquo-complex formation were attributed to ionic splitting and Co(If) formation to homolytic splitting, then
PHOTOCHEMICAL REACTIONS OF SOME C O ( I l l ) AND CR(III) COMPLEXES
211
the ratio of ionic to homolytic fission rose from 0.47 for light of 370 m~ to 4.1 for light of 550 m~. Also, for the series Co(CN)sX -s ( X -- CN, C/o Br) only the aquation product, i.e. supposedly ionic fission, resulted even though ultra-violet light was used, in accord with the ligand field nature of the absorption band involved. With further world, however, serious questions developed. In the case of Co(C20,)8-3, the intra-molecular racemization reaction is conside-
,,,./, 300
4OO
500
60O
Fig. 1 - P h o t o - a q u a t i o n of [Co(CN)61]'--S 550 rap..
rably faster than the thermal decomposition at room temperature, yet absorption in the crystal field band led to some decomposition but not to racemization. In the case of the series Co(CN)~X-4, the quantum yields for aquation decreased in the order I, CN, Br, Cl, which is not the order of the spectrochemical series, as would be expected if the primary act were an ionic fission, but is the order of increasing difficulty of oxidation of the ligand. Furthermore, the results of series 11, 12, and 13 made it clear that iedox reactions could result from light absorption even though the wave length corresponded to a ligand field band, if an easily oxidizable ion was present. Thus yellow light led to a very eiflcient redox decomposition of Co(NHs)~I +2, even though aquation is faster, thermally, in the absence of 1 - ion. [4]. Also striking was the observation that yellow light, which caused aquation of Co(NHs)sCI +2 with a o of 0.007, led to production
212
ARTHUR
W. ADAMSON,
ALFRED
H. SPORER
of Is in the presence of K I , with a e of 0.009, and t h a t Co(NHs)6+' which was quite stable to yellow light by itself, was again able to sensitize iodide oxidation. The active species in both cases was probably the (complex) ( I - ) ion pair, but the point is that the transition involved with light of 550 mtz was presumably still of the ligand field type since ion pairing affected neither the intensity nor the position of the band. It thus appears that in the case of Co(III) coiitplexes, the presence or
0,8
0,6
0.4 O,3
\ 20
4O
Fig. 2 - Photo-aquat|on X
CH C/ Br I
m/nufe8 1
60
[Co(CN)sX'r 8 at 370 m~. ),D
380 380 380 500, 330
lz/O I
mo~/mm 0.56 3.6 2.9 6.5
absence of photochemical redox reactions depends more on the oxidizability of the ligand than on the wave length used or on whether the band should be designated as electron transfer or ligand field in type; also, aquation and racemization reactions, even though favored thermally, are not necessarily favored photochemically even though absorption is occurring in a ligand field band. In the case of the Cr(III) complexes, no redox processes at all were observed, nor were there very large differences in the quantum yields for aquation between 370 and 550 m~.
PHOTOCHEMICAL REACTIONS OF SOME CO(III) AND CR(III) COMPLEXES
213
A general mechanism ]or photochemical reactions o] complex ions. - It has been possible to give a consistent explanation of all of the above results by supposing that the nature of the primary process is the same in all cases and that the type of final products is determined by subsequent or second stage processes. Specifically, it is supposed that the primary act is in every case an electronic excitation to a pre-dissociation state for homolytic bond fission. That is, electron transfer from ligand to metal occurs and, by virtue of the Franck-Condon principle, the resulting state is one of considerable repulsion between the oxidized ligand and the rester the complex. This repulsion energy is shown as A in the scheme below, and is supposed to be dissipated rapidly by exchange of vibrational energy with the surrounding medium, concurrently with a more or less rapid reoil of the ligand from the complex. If the energy requirement to give the homolytic fission product, M(I1)A6.X, is high, then the excess energy, A will be small, the oxidized ligand will separate or recoil only slightly, and the opportunity for the return process (2) will be large. The quantum yield is thus determined by the competition between the first stage processes (2) and (3) ; it should be lower the more difficultly oxidizable the ligand, in accord with the sequence for the Co(CN.)~: -4 series and the C o ( N H s ) ~ ( X - - I, C/, NHs) series, and in agreemnt with Plane and Hunts' [5] oservation of a very low quantum yield for H20 Is exchange with the hexaaquochromic ion and the lack of photodecomposition of Cr(Cz04)s"4 found here. If A is large, then process (3) is favored, in which sufficient separation between the complex and the ligand develops for solvent to be interposed, and various second stage processes can now take place. If electron transfer is energetically feasible, then process (4) can occur, which leads to aquation as the net reaction. Alternatively, continued separation of the homolytic fission products may take place, leading to a net redox reaction (5). Primary act : M(III)A6X + hv - - ~ M(II)A6.X -t- A
(1)
First stage processes :
ligand return: M ( I I ) A s . X ~ separation: M ( I I ) A s X ~
M ( I I I ) A 6 X + A' (favored if ~ small) (2)
M(I I)A6(H20).X (favored if A large)
(3)
Second stage processes :
electron return: M(II)A 5(Hz0).X --~- M(III)AsHzO + X -
(4)
net redox reaction: M(II) As(HzO).X ~
(5)
M(II) + 5A -}- X
214
ARTHUR W. ADAMSON~ ALFRED H. SPORER
TABLE I. - Photochemical Reaction o/ C o ( I / / )
NO.
12
7 11
System
Abs. Max. and extinct. Wave Nature of ]Reaction on which coeff. (m~/~) Length I based
~y
0.014M Co~NHs)o"[-3
475/55; 340/49
370
no reaction observed
same. in O.08M Kl
475/55; 340/98"
370
I , produced
0.7*/
5OO
Z, produced
0.001 s
370
aquation, trace Co+-[-
o.o51
550
aquation
o.ooe s
370
Z. produced
0.40
55O
Is produced
o.ooo
870
SCN-- produeed
o.sx
0.01M Co(NHs)sCI-I-2 same, in 0.008M KI,
530148; 360/45 530/48; 360/80*
0 . 0 0 ~ S|O, 1
Complexes.
0.0025M Co(NHs)sSCN+2
500/170; 330/1000
(
(aquaUon/Co+-l--- 0.47) 55O
s c ~ - - w.xt~eed
0.0084
(aqtmtion/Co@+ --- 4.1) 18
0.01M Co(NR,),Z+2
585/75"380/1880
370
Zl pro4aeed
3.0
550
Z, ~ o d u ~ d
0.47
3101190
370
aqtmtion
0.S,
310/128; 380194
370
aquatlon
0.25
(very little aquation) 8
O.OOeM Co(CN)4--a
5
O.O05M Co(CN)~CP-S
I0
O.O05M Co(CN)sBr--8
6
O.O06M Co(CN)sl.--8 O.ZM co(c|O,)s--a
.395/170 ~0/'~0;
5OO/87
4~o/2~o; 6o51x70
(levo-eomplex)
87O
aquation
o.el
370
aquation
o.os
550
aquatlon
0.%
370
Co++, CO, predueed
550
Co++, COs produced
550
no photoraeeml~Jation
1.o o.oo% (
* for comparison purpmes; no maximum present.
TAsta~ II. -Photochemical No.
System
0~0049M Cr(NHs)sSCN-1"2 0.0078M Co(NHs)t(SCN)4same, in Methanol
Reactions o/ C r ( I I l ) Complexes.
Abs. Max. and extinct. Wave Nature of Reaction on which coeff. (miz/¢) Length ~ based
30016000; 5oo/8o 390191; 5201103
(same)
0.06,
55~
aquatlon
37~
aqtmtion
o.oss
37C
aquatlon
1.4
55C
aqtmtion
0.7
370
0.5
same~ in nltromethane
390/92; 520/118
S~
SCN-- formed SCN-- formed SCN-- formed
0.01M cr(C~O4)z--s
420/97; 570/ 75
37O
no reaction
<0.1
550
no reaction
55O
1.4 0.4 T
PHOTOCHEMICAL REACTIONS OF SOME CO(III) AND CR(III) COMPLEXES
215
An i m p o r t a n t point is t h a t if hÜ is replaced by thermal energy, t h e n the sequence (1), (3), (5) is essentially t h a t for the thermal redox decomposition; one m a y thus expect t h a t if the thermal process occurs readily, the photochemical one should als occur'readily. On the other hand, the sequence for photochemical aquation, (1), (3), (4), undoubtedly does not pass t h r o u g h the sequence of states involved in thermal aquation (or racemization). That is, the intermediate (M(II)As(H20).X, is not an intermediate t h e r m a l aquation [6]. In consequence, the ease of photochemical aquation will depend on quite other factors, such, as ligand oxidizability than does the thermal reaction. It is thus quite reasonable for the redox decomposition of Co(C204)~-3 and of Co(NH3)5I +2to dominate photochemically, while the aquation and racemization dominate as thermal reactions. If ion pairing is present, oxidation of the Y - ion can occur by electron transfer either as a first stage or as a second stage process, as shown by equations (6) and (7).
[ M ( I I ) A s . X ] Y - - - ~ . . [ M ( I I ) A v X - ] Y - - ~ M ( I I ) + 5A + X - + Y [M(I I)A~(H~O).X] Y - ~
[M(II)As(H~O)X-] Y--~-M(II) + X - +
(6)
Y (7)
If (6) can compete favorably with (2), as in the case of Co(NHs)6 +s in K I solution, then photo-oxidation can occur even t h o u g h the complex by itself is quite stable towards light. The proposed scheme also permits an explanation of how light quanta of 50 to 70 Kcal (550 to 370 m~) can be absorbed by a complex ion and degraded to thermal energy w i t h o u t any chemical reaction necessarily occurring. This process must be occurring since there is no fluorescence, as concluded by Plane and H u n t [1] in the case of Cr(H~O)6+3, and by ourselves for Co(CN)sCI -s, so the absorbed light is consequently not reemitted. The sequence (1), (2) returns the complex to its initial state, but the energy ho is dissipated in two stages, as A and A', so t h a t the full a m o u n t is not converted into thermal energy at once. The degree of local ,~heating ,, is thus reduced with the consequence t h a t reactions of even low activation energy need not be photo-induced with any great efficiency. An i m p o r t a n t implication of the scheme is t h a t there is little difference between the excited states produced by absorption in an electron transfer vs. a ligand field band. More specifically, the implication is t h a t absorption in the region of a iigand field band involves an electron transler process, i.e. a radial change in electron density as would correspond to promotion of a ligand electron to an s or p state of the metal ion; this would deny the usual interpretation of these bands as being due to a transition of a metal electron from a T o to an Eg state of the 3d orbital. The m a t t e r m a y be one of degree however. It is generally t h o u g h t t h a t the excited
216
ARTHUR W. ADAMSON~ ALFRED H. SPORER
state in a ligand field transition involves some admixture of s or p states, in order to explain even the small transition probabilities, We would reverse the emphasis to say that ccli~and field ~ transitions are essentially electron transfer processes whose energies and probabilities have been modified by some admixture with E, states. ACKNOWLEDGEMENT
These investigations were supported in part by the U.S. Atomic Energy Commission. BEFERENC.~S [1J M. I.,~*~*~ and M. W m m ~ • Z. a n m ~ ~ ~ •, ~ , 4@ (1951). [2] J. B . m m m ~ C.. J. BJ,.~ousmf and IL Joaamcslm, • Aeta Clmn. Seand. ,, 8, 1275 (1954). [3] L. E. Oaom., • J. ~ Phys. ,, Jm, 1819 (1955). [4] R. G. Y . ~ / ~ , • J. Am. C,hem. Soe. ,, '/5, 1842 (1951). [5] 11, A. ~ and J. P. Hmer, ibid., '19, 3343 (1957). [6] A. W. A~.msoN and F. B~um~, * Aeta Chem. Seand. ,, D, 1261 (1965).
DISCUSSIONS SchJJffer ( C o ~ n ) - A s mentioned b y Orgel (Orgel, L. E. Reports to t h e X t h S o l v a y Council, Bruxetles May 1956), discussing halide complexes of Co(IIl), there is evidence t h a t t h e b o n d i n g orbitals of ~-type have higher e n e r ~ t h a n t h e bonding a-orbitak. Therefure, ff you consider an electron transfer reaction to be the main step in your photochemical process i t will p r o b a b l y be a ~-electron transfer from bonding, essentially ligand orbltals of h v or l ~ s y m m e t r y (or reductions of these to lower symmetries) to the empty, a n t i b o n d l n g el, essentially m e t a l orbitals. I t m a y be qualitatively understood w h y you observe a higher q u a n t u m yield in the vis/ble reg/on w i t h Co(NHs)s j z + t h a n with Co(NHs), SCNZ-I- although the intensity of t h e absorption is the same w i t h b o t h ions. In t h e Co(~VHs)s j z ÷ the first b a n d of high intensity is p r o b a b l y of the t y p e mentioned above, i.e. i t is m a i n l y due to a n electron transfer from the J to the Co. In t h e visibfle r e s | o n you h a v e two possibilities of provoking this transition. F i r s t its probability is n o t zero, t h o u g h v e r y small, as it still has a foot in t h e visible region in question. Secondly, t h e lig~nd field b a n d will p r o b a b l y have gained most of its intensity b y being mixed w i t h the same electron transfer bend. In t h e Co(NHs)s SCNZ-J- the first transition of high i n t e n s i t y has a position which is almost i n d e p e n d e n t of t h e n u m b e r of ~ C N ions present in the complex. This fact indicated t h a t we are dealing w i t h a transition m a i n l y localized within the SCN ion and thus not affecting t h e CoS C N h a n d much.
Co(III)
and Cr(lII) complexes J~RTHUR W. ADAMSON AND ALFRED H. SPORER D e p a r t m e n t of Chemistry, University of Southern California, Los Angeles - USA
ab*_mmLt-y: Studies have been made of the p h o t e e h e ~ s t r y of aqueous C,o(NHt) I + 8,
Co(NH~ X + s ( X
ffi SCN. CZ.Z). Co (CN), . y - - s ( X ffi CN. Ct. ~.. ~. Co (C.O,)s--s.
Cr(NHs)sSCN + 2, ~ N H ~ s (SeN)t-..- , and Cr (CsO4)s--S. Quantum yields were ebtailmd for ~ redox decompositions, aqtmtion, or raeemlzatimt I m m m u s that oeemed, mmally for two wave lengths, one in the ligand field Imnd region, and one in the electron t m m f e r band reg/on of the absot3~km spectra. In addition, qttsnttml yields were e b t a i n ~ tin' the photochemically induced oxidation of iodide ion in the Iresence of Co(NHs) I + a a n d Co(NHs),C! + 2. The m u l t s are interpreted to indicate that the ebmh~,A n a t m of the excited state is qualitatively independent of the wove lemght of the light used. In the other band, the net emmequence of l i l ~ t ~)sorptkm depmlds upon wh/eh of ~ peu/ble emwses |s taken subsquent to the fm~mation of the ptqnm~y exelted state.
The present investigation represents the initial effort of a program to develop a more quantitative understanding of the photochemistry of complex ions, and of its relationship to the thermal reactions of such ions and to the current interpretations of their absorption spectra. Linhard et al [1], observing that light of 366 mtt led to production of It and Ns from the corresponding acido-pentaminecobaltic complexes, concluded that the primary act involved an electron transfer from ligand to metal, in accord with the general interpretation of the near ultra-violet, high intensity. bands as being electron transfer in type. On the other hand, the low intensity bands, usually in the visible, observed with most first row transition metal complexes are interpreted in terms of transitions between 3d levels whose degeneracy has been partially removed by the ligand field [2,3] Thus, for an octahedral field, the d~, d=, and d,, orbitals, whose electron density is directed away from the ligand positions, are lowered in energy relative to the d., and d...~ orbitals, whose electron density is directed toward ligand positions. The absorption of light in the ~[igand field band is thus thought to involve promotion of an electron from a T,, to an E~, 3d orbital, and should lead to a repulsion between the ligand and the metal ion. Qualitatively, then, it might be expected that absorption in the region of an electron transfer band should lead to a redox process, and absor14
210
A R T H U R W. A D A M S O N , A L F R E D H. S P O R E R
ption itt the region of a ligand field band, to a displacement of the ligand and hence to a substitution process. One of the purposes of the presente investigation has been to test this conclusion. Experimental. - The various compounds were prepared by standard procedures, which cannot be described here, for lack of space. For the irradiations, a water cooled, high pressure General Electric A H - 6 mercury arc was used, and wave lengths were selected by means of suitable BairdAtomic interference or colored glass filters. Intensities were measured by means of a bolometer. Absorption spectra were obtained by means of a Cary recording spectrophotometer, and radioactivity measurements by a G.M. or scintillation counter, as appropriate. Referring to Tables I and I I, the rates of reaction were obtained for systems 1, 2, and 3 by a colorimetric test for S C N - ; in system 1, the amounts of aquo-complex and of Co(I I) formed were determined by the use of Co6°.labeled complex and fractional precipitation procedures. In system 7, Cl ~ labelled complex was used, and aquation determined by the amount of AgCP 6 precipitatable from the solution. The reaction was followed spectrophotometrically in systems 5,6,8 and 10; Figure 1 shows the sequence of absorption spectra for system 6, and figure 2, the first order rate plots for the four systems. Decomposition was followed in system 4 spectrophotometrically, and by CoCzO, precipitation, using labelled complex; racemization was followed by the loss in optical rotation, after correction for decomposition. The rate of 12 production was determined-in systems 11, 12, and 13 by back-titration of an excess of standard S z O s - added to the irradiated solution. Complete first order plots were obtained in those cases where the products did not absorb strongly; otherwise only initial rates were used, so that accurate quantum yield could be obtained. Where decomposition occurred, acetic or hydrochloric acid was added in sufficient amount to maintain a slight acidity. Results and discussion. - The results are summarized in Tables I
and II; for the convenience of the reader, the absorption maxima and corresponding extinction coefficients are given for each compound. The first systems studied, as marked by the sequence numbers in the tables, tended to bear out the initial hypothesis, namely that absorption in the region of an electron transfer type band should l~ad to a homolytic breaking of the metal-ligand bond and hence to redox decompositions, while absorption in the region of a ligand field band should lead to an ionic splitting and hence to substitution reactions. Thus, in the photochemical decomposition of Co(NHs)~SCN+z, if aquo-complex formation were attributed to ionic splitting and Co(If) formation to homolytic splitting, then
PHOTOCHEMICAL REACTIONS OF SOME C O ( I l l ) AND CR(III) COMPLEXES
211
the ratio of ionic to homolytic fission rose from 0.47 for light of 370 m~ to 4.1 for light of 550 m~. Also, for the series Co(CN)sX -s ( X -- CN, C/o Br) only the aquation product, i.e. supposedly ionic fission, resulted even though ultra-violet light was used, in accord with the ligand field nature of the absorption band involved. With further world, however, serious questions developed. In the case of Co(C20,)8-3, the intra-molecular racemization reaction is conside-
,,,./, 300
4OO
500
60O
Fig. 1 - P h o t o - a q u a t i o n of [Co(CN)61]'--S 550 rap..
rably faster than the thermal decomposition at room temperature, yet absorption in the crystal field band led to some decomposition but not to racemization. In the case of the series Co(CN)~X-4, the quantum yields for aquation decreased in the order I, CN, Br, Cl, which is not the order of the spectrochemical series, as would be expected if the primary act were an ionic fission, but is the order of increasing difficulty of oxidation of the ligand. Furthermore, the results of series 11, 12, and 13 made it clear that iedox reactions could result from light absorption even though the wave length corresponded to a ligand field band, if an easily oxidizable ion was present. Thus yellow light led to a very eiflcient redox decomposition of Co(NHs)~I +2, even though aquation is faster, thermally, in the absence of 1 - ion. [4]. Also striking was the observation that yellow light, which caused aquation of Co(NHs)sCI +2 with a o of 0.007, led to production
212
ARTHUR
W. ADAMSON,
ALFRED
H. SPORER
of Is in the presence of K I , with a e of 0.009, and t h a t Co(NHs)6+' which was quite stable to yellow light by itself, was again able to sensitize iodide oxidation. The active species in both cases was probably the (complex) ( I - ) ion pair, but the point is that the transition involved with light of 550 mtz was presumably still of the ligand field type since ion pairing affected neither the intensity nor the position of the band. It thus appears that in the case of Co(III) coiitplexes, the presence or
0,8
0,6
0.4 O,3
\ 20
4O
Fig. 2 - Photo-aquat|on X
CH C/ Br I
m/nufe8 1
60
[Co(CN)sX'r 8 at 370 m~. ),D
380 380 380 500, 330
lz/O I
mo~/mm 0.56 3.6 2.9 6.5
absence of photochemical redox reactions depends more on the oxidizability of the ligand than on the wave length used or on whether the band should be designated as electron transfer or ligand field in type; also, aquation and racemization reactions, even though favored thermally, are not necessarily favored photochemically even though absorption is occurring in a ligand field band. In the case of the Cr(III) complexes, no redox processes at all were observed, nor were there very large differences in the quantum yields for aquation between 370 and 550 m~.
PHOTOCHEMICAL REACTIONS OF SOME CO(III) AND CR(III) COMPLEXES
213
A general mechanism ]or photochemical reactions o] complex ions. - It has been possible to give a consistent explanation of all of the above results by supposing that the nature of the primary process is the same in all cases and that the type of final products is determined by subsequent or second stage processes. Specifically, it is supposed that the primary act is in every case an electronic excitation to a pre-dissociation state for homolytic bond fission. That is, electron transfer from ligand to metal occurs and, by virtue of the Franck-Condon principle, the resulting state is one of considerable repulsion between the oxidized ligand and the rester the complex. This repulsion energy is shown as A in the scheme below, and is supposed to be dissipated rapidly by exchange of vibrational energy with the surrounding medium, concurrently with a more or less rapid reoil of the ligand from the complex. If the energy requirement to give the homolytic fission product, M(I1)A6.X, is high, then the excess energy, A will be small, the oxidized ligand will separate or recoil only slightly, and the opportunity for the return process (2) will be large. The quantum yield is thus determined by the competition between the first stage processes (2) and (3) ; it should be lower the more difficultly oxidizable the ligand, in accord with the sequence for the Co(CN.)~: -4 series and the C o ( N H s ) ~ ( X - - I, C/, NHs) series, and in agreemnt with Plane and Hunts' [5] oservation of a very low quantum yield for H20 Is exchange with the hexaaquochromic ion and the lack of photodecomposition of Cr(Cz04)s"4 found here. If A is large, then process (3) is favored, in which sufficient separation between the complex and the ligand develops for solvent to be interposed, and various second stage processes can now take place. If electron transfer is energetically feasible, then process (4) can occur, which leads to aquation as the net reaction. Alternatively, continued separation of the homolytic fission products may take place, leading to a net redox reaction (5). Primary act : M(III)A6X + hv - - ~ M(II)A6.X -t- A
(1)
First stage processes :
ligand return: M ( I I ) A s . X ~ separation: M ( I I ) A s X ~
M ( I I I ) A 6 X + A' (favored if ~ small) (2)
M(I I)A6(H20).X (favored if A large)
(3)
Second stage processes :
electron return: M(II)A 5(Hz0).X --~- M(III)AsHzO + X -
(4)
net redox reaction: M(II) As(HzO).X ~
(5)
M(II) + 5A -}- X
214
ARTHUR W. ADAMSON~ ALFRED H. SPORER
TABLE I. - Photochemical Reaction o/ C o ( I / / )
NO.
12
7 11
System
Abs. Max. and extinct. Wave Nature of ]Reaction on which coeff. (m~/~) Length I based
~y
0.014M Co~NHs)o"[-3
475/55; 340/49
370
no reaction observed
same. in O.08M Kl
475/55; 340/98"
370
I , produced
0.7*/
5OO
Z, produced
0.001 s
370
aquation, trace Co+-[-
o.o51
550
aquation
o.ooe s
370
Z. produced
0.40
55O
Is produced
o.ooo
870
SCN-- produeed
o.sx
0.01M Co(NHs)sCI-I-2 same, in 0.008M KI,
530148; 360/45 530/48; 360/80*
0 . 0 0 ~ S|O, 1
Complexes.
0.0025M Co(NHs)sSCN+2
500/170; 330/1000
(
(aquaUon/Co+-l--- 0.47) 55O
s c ~ - - w.xt~eed
0.0084
(aqtmtion/Co@+ --- 4.1) 18
0.01M Co(NR,),Z+2
585/75"380/1880
370
Zl pro4aeed
3.0
550
Z, ~ o d u ~ d
0.47
3101190
370
aqtmtion
0.S,
310/128; 380194
370
aquatlon
0.25
(very little aquation) 8
O.OOeM Co(CN)4--a
5
O.O05M Co(CN)~CP-S
I0
O.O05M Co(CN)sBr--8
6
O.O06M Co(CN)sl.--8 O.ZM co(c|O,)s--a
.395/170 ~0/'~0;
5OO/87
4~o/2~o; 6o51x70
(levo-eomplex)
87O
aquation
o.el
370
aquation
o.os
550
aquatlon
0.%
370
Co++, CO, predueed
550
Co++, COs produced
550
no photoraeeml~Jation
1.o o.oo% (
* for comparison purpmes; no maximum present.
TAsta~ II. -Photochemical No.
System
0~0049M Cr(NHs)sSCN-1"2 0.0078M Co(NHs)t(SCN)4same, in Methanol
Reactions o/ C r ( I I l ) Complexes.
Abs. Max. and extinct. Wave Nature of Reaction on which coeff. (miz/¢) Length ~ based
30016000; 5oo/8o 390191; 5201103
(same)
0.06,
55~
aquatlon
37~
aqtmtion
o.oss
37C
aquatlon
1.4
55C
aqtmtion
0.7
370
0.5
same~ in nltromethane
390/92; 520/118
S~
SCN-- formed SCN-- formed SCN-- formed
0.01M cr(C~O4)z--s
420/97; 570/ 75
37O
no reaction
<0.1
550
no reaction
55O
1.4 0.4 T
PHOTOCHEMICAL REACTIONS OF SOME CO(III) AND CR(III) COMPLEXES
215
An i m p o r t a n t point is t h a t if hÜ is replaced by thermal energy, t h e n the sequence (1), (3), (5) is essentially t h a t for the thermal redox decomposition; one m a y thus expect t h a t if the thermal process occurs readily, the photochemical one should als occur'readily. On the other hand, the sequence for photochemical aquation, (1), (3), (4), undoubtedly does not pass t h r o u g h the sequence of states involved in thermal aquation (or racemization). That is, the intermediate (M(II)As(H20).X, is not an intermediate t h e r m a l aquation [6]. In consequence, the ease of photochemical aquation will depend on quite other factors, such, as ligand oxidizability than does the thermal reaction. It is thus quite reasonable for the redox decomposition of Co(C204)~-3 and of Co(NH3)5I +2to dominate photochemically, while the aquation and racemization dominate as thermal reactions. If ion pairing is present, oxidation of the Y - ion can occur by electron transfer either as a first stage or as a second stage process, as shown by equations (6) and (7).
[ M ( I I ) A s . X ] Y - - - ~ . . [ M ( I I ) A v X - ] Y - - ~ M ( I I ) + 5A + X - + Y [M(I I)A~(H~O).X] Y - ~
[M(II)As(H~O)X-] Y--~-M(II) + X - +
(6)
Y (7)
If (6) can compete favorably with (2), as in the case of Co(NHs)6 +s in K I solution, then photo-oxidation can occur even t h o u g h the complex by itself is quite stable towards light. The proposed scheme also permits an explanation of how light quanta of 50 to 70 Kcal (550 to 370 m~) can be absorbed by a complex ion and degraded to thermal energy w i t h o u t any chemical reaction necessarily occurring. This process must be occurring since there is no fluorescence, as concluded by Plane and H u n t [1] in the case of Cr(H~O)6+3, and by ourselves for Co(CN)sCI -s, so the absorbed light is consequently not reemitted. The sequence (1), (2) returns the complex to its initial state, but the energy ho is dissipated in two stages, as A and A', so t h a t the full a m o u n t is not converted into thermal energy at once. The degree of local ,~heating ,, is thus reduced with the consequence t h a t reactions of even low activation energy need not be photo-induced with any great efficiency. An i m p o r t a n t implication of the scheme is t h a t there is little difference between the excited states produced by absorption in an electron transfer vs. a ligand field band. More specifically, the implication is t h a t absorption in the region of a iigand field band involves an electron transler process, i.e. a radial change in electron density as would correspond to promotion of a ligand electron to an s or p state of the metal ion; this would deny the usual interpretation of these bands as being due to a transition of a metal electron from a T o to an Eg state of the 3d orbital. The m a t t e r m a y be one of degree however. It is generally t h o u g h t t h a t the excited
216
ARTHUR W. ADAMSON~ ALFRED H. SPORER
state in a ligand field transition involves some admixture of s or p states, in order to explain even the small transition probabilities, We would reverse the emphasis to say that ccli~and field ~ transitions are essentially electron transfer processes whose energies and probabilities have been modified by some admixture with E, states. ACKNOWLEDGEMENT
These investigations were supported in part by the U.S. Atomic Energy Commission. BEFERENC.~S [1J M. I.,~*~*~ and M. W m m ~ • Z. a n m ~ ~ ~ •, ~ , 4@ (1951). [2] J. B . m m m ~ C.. J. BJ,.~ousmf and IL Joaamcslm, • Aeta Clmn. Seand. ,, 8, 1275 (1954). [3] L. E. Oaom., • J. ~ Phys. ,, Jm, 1819 (1955). [4] R. G. Y . ~ / ~ , • J. Am. C,hem. Soe. ,, '/5, 1842 (1951). [5] 11, A. ~ and J. P. Hmer, ibid., '19, 3343 (1957). [6] A. W. A~.msoN and F. B~um~, * Aeta Chem. Seand. ,, D, 1261 (1965).
DISCUSSIONS SchJJffer ( C o ~ n ) - A s mentioned b y Orgel (Orgel, L. E. Reports to t h e X t h S o l v a y Council, Bruxetles May 1956), discussing halide complexes of Co(IIl), there is evidence t h a t t h e b o n d i n g orbitals of ~-type have higher e n e r ~ t h a n t h e bonding a-orbitak. Therefure, ff you consider an electron transfer reaction to be the main step in your photochemical process i t will p r o b a b l y be a ~-electron transfer from bonding, essentially ligand orbltals of h v or l ~ s y m m e t r y (or reductions of these to lower symmetries) to the empty, a n t i b o n d l n g el, essentially m e t a l orbitals. I t m a y be qualitatively understood w h y you observe a higher q u a n t u m yield in the vis/ble reg/on w i t h Co(NHs)s j z + t h a n with Co(NHs), SCNZ-I- although the intensity of t h e absorption is the same w i t h b o t h ions. In t h e Co(~VHs)s j z ÷ the first b a n d of high intensity is p r o b a b l y of the t y p e mentioned above, i.e. i t is m a i n l y due to a n electron transfer from the J to the Co. In t h e visibfle r e s | o n you h a v e two possibilities of provoking this transition. F i r s t its probability is n o t zero, t h o u g h v e r y small, as it still has a foot in t h e visible region in question. Secondly, t h e lig~nd field b a n d will p r o b a b l y have gained most of its intensity b y being mixed w i t h the same electron transfer bend. In t h e Co(NHs)s SCNZ-J- the first transition of high i n t e n s i t y has a position which is almost i n d e p e n d e n t of t h e n u m b e r of ~ C N ions present in the complex. This fact indicated t h a t we are dealing w i t h a transition m a i n l y localized within the SCN ion and thus not affecting t h e CoS C N h a n d much.