JAYARAMA R. PERUMAREDDI AND ARTHUR W. ADAMSON
414
Photochemistry of Complex Ions. V.
The Photochemistry of
Some Square-Planar Platinum (11) Complexes by Jayarama R. Perumareddi1"Iband Arthur W. Adamson Department of Chemistry, University of Southern California, Los Angeles, California 90007 (Received Julu 10, 1967)
The photochemical studies of transition metal compounds in our laboratory have been extended to include complexes of bivalent platinum. The systems studied are the following: cis- and trans-[Pt(NHa)~Cl~] and cis- and tran~-[Pt(NH~)~(H~0)~]~++. Both the geometric isomers of Pt (NHJ2Clz photoaquate releasing the chloride ligands, the quantum yield for the cis compound being much larger than for the trans compound. Although the aqueous solution of cis- [Pt(NH&(Hz0)2]2+decomposes on photolysis, preliminary studies show that the acidified solution of the same isomerize to the corresponding trans complex cation. Quantum yields for the photolysis of platinum(I1) complexes were generally in the range of 0.1-0.5, showed little wavelength dependence, and the temperature coefficients corresponded to 2 to 3 kcal activation energy. During the course of our studies we have also determined the kinetic and thermodynamic parameters for the thermal equilibrium cis-[Pt(NH&C12] HzO cis-[Pt(NH&(HzO)Cl] + C1- and the first and second acid dissociation constants of the geometric isomers of the complex cation [Pt("i)z (HzO)zl2+.
+
Introduction The recent renaissance of the quantitative photochemistry of transition metal compounds centered mainly on the hexacoordinate complexes of trivalent chromium and c ~ b a l t . ~ JStudies on systems with different geometry and electronic configurations have been very f e ~ . ~We - ~report here our findings on the photochemical studies of some square-planar platinum(11) complexes belonging to ds configuration. The complexes studied include the cis and trans isomers of dichlorodiammineplatinum(I1) known as Peyrone's chloride and Reiset's Zweites chloride, respectively, and also the corresponding isomers of the diaquodiammineplatinum(I1) complex cation. Experimental Section Preparation of Compounds. The cis and trans isomers of dichlorodiammineplatinum(I1) compound have been prepared as described by Ramberg.'" According to these procedures, although the yield of the cis isomer is fairly good, that of the trans which is obtained by heating the dry tetrammineplatinum(I1) chloride salt is very low. An alternative procedure originally suggested by Peyrone which has been described in a recent publicationll reportedly results in much higher yield. This latter procedure is based on the action of concentrated hydrochloric acid on tetraammineplatinum(I1) chloride. The cis- and trans- diaquodiammineplatinum (11) complex cations are obtained in aqueous solution by simply dissolving the corresponding diniThe Journal of Physical Chemistry
+
tratodiammineplatinum(I1) compounds in water. The dinitrate compounds have been prepared as suggested by King.12 All complexes were recrystallized and checked for purity by means of their spectra and other physical properties as conductances and so on. Thus the ultraviolet and visible spectra of Peyrone's salt and Reiset's chloride agreed very well with those published in the (1) (a) Taken in part from a dissertation submitted by J. R. Perumaredd1 to the Graduate School of the Southern California University in partial fulfillment of the requirements for the Ph.D. degree in Chemistry, 1962. A portion of this material was presented a t the Symposium on Inorganic Photochemistry, Division of Inorganic Chemistry, a t the 153rd National Meeting of the American Chemical Society, Miami Beach, Fla., April 1967. (b) Mellon Institute, Pittsburgh, Pa. 15213. (2) E. E. Wegner and A. W. Adamson, J . Am. Chem. SOC.,88, 394 (1966),and references therein. (3) A. W. Adamson, J . Phys. Chem., 71, 798 (1967),and references therein. (4) A. W. Adamson and A. H. Sporer, J . Am. Chem. SOC.,80, 3865 (1958). (5) A. W. Adamson and J. R. Perumareddi, Inorg. Chem., 4, 247 (1965). (6) J. R.Perumareddi, Z . Naturforsch.,21b, 22 (1966),and referencee therein. (7) V. Balzani, V. Carassiti, L. Moggi, and F. Scandola, Inorg. Chem., 4, 1243 (1965). (8) P. Haake and T. A. Hylton, 1.Am. Chem. SOC.,84,3774 (1962). (9) I. Lifshitz and W. Froentjes, Z . Anorg. Allgem. Chem., 233, 1 (1937). (10) L. Ramberg, Z . Anorg. Chem., 8 3 , 33 (1913). (11) G.B. H a d m a n and D. 0. Cowan, Inorg. Syn., 7 , 239 (1963). (12) H.J. S. King, J . Chem. SOC.,1338 (1938).
SYMPOSIUM ON INORGANIC PHOTOCHEMISTRY l i t e r a t ~ r e . ' ~ !The ' ~ equivalent conductance measurements on the cis- and trans-diaquodiammineplatinum(11) complex cations concur with King's measurements12 on the same. Apparatus. Equivalent conductances of solutions were measured according to standard procedures. I n the aquation measurements of Pt(NHa)ZClz, the released chloride was titrated potentiometrically against standard silver nitrate solutions using a calomel electrode and a silver-silver chloride electrode assembly. The photochemical runs were carried out using the same general equipment as described previously4 and the same method for determining the intensity of the absorbed light. The cell of about 2-cm path and 25-ml volume was thermostated to =t0.2". Absolute quantum yields were obtained by calibration with standard actinometric solutions.16J6 Spectra were scanned generally by a Gary-14 recording spec1;rophotometer in the wavelength range 750-200 mp. Concentrations of complexes were usto 2.5 X M , and cells ually in the range 5 X of 5-, 1-, and 0.1-cm light path were used. The ultraviolet and visible spectra of all the systems studied in this report are depicted in Figure 1, and the spectral data are summarized in Table I. The bands have been attzibuted to d-d transitions and the assignments based on Dqh point group where the one electron energy level ordering has been assumed to be eg(dzz, d,J < bzg(d,,) < alg(dzz) < blg(dz2-,P) in increasing order of energy." The fitting of the spin-allowed bands and a value of 650 cm-l for B with CIB = 7, where B and C are the well-known Racah interelectronic repulsion parameters, predict the position of the 3Azg spin-forbidden transition within 800 cm-l from the observed low-intensity absorption maximum. l8 The 3Egtransition in some cases is calculated to be close to the energy of the 3Azgtransition and in others to be positioned at, slightly higher energies covered by the intense spin-allowed absorption. The aB1gtransition is then predicted to be placed in the energy range 12,000-14,000 cm-l, which has not been uncovered yet in any of theae systems. Various other ordering of oneelectron energy l e ~ e l s , ~ 3 ~ ~in9 -particular ~1 the ones in which the alg(dzz)molecular orbital has been moved below in energy to bz,(d,,) and to eg(dzz,d,,), were used by others previously to explain the electronic spectra of tetrachloroplatinate(I1) complex anion and similar complexes. In view of the fact that there is no definite proof for any of these one-electron energy level orderings at present, alternative assignments for the observed transitions are possible. The proper assignments given in Table I are not meant to be definite.
Results cis- [Pt(NH3)zCZz]. Thermal Aquation. It has been qualitatively o b ~ e r v e that d ~ ~the ~ ~aqueous ~ solution of
415
2oo
H
2-
I
250
300
350
x,
w
400
450
Figure 1. Absorption spectra of (A), ci~-[Pt(NH~)~C1~1; (B) tT~ns-[Pt( NHs)2C12].
neutral cis-dichlorodiammineplatinum(I1) releases chloride ion and aquates as evidenced by the increase in electrical conductivity. Some further quantitative studies on this system have been made by Banerjea, Basolo, and P e a r s ~ n , ~who ' observed that the thermal aquation has an apparent first-order rate constant 3.83 X 10-6 sec-l at 25". They also noted that a 5 X M solution of the complex comes to an equilibrium when 45% of the total chloride is released. Based on the fact that [Pt(NK3)&1]+ ion does not release any chloride even after several days of standing, (13) J. Chatt, G. A. Gamlen, and L. E. Orgel, J . Chem. SOC.,486 (1958). (14) D.S.Martin, Jr., and R. J. Adams, "Advances in the Chemistry of Coordination Compounds," S. Kirschner, Ed., The Macmillan Go., New York, N. Y., 1961,p 579. (15) C. A. Parker, Proc. Roy. Soc. (London), A220, 104 (1953). (16) F. P . Brackett and G. S. Forbes, J . Am. Chem. SOC.,5 5 , 4459 (1933). (17) J. R. Perumareddi, A. D. Liehr, and A. W. Adamson, ibid., 85, 249 (1963). (18) J. R.Perumareddi, A. D. Liehr, and A. W. Adamson, Abstracts, Symposium on Molecular Structure and Spectroscopy, Ohio State University, Columbus, Ohio, June 1964, and to be published. (19) D. S. Martin, Jr., M. A. Tucker, and A. J. Kassman, Inorg. Chem., 5, 1298 (1966). (20) R. F. Fenske, D. 8. Martin, Jr., and K. Ruedenberg, ibid., 1, 441 (1962). (21) H.Basch and H. B. Gray, ibid., 6, 365 (1967). (22) A. Werner and A. Miolati, 2. Physik. Chem., 12, 49 (1893). (23) H.D.K.Drew, F. W. Pinkard, W. Wardlaw, and E. G. Cox, J . Chem. Sac., 988 (1932). (24) D. Banerjea, F. Basolo, and R. G. Pearson, J . Am. Chem. SOC., 79,4055 (1957). Volume 76, Number 6 February 1968
JAYARAMA R. PERUMAREDDI AND ARTHUR W. ADAMSON
416 Table I: Absorption Spectral Data Xmu,
Ymam
mr
om -1
emax
37,200 33,100 27,300 24,000
92 128 28 12
266 312 370 -450
37,600 32,100 27,000 22,200
78 68 33 8
-260 310 380 (1) -430
38,460 32,260 26 ,320 23 ,260
280 100 20 9
-260 -300 350 -400
38,460 33,330 28,570 25,000
120 45 21 5
Complex
&-[Pt(NH,)zClz]
269 302 366.5 416.5
tran~-[Pt(NHa)*(HzO)1] *+
it has been assumed that in the present case the reaction is the equilibrium
+
kr
ci~-[Pt(NHa)zC12] H2O kn
+ C1-
ci~-[Pt(NH&(H20)Cl]+
(1)
Thus 90% of the original complex transforms to the monoaquo product at equilibrium giving rise to an equilibrium constant K = kl/lc2 = 4.05 X at 25". Later, Reishus and Martin26 made a detailed kinetic study of the aquation and exchange of chloride ligands of the same system and interpreted the aquation due to eq 1 and also
+
ci~-[Pt(NHa)z(H20)zC1]+ H2O
-
10
0
+
ci~-[Pt(NH3)2(HzO)2]~+ C1-
(2)
and obtained the thermal equilibrium constants and the rate constants for both reactions. We have followed the thermal aquation at 25" by three different procedures : (i) by potentiometric titrations of the released chloride against standard silver nitrate solutions; (ii) by measuring the conductance at various intervals of time; and (iii) by studying the changes in the absorption spectrum (the optical density increases at 260 mp as aquation proceeds). Within the experimental errors (see Figure 2), all three techniques agree and give an apparent rate constant for the thermal aquation of 4.0 X 10-6 sec-l at 25". Starting from 2.5 X M complex, the equilibrium concentration of chloride ion has been found to be 1.71 X lopa M , which gives a.n equilibrium constant of 3.86 x 10-3. Table I1 summarizes the equilibrium constant data at various temperatures. We have used conductance measurements for the evaluation of equilibrium conTha Jousncrl of Phymkal Chemistry
@
0
Potentiometric Titrations Conductance Measurements Spectroscopic
100
200
300
400
500
t (min.) Figure 2. Thermal aquation of cis-[Pt(NH&Clz] at room temperature.
stants in this table. The degree of aquation, CY, values at 290 and 303°K were calculated relative to the value at 298°K from the equilibrium conductance readings using Walden's rule126according to which the product of conductivity and viscosity of a solution is constant. We have verified this rule and used the viscosities of water* in these calculations since we are dealing with dilute solutions. Table I1 shows that the heat of reaction is zero. The individual rate constants of eq 1 can be evaluated by using the rate eq 3,28 where a is the initial (26) J. W. Reishus and D. 9.Martin, Jr., J . Am. Chem. SOC.,83, 2467 (1961).
(26) P. Walden, 2.Phyaik. Chem., 55, 249 (1906). (27) "Handbook of Chemistry and Physics," 37th ed,The Chemical. Rubber Publishing Co., Cleveland, Ohio, 1966. (28) L. F. Grantham, T. 8.Elleman, and D. 8. Martin, Jr., J. Am, Chem. Soc., 77,2966 (1966).
SYMPOSIUM ON IWORQANIC PHOTOCHEMISTRY
417
In
concentration and x and xm are the concentration a t time t and at equilibrium, respectively, of the products monoaquo complex or chloride ion. The rate constants k2 for the reverse reaction a t different temperatures were calculated from the slopes of Figure 3. The Table I1: Temperature Dependence of the Equilibrium of the Thermal Aquation of cis-[Pt(NH,)&l~] T,
L x 106,
OK
mhos
a
K x
108, moles/l.
290 298 303
15.94 19.86 21.38
0.682 0.690 0.680
3.66 3.86 3.76
forward rate conatants ICl were then evaluated from the corresponding equilibrium constants. The activation plots shown in Figure 4 yield the activation energies El* and Ez* for the forward and the reverse reactions, Le., the aquation and anation reactions of eq 1. All of the thermal data, obtained so far are collected in Table 111. These results indicate an activation entropy of -6 cal for the forward reaction and 10 cal for the reverse reaction at 25".
Table I11: Kinetic and Thermodynamic Parameters of the Thermal Aquation Equilibrium of cis-[Pt( NHs)zC1z]
T, OK
kl X 108, seo-1
273 298 303
0.095 2.80 5.08
kz x 102, l./mole K X 108, see-1 moles/l.
0.023 0.73 1.35
4.1 3.81 3.8
4t
I
2' 0
I
100
I
I
I
300 400 500 t (min.)(for 25OC ond 3OoC) 200
Figure 3. Evaluation of the rate parameters for the t h e h a l aquation of cis-[Pt(NHa)~Cl~] and its reverse reaction.
100
80 60 -
-
AH,
Et*,
koal
koa1
Ex*, koal
0
22.0
22.6
40 -
20 10
Photochemical Studies. Irradiation of aqueous solutions of Peyrone's chloride showed that more chloride ion is released than is the case in the dark for the same M aqueous solution amount of time. Using 2.5 X of the complex, the photolytic reaction was followed a t 363-mp irradiation simultaneously by measuring the equivalent conductance and immediately titrating the released chloride ion by potentiometry. The results plotted in Figure 5 show that both potentiometric titrations and conductance measurements are showing the same extent of reaction a t all intervals of time. Since the potentiometric titrations measure directly
8 -6-
-
4-
21'
3.2
I
I
I
I
3.3
3.4
3.5
3.6
(1/T)
lo3
3.7
Figure 4. Evaluation of the activation energies for the thermal aquation of cis-[Pt( NH&C12] and its reverse reaction.
Volume 78, Number 8 February 1988
JAYARAMA R. PERUMAREDDI AND ARTHUR W. ADAMSON
418 100
60
..
a
Y
0
Meosurernents
-
20 -
0
I
I
I
I
15
30
45
60
t
(min)
"1 I
I
75
Figure 5. Photoaquation of cis-[Pt( NH&C12] a t room temperature and 363-mfi irradiation.
the amount of free chloride ion, this ensures that there is no complication other than simple aquation under the influence of light and that the conductance measurements also follow the photoaquation only. Temperature Dependence. The data on the photoaquation at various temperatures with 363-mp irradiation as followed by conductance measurements are summarized in Table IV. The data correspond to an apparent activation energy of 2.4 kcal/mole. ~~
Table IV : Temperature Dependence of the Quantum Yield for the Photoaquation of cis-[Pt( NHs)2Clz]'
zx
108,
T, OK
einsteins/ seo
R X 108, moles/sec
(P
thermal)
274 282 290 298
2.12 2.73 1.76 2.16
0.267 0,708 0,550 0.891
0.13 0.26 0.32 0.41
0.12 0.20 0.23 0.25
a Concentration, 2.5 X 10-8 363 mM.
(P
(cor for
M. Wavelength of irradiation,
The thermal correction for photoaquation in Table IV has been carried out by the following two methods. In the first method, the forward rate in the photochemical process, Ri,is written as
Re
= kl(a
I
- 2) + o--(a - z)
=
kl'(a
where
The Journal of Physical Chemistry
- z)
Table V: Wavelength Dependence of the Quantum Yield for the Photoaquation of cis-[Pt(NH&C&] (Concn, 2.5 X M)
(4)
since both complex and the product absorb to the same extent at the wavelength of irradiation, 363 mp. Thus
Ri
Assuming that kz of thermal aquation is the same in the photochemical process also and that kl changes to ICl' where kl' > kl thus giving rise to a new equilibrium constant K', several rate plots are constructed using K' = nK, n > 1, where K is the thermal equilibrium constant. After correcting for intervals of time from the thermal plot, the amounts of aquation at various times of irradiation are matched with the calculated plots. From the K' so obtained, kl' can be calculated and hence Q from eq 6. In the second method, first from the measured conductances for different irradiations, the amount of aquation corresponding to thermal equilibrium is calculated. Using the thermal rate plot, the thermal aquations that will occur during the times of irradiation are read. Subtracting these conductance values from the measured conductances gives the increase in conductivity due to photoaquation alone. The rate of photoaquation can then be evaluated by the usual procedure. Wavelength Dependence. The quantum yields for the photoaquation of Peyrone's chloride have been determined a t three different wavelengths in the range 450-350 mp at room temperature. We were restricted in our photochemical equipment to go below 350 mp in the ultraviolet range, and at wavelengths longer than 410 mp the complex does not absorb enough to be photoactive. The data of these runs are summarized in Table V.29 Although the data show a variation of 0.25 to 0.46 for the quantum yield in the wavelength range studied, the values are subject to perhaps i15% error because of the difficulty of accurate measurement of low absorbed light intensities, particularly that a t 350 mp. However, the general trend of decreasing quantum yield with increasing wavelength indicated by the data is probably real. trans-[Pt(NH~)&'12]. Thermal and Photochemical Studies. Thermal aquation studies on the trans-
(5)
a
A(T°C),
I x 108, einsteins/
mr
350 (24') 363 (26') 410 (25.5')
(P
seo
R X lOS, molee/seo
(P
(cor for thermal)"
2.30 4.01 7.65
1.54 2.05 2.27
0.67 0.51 0.30
0.46 0.39 0.25
See footnote 29.
(29) Correction for thermal aquation in this table has been made not by the procedures described for Table I V but by simply subtracting the apparent thermal rate constant from the total rate to arrive at the photochemical rate, so that the thermal corrected quantum yield values given in the last column are not exact. In addition, the runs were not temperature regulated. Hence, the seeming discrepancy of the quantum yield value, for instance, at 363 mp compared with the value given in Table IV.
419
SYMPOSIUM ON INORGANIC PHOTOCHEMISTRY dichlorodianimineplatinum(I1) have been made by Martin and Adams,l4 who obtained an equilibrium at 25", about one-tenth as large as constant 3 X that of the corresponding cis compound. This means that the equilibrium concentration of chloride ion due to thermal aquation will be very small. The aqueous solution of the complex when photolyzed by light releases chloride ion further, thus increasing the conductance of the solution. The photoaquation of this complex was followed a t 25" and 363-mp irradiation by means of conductance measurements. A value of 3.67 X 10-6 set-1 was obtained for the rate constant with 0.024 1. of 5 X 10-4 M solution. This gives 4.40 X 10-10 mole/sec for the rate of reaction. From the absorbed light intensity of 4.13 X lov8 einstein/sec, the quantum yield can be calculated to be 0.01. In view of such low quantum yield, no further studies on its variation with wavelength and temperature were pursued. cis- and trans- [Pt(NHa)2(H20)2]2+. These complex cations are obtained by disolving in water the corresponding neutral dinitratodiammineplatinum(I1) compounds when the nitrato groups are immediately released and substituted by aquo groups. In order to gather more data on the behavior of these complexes, we have carried out acid-base titrations and evaluated their acid dissociation constants. The pK1 and pK2 values for the cis complex were found to be 5.63 and 9.25 and the corresponding values for the trans complex were 4.23 and 7.30, respe~tively.~~ Photochemical Behavior. The solid cis- and trans-dinitratodiammineplatinum(I1) compounds and their aqueous solutions in which the corresponding diaquodiammineplatinum(I1) complex cations are present have interesting photolytic behavior. Although the solid trans-dinitrato compound is photoactive as evidenced by the darkening of the yellow color of the complex under sunlight, the corresponding trans-diaquo complex cation seems to be very stable to light and nothing apparently happens on irradiation. a1 The situation is the reverse with the cis systems. The solid cis-dinitratodiammineplatinum(I1) is stable to light, but the aqueous solution of the cis-diaquo cation which is pale yellow changes its color to dark yellow on photolysis. There is a change in pH from 3.39 to 3.83 on M solution at 25" for about 2 hr irradiating a 5 X with a light of 363 mp and an absorbed light intensity of 2.16 X lo-' einstein/sec. The complex is probably decomposing with the release of ammonia on irradiation. The photolysis of the cis-diaquo complex was carried out in an acid solution of 0.1-0.12 M HC104, and we observed a gradual change in the absorption spectrum. The absorption spectrum of the irradiated solution shown along with the spectra of the cis- and transdiaquo cations in Figure 6 indicates that the cis complex is being isomerized to the corresponding trans complex
\ 2AO
3bO
3AO X,mp
460
4$0
5
1
Figure 6. Absorption spectra of the cis- and
trans- [Pt(NH&( HzO)~] 2 + and of the photolyzed cis- [Pt( NH&( H20)2]z c complex cation.
on irradiation. Unfortunately, the spectra of these complex cations consisting of diffuse bands do not differ enough to permit a quantitative and detailed study of photoisomerization by spectroscopic means. In B sample experiment which was carried out at 25" and 363-mp irradiation when the absorbed light intensity was 6.23 X einstein/sec, the observed rate was mole/sec from the changes estimated to be 0.81 X in the absorption spectrum at 330 mp. Hence, the quantum yield for the photoisomerization was found to be 0.13, a value within the range of quantum yields observed for the photoaquation reactions of other square-planar complexes.
Discussion The types of photolytic reactions among the family of square-planar platinum(I1) complexes seem to be photoaquation and photoisomerization. Various intermediates have been proposed in the literature' for these reactions. An intermolecular mechanism where the formation of a trigonal bipyramidal intermediate with either the excess of a ligand or solvent is one. Another mechanism involves a planar trigonal intermediate (30) These values agree very well with those obtained by Jensen [K. A. Jensen, 2. Anorg. Allgem. Chem., 242, 87 (1939)]with the exception of ~ Kofz cis for which his value is 7.32. (31) The trans-diaquo does decompose on prolonged irradiation. A 2.5 X 10-8 M solution (25 ml) a t 2 5 O when irradiated by full light beam for about 6 hr becomes cloudy, its pH changing from 3.2 to 3.36. Volume 79,Number I February 1968
420
JAYARAMA R. PERUMAREDDI AND ARTHUR W. ADAMSON
caused by dissociation of one of the ligands. Finally, an intramolecular mechanism involving a tetrahedral intermediate with a triplet (spin) ground state has been proposed. Our results obtained in this work do not permit a definite choice among these different proposals at the present. More data on these and similar systems are needed before speculating a probable mechanism for the photoreactions of square-planar platinum(I1) compounds. Recently, Adamson3 has systematized the photolytic behavior of hexacoordinate octahedral complexes of chromium(II1) by noting some semiempirical rules for predicting which photolytic process will dominate when more than one is possible. It is interesting to examine his first two rules in connection with our observations on the photochemistry of platinum(I1) complexes. His first rule is that in an octahedral complex where one (or more) of the six ligands is different from the others, the axis having the weakest average crystal field will be the one labilized. The second rule concerns the labilized axis containing two different ligands in which case the ligand of greater ligand field strength preferentially aquates. The first rule does not apply to our cis systems since the average crystal fields along both the axes of the square plane are the same. However, in the case of the trans systems, according to the first rule the chloro and the aquo groups should be labilized in the dichlorodiammine and diaquodiammine, respec-
tively. The photosensitivity of both these trans complexes is rather low and hence application of the first rule (and also the second rule) cannot be tested. On the other hand, according to the second rule the ammine group having greater ligand field strength than the chloro should be replaced first in the cis-dichlorodiammineplatinum(I1) and similarly in the cis-diaquodiammineplatinum(I1) ion since the aquo group is of lower ligand field strength. Although irradiation of unacidifled solutions of cis- [Pt ("3) 2 (HzO)2] + suggests release of ammonia to a slight extent, it is the chloride ion that is preferentially released in cis- [Pt(NHa)2C12], which is in direct contradiction to rule 2. It is possible that complexes of chromium(II1) and platinum(I1) having different electronic configurations and geometry react differently under excitation by light. Further experiments on the photochemistry of other platinum(I1) systems, in particular, complexes such as chlorotriamminepla tinurn( 11) and amminetrichloroplatinate(I1) anion, would give better understanding of the semiempirical observations, and thus an insight into these observations may lead to a definite understanding of the chemistry of the excited states and electronic structures. Acknowledgment. These investigations were supported in part by Contract AT 11-1-113 between the University of Southern California and the U. S. Atomic Energy Commission.
The Journal of Physical Chemistry
