Formation and characterization of transitory platinum ...

Formation and characterization of transitory platinum-ammonk complex ions using pulse radiolysis HASAN I'd. KHAN,WILLIAML. WALTZ,'A N D ROBERTJ . WOODS Can. J. Chem. Downloaded from www.nrcresearchpress.com by China University of Petroleum on 06/05/13 For personal use only.

Depnrttncnt of Chetnist~y-vclnif Chemical Engineering und the Saskatr,he~r,un Ac,crlerator LuDor.utoty, Unisenity oJ' Saskiitcheivnrl, Saskntoon, Snsk., Cunnda S7N OW0 AND

JOCHENLILIE Huhrl-Meitn~r-lnstitrrt fir Kcrnforachung Berlin GtnbN, Bereich Strahlenchen~ie,0-1000 Berlin 39. F-v,tiercil Rc7pul~lic oj'Cernrtrrly Received July 13, 198 1 HASANM.K H A N ,WILLIAML. W ~ L T ZROBERT . J. WOODS,and J o c ~ k uLILIE.Can. J . Chem. 59, 3319 (1981) The reactions of tetraammineplatinum(I1) complex ion with the hydrated electron and hydrogen atom have been investigated in aqueous media using the technique of pulse radiolysis coupled with absorption and conductivity detection. Analysis has also been carried on pulse-irradiated solutions for the formation of free ammonia and for changes in concentration of Pt(I1). The hydrated electron and hydrogen atom react with the co~nplexat near diffusion-controlled rates. with the respective rate constant being 1.9 0.1 x l o L o.%-I s i and 2.8 0.3 x 10'O M-' s-I. The nascent products of these reactions are shown to be different transitory species. For the electron reaction, the initial product is Pt(NH,),- in which the metal center is formally Pt(1). In acidic media, there is a subsequent and rapid release of two ammonia ligands. with only the second step being measurable (k = 4.2 1.7 x lo4 S-I). In the reaction of H atom. the results support the occurrence of an addition process, giving rise to a hydrido type product. This species undergoes a first-order reaction (X = 2.2 2 0.6 x lo4 s l ) ;however the process is not associated with a change in conductivity. and thus it is not one involving loss of ammonia. Subsequent to this process. the loss of one ammonia ligand is observed, with k = 2.0 i: 0.6 x 10's-I. The natures of these transients and the long term behavior of these systems are discussed.

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HASANM . K H A N ,WILLIAML . WALTZ,ROBERTJ . WOODSe t JOCHENLILIE.Can. J. Chem. 59, 3319 (1981). Faisant appel h la technique d e radiolyse par impulsion couplee avec la detection par absorption et par conductivite. on a etudii. les reactions de I'ion complexe platine(l1) tetrammine avec l'electron hydrate et I'atome d'hydrogene. On a egalement analyse des solutions irradiees par impulsion afin d'evaluer la formation d'ammoniac libre et les changements de concentration en Pt(I1). L'electron hydrate e t l'atome d'hydrogene reagissent avec le complexe au voisinage des vitesses contrtilees par la diffusion avec des constantes de vitesse respectives de 1.9 0.1 x 10Io .I-(-' s-' et de 2.8 5 0,3 x 10-lo .M-I s-I. On montre que les produits naissants de ces reactions sont des especes transitoires differentes. Dans le cas de la reaction de I'electron. le produit initial est I'ion PtiNH,),' dans lequel le Pt est formellement le metal central. En milieu acide, il se produit une rapide liberation additionnelle de deux ligands d'arnmoniac. dont seulernent la vitesse de la deuxieme etape peut 6tre mesuree ( k = 4,2 5 1,7 x lo4 s-I). Lors de la reaction des atomes d'hydrogene. les resultats confirment I'existence d'une reaction d'addition donnant lieu a un produit de type hybride. Cette espece reagit selon une cinetique d'ordre un (k = 2.2 i 0,6 x lo4 s-'1; cependant on n'associe pas la reaction a un changement de conductivite et dans ces conditions elle ne libere pas d'ammoniac. A la suite de cette reaction, on observe la perte d'un ligand d'ammoniac. avec k = 2,0 5 0.6 x 10' s t . On discute de la nature de ces intermediaires et de leur compo:.tement a long terme. [Traduit par lc journal]

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Introduction ~ ~thet hydrated h electron, e,,-. and hydrogen atom represent two of the most elementary reagents available for investigating the oxidation-reduction mechanisms of transition-metal pounds. These radical species are formed as pri,nary products along with hydroxyl radical when aqueous solurions are exposed to ionizing such as high energy electrons ( 2 - 2 0 ~in ~this ~ work) e[I1 Hz0 e,-, W, OH, HT

-

The hydrated electron and hydrogen atom are also intenelated in the manner of a conjugate acid base pair, [21 H e H+

+ e,,-

' T o whom all correspondence should be addressed.

with pK, of 9.4 (1). Through the use of the fast reaction technique of pulse radiolysis, a number of have been carried Out O n the electron reactions with metal complexes, generally processesleading to the reduction of the metal center (2-4). In some instances, this has afforded the unique opportunity to investigate transitory products in which the metal is formally in an unusual oxidation state. for example platinum(I1, a feature of this study. By way of contrast. relatively few reports exist concerning the reactions of H atom with metal complexes; however, the results do indicate that the processes can involve not only single dectron reduction as for e , , but also ones of addition and atom transfer (3, 5). The potential differences as well as similarities in reaction mechanisms between e,,- and g% atom are exemplified in their

0008-40421811243319-07$01.00/0 0 1 9 8 1 National Research Council of CanadaIConseil national de recherches du Canada

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C 4 N . J. CHEM. VOL. 59. 1981

reactions with square-planar platinum(I1) complex helium. or nitrous oxide gases. The pH of the solutions was adjusted by addition of reagent grade perchloric acid or sodium ions. m l s e radiolysis studies using uv-visible hydroxide, and was measured by pH-meter, calibrated with absorption detection have shown that a common certified buffer solutions. ,411 other materials were of reagent product arises from their reactions with Pt(CI)4Z- grade. (6.7). With complexes containing bi- and tridentate Product Analysis amines or glycine as ligands and that of Pt(CN)42-, Pulse-irradiated solutions were analyzed for uncoordinated the nascent products are different, implying a ammonia and for changes in total Pt(I1) content by the following difference in mechanism (7-10).2 These studies spectrophotometric methods. The determination of free ammoand one on the reaction of e , , with c~s-A(NH,),(CI)~ nia was based upon its reaction with hypochlorite and phenol to indophenol. the absorption of which was measured at (1 1) indicate that the initial products are capable of give 625 nm in basic media at 25°C (15. 17). Analysis for changes in undergoing further and rapid reactions involving, total Pt(I1) concentration upon irradiation made use of the for example, ligand substitution. The final products oxidation of Pt(I1) to Pt(1V) in 0.2.M HCl and 0.2.W NaCl from these reactions have generally not been solutions at 25°C by K,[lr(CI),] (15. 18). The decrease in the characterized, due in part to the very low concen- intense absorption peak of the oxidant. Ir(CI),'-. at 487 nm. a wavelength where other species present do not absorb signifitrations formed in pulse radiolysis experiments; cantly, was used to measure the concentration of Pt(I1). While however, based upon some of the kinetic results some colloidal platinum metal was formed upon irradiation, its and the detection of metallic platinum, dismutation presence under our conditions did not appear to interfere with processes have been proposed. Additional interest the analysis: removal by tiltration of the suspension prior to did not alter the results. Our findings for solutions in the occurrence and properties of Pt(I) species analysis containing Pt(NH,)," (50-250~M) or ~ ~ . Y - P ~ ( N H , ) , ( H , O ) , ~ ~ has been engendered by the role of platinum (123 F-M) and that of others on related Pt(I1) complex ions (18. compounds in radiation sensitization of cellular 19) would indicate that this analytical test provides a valid materials where the sensitizing action may be in measure of the total concentration of Pt(II).I part derived from the reactions of Pt(1) with organic Pulse Radiolysis Apparatus and Dosimrtn~ substrates and/or the release s f toxic ligands such The apparatus employing conductivity and optical detection methods and the associated dosimetry have been described as ammonia (11. 12). The present study was undertaken to compare previously (16). Of note is that the transient signals represent differences between those of the unirradiated and the pulsed the reactions of e,,- and H atom with Pt(NH,),?+ solutions. Such differences were calibrated against one of the and to characterize the reaction products and their following dosimetry solutions: ferrocyanide, thiocyanate. or subsequent chemical behavior. In so doing, we tetranitromethane. The absorption spectra were recorded on a have endeavored to enlarge the scope of such point-to-point basis, generally at lOnm intervals, using an optical cell of 3.93 cm light path. Contributions to the absorption investigations in part through the use of conductiv- changes owing to the absorption of Pt(NH,),'+ were minor as ity as well as absorption detection techniques. The this complex ion absorbs only weakly over the wavelength range former has been particularly useful in elucidating investigated. and thus the spectra have not been corrected for the nature and reaction properties of the intermedi- this (or for minor contributions by alcohol radicals below 300 nm ates, for which the platinum center is formally in an (20)). The G-values (number of a given species formed or lost per 100eV of energy absorbed) used were G , = 6,- = 2.66 and unusual oxidation state. The kinetic studies have 6, = 0.55 (21). In the study of the reaction of H atom and been supplemented by product analysis on pulse- Pt(NH,),,+. conditions were maintained so that e , , reacted irradiated solutions for the formation of free am- predominantly with the proton, and thus the effective G-value monia and for changes in total A(1I) concentrations. for H atom was now the sum of G , and GH.The equivalent ionic Experimenbl Section

conductivity values for H+, OH-. and NH,+ were taken as 350. 198, and 74R-'cm2equiv-I respectively, and those for the platinum complex ions were anticipated to be about S O P ' cm2 equiv-I (22, 23).

Materials and solution^ Tetraammineplatinum(I1) perchlorate was prepared from the chloride salt, obtained commercially (Platinum Chemicals, Alfa Division of Ventron Corp., K and K Labs of ICN Pharmaceuticals. Inc.) (l3), and the compound was characterized by comparison of its uv absorption spectrum with that reported (14) and by elemental analysis (15). All solutions used for pulse radiolysis experiments, carried out at about 23"C, were prepared just prior to use with triply distilled water or water purified by a Millipore Super-Q system. The solutions were deaerated by procedures which are described elsewhere (16) and which employed argon.

Results Reactions Associated with e,,- and Pt(NtP,),*+ The reaction of the hydrated electron with Pt(NH3)42+was followed by monitoring the decrease in the electron absorption at 578nm in solutions at a natural pH of about 5.6 and containing 0.1 M isopropanol. This alcohol is a facile

ZResultsof a recent low temperature, esr study on Pt(CN)qZindicate that in methanol the electron product, Pt(CN)43-. undergoes proton addition to yield the same complex, HPt(CN)42 , as formed by H atom reaction in aqueous sulfuric acid matrices (10).

jOwing to the low concentrations of other Pt(1I) complex ions which might be formed under pulsed-irradiation conditions, it was not feasible to identify their specific compositions although such possible products are likely to be aquated species such as trans- and ~ ~ S - P ~ ( N H , ) , ( H , O (11). ),~+

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K H A N ET AL.

scavenger of both hydrogen atom and hydroxyl radical (5, 24). Under the condition where the concentration of complex ion (15-174pM) was considerably in excess of that for e,,-, the optical decay obeyed a pseudo first-order rate law. A plot of the observed rate constant versus complex concentration was linear. and from the slope of the plot, the second-order rate constant was calculated to be 1.9 0.1 x 10'OM-ts-'. The products derived from this reaction exhibited optical absorption at shorter wavelengths as shown in Fig. 1. The studies of these products were carried out over a p H range of 4.1 to 11.4 on solutions containing 0.1-0.2 M isopropanol or 0.2 M terr-butyl alcohol. The concentration of Pt(NH,),'+ used was now in general higher (100-524 FW) than that employed to study the disappearance of e , , so as to minimize any possible interference by the electron reaction to subsequent events. For these conditions, a prompt increase in absorption occurred following the irradiation pulse (Fig. 1 at 1 p s ) Following this, there transpired a decrease in absorption at wavelengths below 350 nm and a concomitant increase at longer wavelengths. Addition of N,O, used as a scavenger of e , , (4), eliminated these changes except for small levels of residual absorption at shorter wavelengths, having the properties characteristic of the alcohol radicals that are formed by the reactions of OH and H with

"

250

290

330

370

410

Wavelength, nm FIG. 1. Absorption spectra arising from reaction of e,,- and Pt(NH3)4Z+. (0) at 1 ps; (A) at 48 ps for 408 p l 4 Pt(NH,),Z', ca. 2.8 pW e,,-, 2.0M isopropanol at natural pH of ca. 5.6.

the alcohols (5, 20, 24). This observation confirms that the species observed in Fig. 1 are ones derived from the electron reaction. Furthermore, it implies that the alcohol radicals are unreactive towards Pt(NH,),,+, a feature that has also been noted for other Pt(I1) complex ions (6-9). Below pH 7, the absorption changes obeyed a first-order rate law with a rate constant of 4.9 1.7 x lo4 s-I. This value was found to be independent of dose (10-fold), concentration of complex (5-fold), the nature and concentration of the alcohol although the values were somewhat higher (ca. 50%) in the region of 300-340 nm. On the same time scale as the optical change, there was a decrease in conductivity to - 190R-' cmZequiv-I as shown in Fig. 2a. This change exhibited first-order kinetics with a rate constant of 3.6 f 0.1 x lo4 s-I. which is within experimental error the same as that for the absorption change. At longer times (0.1 s scale), relatively minor alterations in absorption and conductivity took place. This situation may, however, be more apparent than real because both here and in the case of the H atom reaction with Pt(NH,),'+, suspensions of metallic platinum were observed after irradiation.

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Ti me FIG. 2. Equivalent conductivity changes with time: ( a ) and (b), reactlon of e , , and Pt(NH3),Z+:495 pM Ft(NH,),*'. 2.0 ,M isopropanol, pH 4.1 and 11.4 respectively; ( c ) reaction of H atom and Pt(NH,),Z+: 9 6 p M Pt(NH,),Z+, 0.10M tert-butyl alcohol. pH 2.8.

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C A N J CHEM VOL 59. 1981

The foregoing absorption results show that the nascent product of the electron reaction, which is expected to be Pt(NH,),+ (2, 3). undergoes subsequent reactions. The observed decrease in conductivity at short time scales demonstrates that more than one ammonia ligand is released from Pt(NW,),+, even though only one kinetic step could be discerned. If only one NH, had been lost from this Pt(1) species. then the rapid reaction of the NH, with the proton formed via the irradiation process, reaction [ I ] , would have resulted in essentially no net change in conductivity in contrast to the observed decrease. Support for the presence of free ammonia is also provided by its detection in substantial amounts as a final product. For product analysis, solutions containing 60-260 pA4 R(NH,)42+ and 0.1-2.0 ,M isopropanol at a pH of ca. 5.6 were pulse-irradiated (1-1 1 successive pulses) to an extent of about 15 to 75% reaction. based upon the amount of the electron reacted. For three separate measurements, the concentration ratio of free ammonia formed to that of the cumulative amount of e,,- was (3.7 k 0.6)/1, and the corresponding ratio for loss in Pt(I1) relative to that of the electron way (1.1 k 0.1)/1. In alkaline media (pH 11.4 and 2.0 M isopropanol), the absorption features were essentially the same as those at lower pH (Fig. I). There was however an immediate decrease in conductivity to about -240 R p l cm2e q u i v l at the end of the irradiation pulse. followed by a further decrease of - llORpl cm2equivp' (Fig. 2b). (No subsequent changes occurred at times up to 1 s.) For the latter process, both the absorption and conductivity signals obeyed first-order rate laws with a rate constant of 1.2 8.3 x lo4 s-I. a value smaller by three-fold than that obtained at pH 4.1. At pH 11.4, free NH3 will not undergo protonation as the pK, of NH4+ is 9.24 (259, nor will there be extensive ionization of isopropanol radicaI wieh a pK, of 12.2 (28). While it was not feasible to carry out investigations at higher pH, this secondary decrease in conductivity suggests that the platinum transient(s) has acidic properties. The prompt decrease of -240 Rp' cm2equiv-' is accounted for by the rapid neutralization of the proton formed during the irradiation pulse, and Gy the reaction of e , , with Pf(NH3)42+.

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Reactions Associated with HArom and Pb(NH3)42' These investigations were carried out at pH's between 1.1 and 2.9 and at low enough levels of Pt(NM,),'+ to insure that the predominant electron reaction was with proton to give hydrogen atom (4).

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333

"2.d

r

"CJ

"53

5(3

Wavelength, nm

FIG.3. Absorption spectra arisingfrom the reaction of H atom and Pt(NH,),'+. (0) at 2 ps; (O) at 200ps; (A) at 1500ps for 112 FM l't(NH,),2+. ca. 3.1 p,W H atom. 0.21 .M tert-butyl alcohol at p H 2.3.

Sufficient tert-butyl alcohol (0.1-0.2 M ) was present to scavenge OH but not to react unduly with H atom (5, 24). The concentrations of the platinum complex ranged between 15 and 245 piW. The results described below were found to be independent of the concentration of alcohol, pH, and dose (5-fold). As shown in Fig. 3, irradiation led to the formation of absorption spectra in the region of 260 to 500nm. Addition of 2.1 ,IM isopropanol, a scavenger of H atom, eliminated the absorption in this region aside from minor amounts at short wavelengths, which are attributable in the main to the alcohol radicals (20). The rates of the initial growth in absorption were measured at 260-300. 420, and 425nm. Under the condition where the complex concentration (15- 100 pM) was considerably in excess of that for H atom. the growth in absorption followed a pseudo first-order rate law. The values of the associated rate constant increased linearly with the concentration of complex, and from this behavior. the second-order rate constant was determined to be 2.8 k 0.3 x 101°iMp' sp' for the reaction of H and Pt(NH,)42+. The initial absorption spectrum (Fig. 3 at 2 ys) is considerably different in its features (peaks at about 310 and 430nm) and in its level of absorption (ca. 4-fold less on a normalized basis at 260 nm) from that of the electron case (Fig. I). The differences between the two cases also extended to the kinetic aspects. For the hydrogen atom situation, two intervening absorption changes were observed (Fig. 3), wieh both obeying first-order rate laws: a general decrease in absorption at intermediate

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K H A N ET AL.

times (k = 2.2 f 0.6 x lo4 s - ' ) , ~followed by a slower growth in absorption at wavelengths below 400nm (k = 1.9 k 0.6 x 10' s-I). In contrast to the two absorption changes, only one conductivity step was found as shown in Fig. 2c (An,,,,, = -260 R-I cm2equiv'). and this process conformed to a first-order rate law with k = 2.1 f 0.2 x 1Q7s-'. Of note is that the conductivity step correlates with the ~ e c o n dabsorption change (growth). At longer times, therc was a general decrease in the absorption spectrum. the kinetics of which were not describable by simple rate law expressions. The conductivity was now constant up to 0.1 s. As with the electron reaction, this situation may be complicated by the development of the observed platinum suspensions. Pulse-irradiation (6- 1 1 successive pulses) of 200 pM Pt(NH,),*+ solutions to an extent of about 25 to 80% reaction of the complex also gave rise to free ammonia as a final product. The ratio of ammonia concentration to that of H atom formed was (1.6 f 0.4)/1 (3 trials). Relatively small losses in Pt(1I) concentration occurred, and the results of two trials suggest that the ratio of Pt(I1) loss to that of H atom formed probably does not exceed 0.311. Discussion The foregoing kinetic results and product analyses indicate that the reactions of e , , and H atom with Pt(NH,),2+ give rise to situations that are different from each other, both from the perspective of the short- and long-term processes involved. For the electron reaction, the nascent product is expected to be the platinum(1) species, Pt(WH,),+ (2-4). The events associated with its occurrence show that at fast time scales, this species undergoes ligand substitution. In particular, the conductivity results indicate the release of two ammonia ligands occurs sequentially

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131 Pt(NH,),+ + H 2 0 -. F't(NH,),(H20)A NH, [4] Pt(NH,),(H2O)+ + H 2 0 + Pt(NH,)2(H20)2++ NH,

The first step, represented by reaction [3], is presumed to be fast based upon the following observations. At high concentrations of Pt(NH,),*+ where the rate of electron reaction with the complex is very rapid (half-life < 0.1 ps), only one kinetic step was discernible subsequent to the prompt development of absorption in the near-uv region (Fig. 1). In addition at pH 4.1, no net 4Between 290-310nm, the rate constants were higher b y a factor o f three than ihat cited here for higher and lower wavelengths, and this may indicate the presence o f an additional transient.

3323

conductivity change was observed immediately after the irradiation pulse. This latter feature is consistent hith the rapid release of one ammonia whose reaction with an amount of proton equivalent to that formed by irradiation (reaction [I]) forestalls a noticeable change in conductivity. (In thc abscncc of such a neutralization process, a substantial increase in conductivity, due primarily to the radiolytically generated proton, would have been recorded.) Within this context, the initial absorption spectrum of Fig. 1 should be representative in the main of that for Pt(NH,),(H,O)+. The substitution of an ammonia ligand in this species is associated with the observed changes in conductivity and absorption, having an average first-order rate constant of 4.2 f 1.7 x lo4 s-I. Protonation of the released ammonia is anticipated to lead to an overall decrease in conductivity of about -250 R ' cm2equiv-I, in reasonable agreement with that found, Fig. 2n. This two-step sequence parallels that observed in the case of the reaction of e , , and cis-Pt(NH,),(Cl), although in this instance the steps pertain to chloride substitution (1 1). The loss of the first chloride ligand was prompt, and was followed by release of a second chloride ion (k = 4.9 x 10' s-I). In both studies, the resulting diamino Pt(I) species appear to be relatively stable within the time domain of pulse radiolysis. With reference to their compositions, we have portrayed them above as being four coordinate, on the basis that other d 9 metal complexes such as those of Ag(1I) exhibit this (or a higher) coordination number (26,27), and that our results found in basic media suggest the presence of coordinated water. In this latter circumstance (pH 11.4).protonation of free NH, will be minimal (25). The level of the prompt decrease in conductivity (-240R-Icm2 equiv-') shown in Fig. 26 is near that expected (-250 R-' cm2e q u i v ' ) owing to the electron reaction and the neutralization of radiolytically produced proton. under the assumption that extensive ionization of Pt(NH,),(H,O)' did not occur. The subsequent absorption and conductivity changes, which proceeded at a somewhat slower rate (ca. 3-fold) than that in acidic media, suggest two additional facets, namely that reaction [3] may now reflect a rapid approach towards equilibrium and that Pt(NH,),(H,O),+ undergoes ionization to yield a base form such as Pt(NH,),(H,O)(OH). Rapid formation of this base form would give rise to the observed decrease in conductivity. The magnitude s f this change (- 110 R 1cm2e q u i v l ) would imply a pKa of the order of 11, which might reasonably be

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anticipated from the pK, values (4-8) encountered for the analogous platinum(I1) complexes of Pt(NH3)2(H,0),2+(28). Both the hydrated electron and hydrogen atom can function as strong, single-electron reducing agents although for the latter other modes are also possible (2-5). For their reactions with Pt(Cl)42-, a common initial product was observed optically (6, 7) whereas with platinum(I1) complexes containing organic amines. glycine or cyano ligand systems, the products arising from the electron and H atom reactions exhibited different optical and kinetic properties (7-10). Our conductivity and absorption results concerning the corresponding reactions of Pt(NH,)42+place this system in the second category. If the reaction of H atom with this complex was to involve a charge transfer mechanism (in analogy to the electron reaction), then the proton so generated would cause in acidic media a prompt and large increase in conductivity. in contrast to our observation of essentially no net, initial change. While the absence of such a change might be masked in a kinetic sense by a rapid release and protonation of NH,. in a process analogous to reaction [3], the observed product should be Pt(NH,),(H20)+. If so. then common spectral and kinetic features (as well as final products) should now prevail between the two situations. This expectation is not in accord with our findings. A mechanism that encompasses the findings, in particular the occurrence of no initial conductivity change and the discernment of two subsequent absorption steps with the slower one coinciding with a process involving a conductivity decrease. is [5] H

+ Pt(NH,),Z'

[6] H-Pt(NH3)42+

[7] [H-Pt(NH3),12+

N-Pt(NH,),'+

[H-Pt(NH,),12+

(rearrangement)

+ H 2 0 + H-R(NH3)3(H20)L+ + NH,

The near diffusion-controlled rate constant associated with reaction [5] is compatible with this step being one of hydrogen atom addition to the metal center. (Such addition reactions are frequently faster than those pertaining to abstraction and charge transfer processes (5).) The resulting five coordinate product, H-Pt(NH3),2', may undergo structural rearrangement such as a transformation from a square-pyramidal form to one of a trigonalbipyramidal type. Such a process would be in keeping with the observed, first absorption change not being accompanied by an alteration in conductivity. The products of reactions [ 5 ] and [6] formally contain a hydrido ligand, a species well known for its trans activating capacity in ligand substitution reactions. Its presence in a complex of

trigonal-bipyramidal form would be expected to promote labilization of the ammonia ligand in the t r a m position to it, and so give rise to reaction [7]. followed by rapid protonation of the released ammonia (pH 2). The latter reaction would yield a conductivity decrease of about -270 R ' cm2e q u i v l , in good agreement with the experimental value of -260 R ' c m 2 equiv-'. For both the electron and hydrogen atom cases. the subsequent processes leading to the disappearance of the intermediates are less readily characterized, in large measure because only minor changes in absorption and conductivity occurred during the longer time periods (upwards of 1 s). Clearly changes did eventually occur as evidenced by the development of colloidal platinum metal (an indication of further reduction) and by the detection of amounts of free ammonia in excess of those that can be accounted for by the intervening processes of reactions 131, [4], and [7]. h l s e radiolysis results concerning the behavior of related platinum complexes have intimated that Pt(I) species decay by disproportionation to yield equal amounts of Pt(0) and Pt(1I) (6-9, 11). Within this context. the expected ratio for loss in Pt(II) relative to each e , , reacting to produce Pt(I) would be 0.511. Our observed ratio of 1.1 f 0.1 for the electron reaction is substantially higher, and as such implicates for the case of Pt(NH,),2+ the occurrence of either processes in addition to disproportionation or a different mechanistic sequence. One possible sequence could involve the oxidation of water (or ammonia) and concomitant reduction of Pt(I) to metallic platinum (Pt(1) is isoeleclronic with Ag(I1). a strong oxidant (26)). This scheme would be representative of a noncomplementary redox situation. A likely consequence would be the occurrence of several reaction steps, the rates of which may be slow and thus not necessarily observable on our time scales. By way of contrast to the electron case. the overall loss in Pt(I1) and formation of free ammonia associated with the hydrogen atom reaction were substantially less. These differences not only underscore the dissimilarity between the two situations but also suggest that decay of the hydrido species leads in part to regeneration of Pt(II), perhaps in association with the formation of molecular hydrogen. Acknowledgements One of us (H.M.K.) wishes to thank the University of Saskatchewan for a Graduate Scholarship. Assistance from the Natural Sciences and Engineering Research Council of Canada is also acknowledged.

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I. A. HENGLEIN.W. SCHNABEL,and J . WENDENBERG. Einfuhrung in die Strahlenchemie, Verlag Chemie GmbH. WeinheimIBergstr. Germany. 1969. p . 140. 2. E. I . HART and M. ANBAR. The hydrated electron. Wiley-Interscience. New York. 1970. Chapt. 5. 3. G . V. BUXTONand R . M. SELLERS.Coord. Chem. Rev. 22, 195 (1977). 4. (CI) M. ANBAR.M. BAMBENEK. and A. B. Ross. Natl. Stand. Ref. Data Ser. U.S. Natl. Bur. Stand. NSRDSNBS43. 1973: ( b ) A. B. Ross. Natl. Stand. Ref. Data Ser. U.S. Natl. Bur. Stand. NSRDS-NBS43. Supplement. 1975. and A. B. Ross. Natl. Stand. 5. M . ANBAR,FARH-ZTAZIZ. Ref. Data Ser. U.S. Natl. Bur. Stand. USRDS-NBS51. 1975. and B. D. MICHAEL. 6. G . E. ADAMS.R. K . BROSZKIEWICZ, Trans. Faraday Soc. 64. 1256 (1968). and E. J . HART.Int. J. Radiat. 7. A . S. GHOSH-MOZUMDAR Phys. Chern. 1. 165 (1969). 8. D. K . STORER,W . L . WALTZ,J. C. BRODOVITCH, and R. L . EAGER.Int. J. Radiat. Phys. Chem. 7, 693 (1975). 9. W . L. WALTZ. J. C. BRODOVITCH, and K . P. K U N D U . Radiat. Phys. Chem. 10, 77 (1977). , J. L. WYATT.J . Chem. 10. M. C. R. SYMONS,M. M. . ~ L Yand Soc. Chem. Commun. 176 (1981). 11. R. C. RICHMOND and M. G. SIMIC.Br. J . Cancer, Suppl. 111. 37. 20 (1978). 12. E. B. DOUPLEand R . C. RICHMOND. In Cisplatin: current status and new developments. Edited by A. Prestakeys. S. T. Crooke. and S . K. Carter. Academic Press, New York. 1980. Chapt. 8. 13. W . R. MASONand R. C. JOHNSON.Inorg. Chem. 4, 1258 (1965).

3325

14. W. R. MASONand H. B. GRAY.J. Am. Chem. Soc. 90.5721 (1968). 15. H. M. K H A N ,W. L . WALTZ.J . LILIE,and R . J. WOODS. Inorg. Chem. T o be published. 16. W. L . WALTZ,J . LILIE,R. T . W ~ L T E R and S , R. J. WOODS. Inorg. Chem. 19. 3284 (1980). 17. W. T. BOLLETER.C. J . BUSHM.AN. and P. W. TIDWELL. Anal. Chem. 33. 592 (1961). 18. J . HALPERNand M. PRIBANIC. J . Am. Chem. Soc. 90. 5941 (1968). 19. A. V. LOGINOV.V. A. YAKOULEV. and G. A. SHAGISULTASOVA.Koord. Khim. 5 . 733 (1979). 20. M. SIMIC,P. NETA, and E. HAYON.J. Phys. Chem. 73. 3794 (1969). 21. K. SEHESTED,H. CORFITZEN.and H. FRICKE.J. Phys. Chem. 74. 21 1 (1970). 22. W. J . MOORE.Physical chemistry. 3rd ed. Prentice-Hall. Englewood Cliffs. N J . 1962. p. 337. 23. R. C . JOHNSON, F. BASOLO.and R. 6 . PEARSON. J . Inorg. Nucl. Chem. 24. 59 (1962). and 6 . E. ADAMS. Natl. Stand. Ref. Data 24. L. M. DORFMAN Ser. U.S. Natl. Bur. Stand. NSRDS-NBS46. 1973. 25. L . G . SILLENand A. E . MARTELL.Stability constants of metal-ion complexes, Special Publication 17. The Chemical Society. London. 1964. 26. F. A . COTTONand G. WILKINSON. Advanced inorganic chemistry. 4th ed. John Wiley and Sons, N e u Yo1.k. 1980. 27. 4 . KUMARand P. NETA.J . P h y s Chem. 83. 3091 (1979). 28. F . BASOLOand R. G . PEARSON.Mechanisms of inorganic reactions. 2nd ed. J . Wiley and Sons, Inc., New York. 1967. pp. 32-33.