platinum( II ) complexes are small as in tim present cases, the redox rate depends strongly .... [32] V.E. Mironov, N.N. Knyazeva, Russ. J. Phys. Chem. 47 t 1973) ...
ELSEVIER
lnorganica Chimica Acla 282 ( 1998 ) 55--60
Kinetics and mechanism for reduction of trans-dichlorotetracyanoplatinate( IV ) by tetraammineplatinum (II) and bis(ethylenediamine)platinum(II) Tiesheng Shi, Lars |. Elding
*
hmr~amc Chemistry" 1. Chemicul Ccnter. Lmtd I,'mvcr.~itv. P.O. Box 124. S-221 qk9 Ltmd. Sweden
Received 4 December 1997; recci~ed in re,,ised form 23 January 1998: accepted 27 March 19'98
Abstract Reduction of trons-I Pt(CN) ~CI_,I-" by [Pt(NH~)4]z " and | P t ( e n ) : l : " was studied at 25"C in the range 0 < [CI " l >KICI 1.k:,--k, andkh--k.K. Ass(~-ialion constants for equilibria of the type of Eq. ( 5 ) have been determined from solubility measurements at 25°C and I.(hq M NaCIOt as 0.25 and 0.45 M t for interaction between chloride and [Pt(NH~).,I-'" and IPt(en)_~l-". respectively [321. We have performed some exploratory spectrophotomelric measurements in the case of [ Pt(en)_. !-" '. Spectra were recorded between 2~) and 3(XI nm at 25°C for a series of solutions with l P t ( e n ) : - " I = 2 . ( K I m M . 2 4 < [CI i < 9 0 4 mM, and INaCI/NaCIO.,I = l.O(I M. For each spectrum, the ionic m e d i u m without Pit !I ) was used as reference. Absorbance changes are significant between 200 and 215 nm. Absorbance data as a function of [CI I can be rationalized in terms of Eq. ( 101. where A,,h.d denotes the absorbance measured, and A,, and A t a r e the absorbances of 2.00 m M I Pt( en ). 1-' ' and I Pt( en ) _.CI l ' • respectively. A lit
+CI
I PtN.~Cll ' + ! Pt( CN ) ,CI_, ]:
dl PI(CNH;
')
~ 3.3 + 0 . 4 ) x 1 0 ' " 380 :t 4 0 " 4(iO _. iOnot a b s e n e d not o h s e n ' e d not o b s e r v e d
A,,,,.d =
[ P t N ~ C I I H ; O I I " +C1
's
I 18 I-
Table 2 Ionic ',trength d e p e n d e n c e of k., al 25~C for r e d u c l m n of I PH C N ) ~CI. I" by I PII NH~ i., ] : ' in the absee,.'e o f chloride I(M)
'
and
1
i
Fig. 5. A b s o r b a n c e at 205 n m .'is a function o f [ CI I for interaction b e t w e e n I Pt( en ) : 1 : ' and chloride at 25°C and 1.00 M ionic strength. T h e solid line results f r o m a non-linear least-squares fitting o f Eq. ( I O) to the experimental data.
T. Shi. L I , &tdu.g / Inor.gunicu Chirnica At'to 21~2 r 1998155-f~)
of Eq. (10) to the experimental data in Fig. 5 by use of a weighted non-linear least-squares method (an error of I C/~ was given to each individual absorbance) gives a value of K - - 0 . 5 3 + 0 . 1 4 M ~ at 25°C, which is in reasonably good agreement with that reported [ 32 l. Reduction of platinum(IV) halide complexes is usually interpreted in terms of halide-bridged electron transfer [ I 23]. In the present work, reaction is assumed to take place by interaction between the filled d.: orbital of [PtN~[ -~" and [ PtN ,Cl ] ' and empty orbitals on coordinated chloride in the platinum(IV) complex, thus forming the usual type of chloride-bridged transition states, l H_,O...Pt 'l.. .CI.-.Pt sv-- -CI] and [ CI.. "Pt It'" "CI'" .Pt Iv-. -CI] ". The ionic strength dependence o f k ( i.e. k~ ) gives a value of Z.~Zt~ of about - 3 . 8 _ + 0.5. supporting a direct reaction between [PtN.,[ 2~ and [Pt(CN).;CI~_] -~ in the rate-determining step of reaction (6). Formation of long-lived intermediates with a structure similar to the bridged transition states is therefore less likely. This is in agreement with the rapid-scan spectra shown in Fig. 2. Since the rate constant k2 is not kinetically separable from the equilibrium constant K in the present systems, the ionic strength dependence of k,, will not provide much information about the transition state of reaction ( 7 ). However, reactions (6) and ( 7 ) are expected to follow the same reaction pattern and to proceed through similar transition states. It has been demonstrated by Peloso [18,19,21-231 that both electronic and steric effects of the coordinated ligands in the platinum( ll ) complexes markedly affect their oxidation rates. Since the steric effects of the coordinated ligands in [ Pt( NH~)~ l " ' and [ Pt( en ), I: ~ are small and similar, the reactivity differences observed in the present cases should, in principle, be due to the different o - d o n o r properties of ammonia and ethylenediamine as measured, for instance, by their protolysis constants. The stronger it-donor properties of ethylenediamine will result in a higher electron density in the d.: orbital of [ Pt ( en ): ] : ' compared with [ Pt( NH,)4 ] 2 . , which in turn will facilitate the formation of the chloride-bridged transition state and the transfer of Ci ' from the platinum( IV ) complex. This model is further supported by the observation that the reactivity ratio k,( [ Pt( en ) 2 ! -" * ) / kt,([Pt(NH~)4] 2. ) for reduction of [ P t ( C N ) ~ C I _ , ] : is almost identical to that tbr reduction of [Pt( NH.~)2CI4], cf. data in Table 1. The redox reaction between [Pt(CN).~CI_,]~" and [Pt(en)_~i-" ~ is the fastest one observed so far among the various Pt( I V ) - P t ( I I ) redox couples studied [ 16-23]. The several hundred times higher reduction rates of [ Pt(CN).,CI, ] 2 _ compared with the previously studied plati n u m ( i V ) ammine complexes (cf. Table I ) can be rationalized in terms of transition state stabilization due to the strong o'-donor and -rr-acceptor properties of the cyanide |igands. Similar reactivity differences between complexes coordinating ammonia and cyanide have been observed in gold( Ill ) chemistry [ 33 ]. "
59
Table ! lists some rate constants and formal redox potentials with [Pt(en)_,]-'" and [PttNH~).=] 2. as re.d, tctants. When the steric effects of the coordinated ligan-~ in the platinum( II ) complexes are small as in tim present cases, the redox rate depends strongly on the thermodynamic driving force. This behavior is very similar tO what has been observed in the reduction of [Pt(CN).,CI_~]-" - by thiols [ 1,34] where the redox rate is correlated to the basicity of the r,-ductants. These observations indicate that the ptlv-Cl bonds in the transition states proposed above are significantly weakened [341. In the present systems, like most of those studied previously by Peloso [ ! 6.18,10,21-23 ], the platinum( II ) complexes are doubly positively charged cations, which means that. according to Eq. ( 5 ) . rapid pre-association equilibria will be of importance in the reaction mechanism. In other redox systems, where negatively charged platinum( II ) complexes have been used [ 20]. the contribution from pre-e,~,-~ libria of the type of Eq. ( 5 ) is expected to be much smaller. resulting in a disappearance of the third-order pathway.
4. Supplementary. material Observed rate constants as a function of hydrogen ion concentration (Table S! ) and numerical values of the rate constant k' as a function of chloride concentration (Table $2 ) are available from the authors on request.
Acknowledgements Financial support from the Swedish Natural Science Research Council is gratefully acknowledged.
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