UMBERTO BELLUCO, LUCIOCATTALINI, A N D ALDOTURCO
226
[CONTRIRUTION FROM
THE
ISTITUTO CHIMICA GENERALE, USIVERSITA'
DI
Vol. Mi
PADOVA, I'ADOVA, ITALY]
Rates of Substitution Reactions of Square-Planar Platinum (11) Complexes. I. The Reaction of fvans-Dichloro-bis-(triethylphosphine)-platinum(II) with Radiochloride and Nitrite Ions B Y UMBERTO BELLUCO, LUCIOCATTALINI, A N D ALDOTURCO RECEIVED JULY22, 1963 Rates of chloride replacement by nitrite and radiochloride in the platinum( 11) complex trans- [ P t (PEt.j)zC1?] are reported. The results show t h a t both first-order and second-order reactions occur. The first-order rate constant for the reaction with NO,- is obtained by applying a stationary state treatment. The relative reactivities of CI- and NO?- are compared with those of similar amino complexes of platinum(I1). The bimolecular reactivity of K O * - decreases in the reaction with tians-[Pt(PEt,~)zCl,] and this is interpreted in terms of the electrophilic character of the nitrite ion and of the electronic structure of the complex.
The kinetics of nucleophilic substitution in square complexes generally take place according to two simultaneous paths, one of which is first order and the other is second order.'-I3 This results in the general kinetic equation'?
sis, we have studied the reactions of C1- and of NO2with trans- [Pt(PEt3)2C12].Similar complexes containing cyclohexylphosphine or phenylphosphine proved to be unsuitable for the present work owing to their limited solubility.
Experimental where kobsd is a pseudo-first-order constant, kl and kz are constants of first and second order, and Y is theentering group. The first-order path of the reaction is generally assumed to involve a nucleophilic displacement by the solvent followed by the coordinated solvent being rapidly replaced by the entering reagent. Hence k l , a pseudofirst-order rate constant, is independent of the nature of the reagent. The second-order path of the reaction, ascribed to kt, is interpreted as a direct bimolecular reaction between the complex and the entering group. A comparison of the specific rate constants k 2 for some reactions of C1- and NOa- with Pt(I1) complexes has shown t h a t C1- reacts more rapidly than NOz- with positively charged complexes, whereas with negatively charged and neutral complexes the opposite occurs. l a This leads to the conclusion that the NO2- attack can be partially electrophilic and t h a t of Ci- is primarily nucleophilic. This is reasonable if we consider the ability of NO?- to stabilize the transition state by using electrons in the 5d-orbitals of Pt(11).13 I t is generally accepted that filled nd-orbitals of transition metals are used in the formation of r-bonds with the empty Bd-orbitals of phosphorus.14 This type of interaction is formally equivalent to transferring some negative charge from the metal to the ligands and hence to render i t more "positive." Thus the effectiveness of NO2- as an electrophilic reagent is expected to be reduced in complexes of platinum(I1) with tertiary phosphines. Accepting this view as a working hypothef l ) I< I. Rich a n d H T a u h e , J P h y s . C'hPm., 68, 1 (1451). ( 2 ) I F G r a n t h a m . T S Elleman, a n d I ) S M a r t i n , J r , , J . A m . Chrm Sor , 77, 296.5 (195% CO I ) Ranerjea, F Basolo, a n d I< G . Pearson. tbui , 7 9 , 40.55 (1957). (4) T S Elleman. J u' Iieishus, a n d 1) S Xlartin, J r . , ibid , 80, 536 ( 1 9.58)
( 5 ) T S E l l e m a n , J U' Iieishus. a n d 1 ) S M a r t i n , Jr , ibid., 81, IO ( 1 !1T,9) (A) .4 A . Grinherg. Ru.rr J I n o v f ( ' h e m , 4 , 1:i9 (1959) ( 7 ) I< C:. Pearson, H R G r a y . a n d F Basolo, J A m C h r m . Soc , 82, 7 8 i (lotio) ( 8 ) I ) S M a r t i n . J r , in I< I ) . Adams, "Advances in t h e Chemistry of t h e Co]
is shown in Fig 3 The marked departure from linearity can be attributed t o the competition between NOz- and the C1- ion formed during the course of reaction This was proved b y carrying out some experiments in the presence of variable amounts of LiCl I n crease of chloride concentration results in a decrease in kobsd
Gray1? found, for the reaction between [Pt(dien)Br]Br and pyridine, that the value of kl was smaller than the values obtained with other nucleophilic reagents He attributed this difference to a competitive
The product of the reaction between trans- [Pt(PEt3)?Clz] and XO2- is [ P ~ ( P E ~ S ) ~ ( N O However, ~)~] the rate-determining stage must be the replacement of the first C1- of the complex, leading to the formation of the intermediate species [Pt(PEt3)7ClS02]. Displacement of the second C1- must occur a t a much higher rate in accord with the greater trans effect of X O j compared to C1I t was not possible to detect the presence, in solution, of the chloronitro complex Spectrophotometric studies showed that the rate of appearance of the chloride ion is twice t h a t of the consumption of the dichloro complex, thus showing that no other ahsorbing species is formed in appreciable concentration during the course of the kinetics The compound [ P t (PEt3!2C1Y02] is expected t o absorb in the range 12004600 A used in the present investigation (shorter wave lengths are useless owing to the strong absorption of NO,-) considering t h a t the absorption curve of [Pt(PEt3)2(K02)2] and $hat of trans- [Pt(PEtl)?C12]start a t -3600 and -5000 A , respectively
UMBERTO BELLUCO, LUCIOCATTALINI, AND ALDOTURCO
228
The over-all reaction may be summarized as
Since [RS] is always small we can apply a stationary state treatment so that
trans- [Pt(PEt3),ClSO2] fast (kz)
7+
dRS/dt = kl[RX]
+
1
-
kL[RS] [XI - ka[RSl [Yl sz 0
NO*-
[MI = ki[RXI/(k-i[Xl
+
tr~n~-[Pt(PEt3)2Cl(CHaOH)lC1ki
Vol. 86
+ k3W1)
Then
fast ( k - L)
+
tr~ns-[Pt(PEt3)2Clz] CHaOH
If Y is in large excess [Y] remains constant for a particular experiment and we can write
+ XO?
klJ
kz[Y] =
trans- [Pt(PEta)2CISOl]
The r a t e constants kz were calculated from t h e slopes of the curves obtained b y plotting k o b s d (Table 11) vs. nitrite concentration. TABLE I1 RATESOF REACTIONS OF trans- [ P t (PEt3)zC12]COMPLEX WITH NaSOo IN CHBOHAT 55" Complex concn. trans-[Pt(PEt3)2C12] = 0.005 M Reactant, concn. M
kobsd
Added substance
0.165 M LiN03 .01M toluenesulfonic acid 11 111 Li?\:oa ,015 hI toluenesulfonic acid ,055 M L i S 0 3 02 M toluenesulfonic acid 025 M toluenesulfonic acid
0.10 .15
.20 ,25
x
RX
kg + Y -+ RY + X
+ S+
ki
RS
+X
So for a particular concentration of Y
Obviously this is not a simple first-order process. write the rate in terms of x
If we
10'
sec. - 1
3.62
Therefore
4.15
- d log ( a
4.55
-
1/2x)
+
=
dt
5.10
The best values of k 2 obtained from experiments with the highest nitrite concentration were used in order to calculate the rate constant k l . This was obtained b y using the following mathematical treatment suggested by Tobe. The general equations 1, 2, 3, and 4 can be written ignoring for the moment the fact t h a t there are two replaceable chlorines (but this must be remembered later on). RX
ka[Y] = ka'
k2';
(1)
If we plot log (a - I/*x) against time, the slope is not constant but is equal to -__
2.303
i k-i[Xl
+
k3'
The rate constant kl can be calculated with the following expression, by measuring the slope 'at various times and knowing the value of [XI a t the same times
-
1 2.303"slope"
+ k2'
k-i[Xl -~
+
klka'
1 k,
+
Plotting - 1/(2.303"slope" k l ' ) against x one should have a straight line of intercept 1l k l .
(2) TABLE 111
RS+X
% R X + S
(3)
ka
RS+Y+RY+S
(4)
CALCULATED FIRST-ORDER RATECOYSTANTS Complex concn. = 0.005 M R e a c t a n t concn , .M
where R X = [Pt(PEta)nCIz]
KY
=
[Pt(PEta)z(~oz)z1
S = CHBOH
Y
=
NO1
Since steps 3 and 4 are known to be much faster than 1 and 2, the concentration of RS is always very small. Then dRl'/dt
=
- dRX/dt
=
"rate" = (ks[RS]
dx/2dt z z "rate"
+ k , [ R X l \ [Y]
118) M L T o b e , private communication
0.10 .l5 .20 .25
k l , sec
-1
x
10s
3.45 4.00 5.45 6.25
The calculated rate constants kl are presented in Table 111. Figure 4 shows that a good linear relation is obtained for the curves a t various nitrite ion concen0 trations. The extrapolated values of l/kl for [CI-] are not exactly the same. However, they can be considered satisfactory, except perhaps for the value a t the highest nitrite concentration. This may result from the fact t h a t a t highest nitrite concentrations most of the reaction goes by the kp path so t h a t the errors in k , become appreciable. The average kl in Table IV has
-
ANATIONO F ~~U~S-AQTJONITROBIS(ETHYLENEDIAMINE)-COBALT(II1)I O N
Jan. 20, 1964
been calculated neglecting the value obtained for the highest nitrite ion concentration. It can be seen t h a t the agreement between the k l S O 2 - and klcl- is extraordinarily good. If the neglected value a t highest [NOZ-] were also used, then the average k1 becomes 5.05 X lop5,which is still a very satisfactory value.
-
TABLE IV
m
229
21
Y
N
1
0.5
+
2
0,4
VI
8
RATES CONSTASTSFOR THE REACTION OF trans- [Pt(PEt3)zC12] WITH 36~1-A N D ~ 0 IN ~ CH~ - O HAT 55' Reactant 3 6 ~ 1 -
so*-
kl,
s e c - 1 X 105
k21 M-1
4 5 4 4i
sec
-1
X 104
0,2 0.1
4 20
1 1 2
0 97
+
THE
+
6
8
4
tCL'IXl0'
The kinetic data for several Pt(I1) complexes in water indicate t h a t the effectiveness of NO*-as a reagent, as compared to C1-, is much greater for the neutral and anionic complexes, e.g., [Pt(NH3)2C12] and [PtCl4l2-, than for cationic complexes. Indeed, for [Pt(dien)H20I2+the reactivity order is inverted. A comparison of kz values between the substitution and chlorine exchange reactions is given in Table IV. The ratio k2s02-,/kzC1- is of the order of magnitude found for positively charged complexes in water. l 3 T h a t NOz- can act as an electrophilic agent can easily be understood if one considers that it has an empty antibonding a-orbital where charge can be transferred from the metal atom. If 14 of the 18 outer shell electrons of the nitrite ion are p u t in its bonding orbitals and lone pair orbitals, there are 4 electrons left for occupation of three a-orbitals. These are orbitals XP,N (bondarising from the combination (pz, pz,) ing, filled), pzl -. p72 (nonbonding, filled), and X(p,l pZz) - pZx (antibonding, empty), written in order of increasing energy. The antibonding orbital is probably of too high an energy to combine with the a-platinum orbitals. However, the drift of electrons in the N -+ Pt u-bond will cause the energy of the antibonding or-
[CONTRIBUTION FROM
0.3
N
+
Fig. 4.-Graphical derivation of kl using stationary state treatment ( Y = NOz-): a, [Y] = 0.1; b, [ Y ] = 0.15; c, [ Y ] = 0.2; d, [Yl = 0.25.
bital to be lowered, becoming thus accessible for bond formation with platinum. This empty orbital must be more Igcalized on the less electronegative nitrogen atom. In this way the electrophilic nature of the nitrite ion may arise from a sort of synergic O ~ N& P t r
interaction. However, when the platinum atom is bonded to strong a-electron acceptor ligands, the energy of its a-electrons will be lowered and the aattack of the nitrite ion will become less favorable, The results presented here are consistent with the assuniption made that a-bonding may be of importance in determining the effectiveness of NO2- as an electrophilic reagent. Acknowledgments.-We thank Professor F. Basolo for helpful discussions and for his interest in this work, Professor R . G. Pearson for helpful suggestions, and Dr. 11.1. L. Tobe for stimulating discussions. This work was supported by the Italian Council for Research (C.N.R. Rome).
MOORELABORATORY O F CHEMISTRY, AMHERST COLLEGE, AMHEPST,MASS.]
Anation of the i!rans-Aquonitrobis-(ethylenediamine)-cobalt(III) Ion in a Tetramethylene Sulfone-Water Mixture1 BY C . H. LANGFORD AND M. P.
JOHNSON
RECEIVEDAUGUST26, 1963 The rates of entry of the anions C1-, Br-, SO3-, and S C N - into t r a n s - [ C o ( e n ) 2 ~ 0 ~ 0 H * ](en = ethylenediamine) were found t o differ by no more than a factor of four in the solvent 7.8yGaqueous tetramethylene sulfone. The reactions were nearly zero order in the anion concentration Conductance d a t a on a model system strongly suggest t h a t the observed reactions take place within outer-sphere complexes. The insensitivity of the rate t o changes in the nature of the entering group suggests no more than weak bonding of the entering group t o cobalt in the transition state. ++
Introduction Hydrolysis reactions of Co(111) amine complexes have been the subject of intensive study,2 b u t the reverse reaction, "anation," has received much less attention. Yet anation has not only its intrinsic in( 1 ) From a thesis b y M . P. J. submitted in fulfillment of a requirement for t h e B.A. degree with honors, Amherst College, 1963. This work was supported by a a r a n t from t h e Research Corporation and presented a t t h e 145th National Meeting of t h e American Chemical Society. N e w Y o r k , K . Y . , Sept. 8-13. 1963. ( 2 ) F . Basolo and R . G. Pearson, "Mechanisms of Inorganic Reactions," John Wiley and Sons, I n c . , New York, N. Y . , 1958, Chapter 3.
terest but also offers information about the transition state of the hydrolysis process through application of the principle of microscopic reversibility. The kinetics of reaction 1 in aqueous solution using the cis isomer were investigated by Basolo, Stone,
+
+
[Co(en)2h-O20H*I++ X - + [Co(en)*NOzX]+ H 2 0 ( 1 )
Bergmann, and P e a r ~ o n . ~They found t h a t the reaction was first order in the complex, first order in the (3) F Basolo, B D Stone, J G Bergmann, and R G Pearson J A m Chem Soc , 1 6 , 3079 (1954)
