For octahedral compleses t8herates of hydrogen exchange of metal ammines decrease mith decrease in cationic. charge. .Just, the opposite is true of square ammines and this behavior is discussed in terms of &onding anti solv:it,ion rffects. Two rates of hydrogen exchange are observed for cis-Co( XHq)4(NO?)2+ whereas for the trnns isomcr. d l hytlrogens nrr tqiiiwlent. Two rates are also foiind for I't(dien)X + complexes where the more labile hydrogen is that ol);)ositca S . T,ikv\vis(. t l i c hydrogen lability increases with an increase in the "trans" effect. of S.
Hydrogen exchange reactions of S-H bonds are often extremely rapid. However, t.he base-catalyzed hydrogen exchanges of certain metal ammines in acid buffers are slow and can he studied by con~ ~ m t ' i o ntechniques. al The rate of exchange can he followed by measurements in the near infrared on solutions originally containing hydrogen-1 met'al ammines in hydrogen-2 acetate buffer in heavy Reported in this paper are the rates of hydrogen exchange of several acidoamininecotxiIt~(II1~:tridoammiiieplatiiiumiII) complexes. Experimental Preparation of Compounds.-The cobalt(II1) complese9 were prepared by nwthods descrilied in the literat.ure.4 The cornpounds were characterized hy analysis for a t least one element and by a comparison of absorption spectrum wit,h that reported for the known compound. Syntheses and analyses of the platiniim(I1) :tiid palladium( 11) compounds were reported elsewhere.5 Rates of Exchange.--The met,hod used to follow the rate of hydrogen exchange in these systems is the same as t'hat described previously.3 Briefly it, involves measurements of optical density a t 1.65 p to determine t.he increase in OH c-oncent,rationwith time for reaction mist,ures originally containing a hydrogen-1 nirt:d ammine in an acetate buffered heavy water solution. The buffer solution was prepared by t,e and acetic anhydride to itdding anhydrous sodiiim :t( 99.57, DpO to give 0.1 .lf S HI02and 0.1 ill DC2H302. lleasurements mere nxidf. 'i a Reckman Model DK-2 recording spect,rophotometer which was equipped with a cell holder maint:rined :at consta:mt, tcniperature ( 1 0 . 1") b j r circulating m:itcr through it from :t thermostated waterhath. The ratrs of hydrogtn csrliange were determined by plotting log ( H , - H + )71.7. t where H , is the optical density :it, infinite time and H , is the optical density a t time t . The product, of the slopcs of these curves and - 2 . 3 gave the pseudo first-order rate constant il.,bsd, which when divided 1)y [OD-] gave the reportled serond-order rate constants, k . The values of k were det'ermined wit,h a precision of 1smc. The [OD-! calcu1:tted on the basis of a p K , of 5.26 for DC2H a 0 2 and a ICi for D?06of 0.15 X lO-'4 was adjusted in each case for t.he small (never more than 0.2 pH unit) variation in ptI resulting from the addition of thr complex to acetatc buffered HzO solutions.7 As shown in Fig. 1(.i) t h plot ~ of log ( H , - H,) ~ s t .for cis-Co(NH:,)4(h-O:!)~ is not linrar siiggeeting that thc +
.__
(1) A. Berger, A . Loeivenstein and 8. Ifeihooni. J .
87n.
Chem. Soc.,
81, 62 (1959).
( 2 ) J. S. Anderson, H. V. -1.Briscoe and N. F. Spoor, J . Chem. Soc.. 561, 367 (1943): 51. Block and T'. Gold, ibid., 966 (I!&?). (3) J. W. Palmrr. F. Rasolo and R. C:. Pearson, .I. A m . Chem. Soc., 82, 1073 ( m n ) . (4) Gnielin, Handbueh a n o r g . Chem., 58B, (19:W). ( 5 ) F. Basolo. H. B. Gray and R. G. Pearson, J . Am. Chem. SOC., 82, Aug. (1960). (6) S. Korman a n d T'. K. Lahler, ibid., 68, 1403 (1936); R. KinarrIvy and V. IC. Lahfer, ibid.. 83. 3260 (1941). (7) I.. $1. Cohh a n d J . S. Anderson, Trans. Faraday S o c . , 40, 145 (1913).
hj-drogrns ma:- hc exchanging 31 tiifi'ercbnt rat('>. In this< striictiire there are trio typrs of hydrogens, thosr on thv two smmonias frnns to each othtlr fiiit ris t o hoth nitro groiips and those on t,hr two animonim trans to thr nitro group. sliming two different rates of hydrogm cvh:inge, thr, d* were ana1r;zed using nicthotls : ~ p p l i w h lto ~ lirrc:w j)ar:illel rntcsg (Fig. 1(B)).
Results and Discussion Octahedral Complexes.--T):itn in Tnhlc I slionthat the rates (of hydrogen exchnngc for. :icido:rmminecobalt (111) coniplexe,< derrcase i l l the order C O ( N H &+~> Co(S€-13),S2 + > Co ( S F I & +. Such an effect is expected on the basis of electrostatic considerations, the more positive cation containing the more acidic or rearti7.e hydrogens. Infrared studies reported for these complexes shiw :in inT.kBLE 1 HYDROGEN EX('€I.ISGE OF OCT.\IIEDR.\I. I\II.:T.IL AMMIXES IS 0.1 .If I)CI.H:I02-C,H.j0,- ! ~ T - F F E R A T 25'
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+
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+
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. 13 . I(i
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ow
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Ir( I'I"a)6ClLCo(en)33' 2.4 trans-Co(SH3)4(SOz)?+ 0 , 1 5 12 i :: :1 cis-Co(r\"3)4(SO?)?. 12(0.45)" 1 I =k 2' I frans-Co(en)2(N02)?+ . OG3 cis-Coien)2(?;02)? .o n ( o .5 jb trans-Co(en)zF? ,001 Calcnhted from rate at 48.5' :md Ea = 23 ked. bRat,es in parentheses are presiimahly for hytlrogcZn.*t r m x t>o SO?-. See Fig. 1 and t,est for discussion. E:, a n d AS'* values arc for exchange of the less lnhile hgtirogciis.
-
+
crease in t'he X-H &retching frequency with a decrease in cationic charge.9 These results, interpreted as a greater K-€I bond strength, wrrelate the slower rates of hydrogen exchange. The difference in rate is relatively small for each unit of positive charge on the cation. This is in aword with the observations that the acid st,rciigtlis of metal ammines do not decrease very much n-ith a decrease in cationic charge, q., p K , = i.!l : L I ~ 8.1 for l't(KH3)84+ and Pt(NH3'i6(lti-%respective1y.I" 111 contrast to this, t,he r*li:irge ( 8 ) A . A . Frost and R. G. Pearsori, "Kinetirs :tnd >It .John Wilep a n d Sons, I n r , , Neir- York, N. I-.,105:l. 1). 150. (9; G. F.S r a t o s , C. Ciirian and .I. V. Q:iogliono . I . . I in. Ci,r,n. .
