Znorg. Chem. 1980, 19, 3183-3185 rearrangement of eq 3 as a function of K gave (5) and (6). kl
+ k2KA[H+]-' = k,K(K[X-] + l)-'
(5)
+ k-&~'[H+l-' = k,,(K[X-] + I)-' (6) The plots of k,K(K[X-] + l)-' and k,(K[X-] + l)-' against k-1
[H+]-' were linear as shown in Figures 1 and 2 and gave the values kl = 0.10 (NCS-) and 0.20 M-' s-l (N3-), kzKA= 42 X s-l (N3-), kl = 0.024 (NCS-) (NCS-) and 3.8 X and 0.001 s-l (N3-), and k & ~ ' = 52 X (NCS-) and 1.1 X M s-l (N3-).
3183
Contribution from the Department of Chemistry, Northwestern University, Evanston, Illinois 60201
Comparisons of Ir-CI Stretching Frequencies and 31P Chemical Shifts in tran~-IrCl[P(C~H~)~l~L Complexes K. Dah1 Schramm, T. H. Tulip, and James A. Ibers*
Received March 6, 1980
Many techniques have been used to probe the trans and, recently, cis (bond-weakening) influences of ligands in transition-metal complexes.' For square-planar Pt(I1) complexes in particular extensive direct ( s t r u c t ~ r a l )and ~ ~ ~indirect Discussion (spectroscopic) information has been amassed. These indirect indicators include Pt-halide and -hydride stretching freAlthough there are two replaceable aquo ligands in the quen~ies:.~ 195Pt-31Pand -'H coupling and lg5Pt complex [Cr(~alen)(H~O)~]+, only monosubstitution occurred and 'H chemical ~hifts.89~ Although analogous square-planar on the stopped-flow time scale. This is consistent with the Ir(1) complexes have attracted much attention,1° trans-influwidely differing Cr1ILOHZbond lengths in the complex, viz., ence data for these compounds are scarce. Many of the 1.923 and 2.085 With the conjugate base of [Cr(salspectroscopicindicators available in Pt(I1) systems do not apply en)(Hz0)2]+,the observed single-stage kinetics is to be exto Ir(1) complexes. In the absence of structural results, of pected because the direct substitution of OH- by other nuwhich there are only a few reports, we have been forced to use cleophiles is considered unlikely in this class of compounds. the Ir-C1 stretching frequency as our sole indicator of trans The observed direct dependence of the forward rates on [X-] influence in complexes of the type ~rans-IrCl(PPh~)~L [Ph = at any [H'] does not provide compelling evidence for dissoC,H,; L = NzAr+,N2C5X4(Ar = aryl; X = C1, Br)].11,12 In ciative mechanisms for the reactions of either Cr(sa1an effort to find additional trans-influence probes for these en)(H20)2+or its conjugate base, though X-ray structure data complexes, we have examined their 31PN M R chemical shifts. would suggest dissociative processes. However the similarity We reasoned that within a closely related series of complexes of the kl terms for the Cr(salen)(H20),+ reactions with NCSdifferences in this parameter will arise predominantly from (0.10 M-' s-l) and N3- (0.20 M-' s-l), after making allowance variations in the inductive-shieldingcontribution of the metal. for the statistical factor for N3-,9 is in keeping with a dissoThese shieldings should reflect relative metal-centered electron ciative Id-type process with small ion-pairing constants. Asdensities which, although not themselves direct measures of sumption of low ion-pairing constants for the reactions of the trans influences of the ligands, will be closely related as uninegative ions like NCS- and N3- with C r ( ~ a l e n ) ( H ~ O ) ~ + both are functions of the effective L M charge transfer. and its conjugate base will be consistent with the low charges The obvious ease with which the 31P N M R method can be on the complex reactants. The reverse rates for the complexes applied to complexes difficult or impossible to isolate or stable [Cr(salen)(H,O)X] with X- = NCS- (0.024 s-l) and N3only at low temperature and even to mixtures of complexes (0.001 s-l) and their equilibrium constants [NCS- (2.2 M-l) makes it a very attractive alternative to infrared spectroscopy and N3- (50 M-l)] are also in keeping with a dissociative which generally requires isolable, relatively pure solids. Here mechanism involving rate-limiting M-OH2 bond dissociation. we describe the results of our comparisons of Ir-C1 stretching On the other hand, magnetic measurements on the conjugate frequencies and 31Pchemical shifts for a series of complexes, base of C r ( ~ a l e n ) ( H ~ Oas ) ~a+solid indicate a five-coordinate truns-IrC1(PPh3),L. [Cr(salen)OH] structure in the ground state.7 If the conjugate base were to be six-coordinate in solation and the mechanism Experimental Section involved rate-limiting loss of the sixth ligand, viz., water, the All procedures were carried out under inert atmosphere. The two similarly charged anions NCS- and N3- should have complexes tr~ns-IrCl(PPh~)~L, where L = CO,I3CS,I4N2,I5C2H4,I6 similar kzKAterms. The values obtained for k2KA(4.2 X were prepared by literature PPh3,17NO+,'8N2Ar+,liand N2C5C14,12 M s-l for NCS- and 3.8 X M s-l for N q ) were markedly different. On the other hand, if the conjugate base were to Appleton, T. G.; Clark, H. C.; Manzer, L. E. Coord. Chem. Rev. 1973, be a five-coordinate [Cr(salen)OH] complex in solution also, 10, 335. the nucleophiles such as N3- and NCS- may undergo bimo(a) Mason, R.; Meek, D. W. Angew. Chem., Int. Ed. Engl. 1978, 17, 183. (b) McWeeny, R.; Mason, R.; Towl, A. D. C. Discuss. Faraday lecular addition reactions to give six-coordinate products with SOC.1969, 47, 20. large variations in the k2 term. In light of the recent reports ManojloviE-Muir, Lj.;Muir, K. W.; Solomun, T.J. Organomet. Chem. on the interesting features of complexes with a five-cdrdinate 1977, 142, 265. Adams, D. M.; Chatt, J.; Gerratt, J.; Westland, A. D. J. Chem. SOC. structure," further work is in progress to distinguish between 1964,134. the two possible mechanisms, viz., nucleophilic substitution Chatt, J. Proc. Chem. SOC.,London 1962, 318. and bimolecular addition for the reactions of the conjugate Tau, K. D.; Meek, D. W. Inorg. Chem. 1979, 18, 3574. (a) Mather, G. G.; Pidcock, A.; Rapsey, G. J. N. J . Chem. SOC.,Dalton base of [Cr(salen)(H,O),]+.
-
Acknowledgment. The authors thank the Director of CLRI for his permission to publish the work and for a grant to D.R.P. Registry No. Cr(~alen)(H~O)~+, 47248-17-9; NCS-, 302-04-5; N
