Mechanistic information from pressure and temperature effects on the rate of transfer of oxygen-18 from aquopentaamminechromium(III) and aquopentaamminerhodium(III) ions to solvent water

Abstract

The rates of exchange of bound water in (NH3)5MOH23+ with solvent water are characterized by the activation parameters AV* = -5.8 ± 0.2 cm3 mol-1, AH* = 23.2 ± 0.5 kcal mol-1, and AS* = 0.0 ± 1.6 cal deg-1 mol-1 when M = Cr; the corresponding data for M = Rh are -4.1 ± 0.4cm3mol-1,24.6 ± 0.3 kcal mol-1, and +0.8 ± 1.1 cal deg-1 mol-1. These figures suggest an associative mechanism for both reactions. Some theoretical implications of this conclusion are discussed.

Pressure Effects upon the Rates of Aquation of Halopentaamminechromium(III) Ions: Evidence for an Associative Mechanism

“…This can be independently checked for the case X-= NCS-(but not C1-) by plotting the reciprocals of Duffy and Earley's observed pseudo-first-order rate coefficients k,,, against [NCS-I-'; the intercept on the k,,,-I axis gives k, = 3.7 x 10-' s-I at 45", while the slope (= l/kiK) gives K -12 (which is close to the spectrophotonietric value of 10). Our aquo-exchange rate data (5) show that NCS-entry into the first coordination sphere from within the ion pair is thus about 18 times slower than aquo-replacement at 45".…”

“…in which a water molecule from the second coordination sphere (solvation sheath) of the complex displaces an existing aquo-ligand from the first coordination sphere in a synchronous process on a time scale which is short co~xpared with the time scale of the exchange of water molecules in the second coordination sphere with bulk solvent. In this mechanism, as in the dissociative mechanism proposed for the Co(III) system, only the water-molecules of the relatively iiicon~pressible first and second coordination spheres are involved, independently of bulk solvent, so that AV'# is not a function of pressure in either case (4,5). Since it has become cominon practice to assume clissociati~e activation to be operative in octahedral substitutions in general, we have sought further evidence for an associCan.…”

“…On the other hand, the AV" values for the aquo-exchange reactions of Cr(H,0), 3' and C~( N H , ) , O H ,~+ are respectively -9.3 and -5.5 cin3 mo!-' and are apparently pressureindependent (2,4,5). It would be very difficult to rationalize these latter data except in terms of an associative interchange (I,) mechanism (7).…”

“…821 (1973) We have recently shown (1)(2)(3)(4)(5) that the effect of pressure P upon the rates of substitution reactions of transition-metal complexes in aqueoi;s solution can lead to useful mechanistic information if either the reaction is suitably simple or there is an independent criterion with which the derived parameters (the volun~e of activation, AV", and its pressure dependence, if significant) can be compared. Thus, for the ideally simple aquo-exchange reaction of CO(NH,),OH,~+, AV* is + 1.2 cm3 mol-' and is pressure-independent within the experimental uncertainty (6), which suggests a dissociative mechanism.…”

The comparative chemistry of ammine and methylamine complexes of rhodium(III) and cobalt(III)

“…Chloropentaamminerhodium(III), aquopentaamminerhodium(III), and aquopentaamminecobalt(1II) perchlorates were made as described previously (15,16). Chloropentakis(methylamine)wbalt(III) chloride was made from dichlorotetrapyridinecobalt(II1) chloride (17) by the method of Mitzner et al (18), with minor variations in reagent quantities and reaction conditions between successive preparations.…”