Palladium complexes anation reactions

Kinetic studies on palladium complexes of dien, and of its more bulky N-substituted forms, continue to furnish valuable mechanistic information. Iodide anation reactions of [Pd(OH2)(R5-dien)] (R = H, Me, or Et) at pH values between 7 and 12 throw light on previously reported deviations in kinetic data above pH The value of kobs increases with increasing [I ], and decreases with increasing steric bulk of Rs-dien and increasing pH. For pH values up to 9, plots of kobs against [I ] are linear and pass through the origin, and can be accounted for by the reactions of Scheme 2 where [Pd(OH)(R5-dien)] is kinetically inert. 

Unidentate Entering Groups. Elding has continued his comprehensive study of the aquation and halide anation reactions of palladium(ii) halide and aquo-halide complexes with a report on the reactions of cis- and /ru 5-[PdX2(H20)2] (X = Cl or Br). The Pd complexes react ca. 5 x 10 —5 x 10 times faster than the analogous Pt complexes, and the activation enthalpies are 8—10 kcal mol smaller for Pd (see Table). The relative trans effects (C1/H20 and Br/H20) are six times smaller for Pd than for Pt, and the cis effects (H2O/CI and H20/Br) somewhat greater. Equilibrium constants for the cis-trans equilibria in [PdX2(H20)2] are 2.1 0.3... 

Similar kinetic rate laws have recently been found for the reaction between [PdCl(dien)]+ and inosine, i, k-i, and ks all being of similar magnitude (32). With adenosine instead of Ino, A3 is diminished to the extent that Ai and A 1 can be treated as a pre-equilibrium, and with the unreactive nucleobase uridine, even the reverse anation step, A 3, can again make a contribution. This type of complication has been found in reactions of palladium-dien complexes with the common buffering agent tris(hydroxymethyl)aminomethane (33), which acts as a poor nucleophile. 

Activation parameters for the anation by chloride of the sterically hindered [Pd(MeEt4dien)(OH2)] + have been determined using a newly developed high-pressure stopped-flow instrument for moderately fast reactions. A negative activation volume and a zero activation entropy are compatible with an associative mechanism, probably 7a. This study is the first in a series in which the effect of pressure on such relatively fast reactions will be studied. A summary of published and current work on pressure effects on the kinetics of sterically hindered palladium(ii) complexes by the Frankfurt group has appeared. ... 

Table Rate constants (25 °C) and activation parameters for anation and aquation reactions of palladium(ii) and platinum(n) complexes from reference 60). Ionic strength 1.00 mol 1 (Pd) and 0.50 mol 1 (Pt). Values of in kcal mol (with standard deviation 1). Values of A5 in cal mol with standard deviation 3)...

Palladium(n).—Solvolysis and Anation. Kinetics of solvolysis of sterically hindered (pseudo-octahedral) palladium(ii) complexes catalysed by mercury(ii) have been monitored in methanol. A relationship holds between the catalytic efficiency and the stability constant of the complex between the mercury(ii) and the leaving group. Reaction of Pdaq with ethene is thought, by analogy with earlier results on the reaction between Pd q and quinone, to be the key step in the kinetics of oxidation of ethene to acetaldehyde catalysed by palladium(ii) in the presence of phospho-molybdovanado-heteropolyanions. ... 

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