INDUCED ACCELERATION OF PHOSPHINE EXCHANGE IN METAL-CARBONYLS BY PENDANT GROUPS OF COORDINATED POLYPHOSPHINES - 2 DANGLING PHOSPHINE ARMS ARE MUCH BETTER THAN ONE

The kinetics and thermodynamics of isomerization of the linkage isomer s (OC)(5)W[eta(1)-PPh2CH2CH(PPh2)(2)] (1) and (OC)(5)W[eta(1)-PPh2CH(P Ph2)CH2PPh2] (2) in chloroform have been studied by P-31{H-1} NMR over the temperature range of 10-55 degrees C. Conversion of 1 to 2 is exo thermic (Delta H = -12.25 +/- 0.1 kJ mol(-1)) and is accompanied by a large decrease in entropy (Delta S = -28.2 +/- 0.35 mol(-1) K-1). The reaction proceeds much faster than expected [k = (1.18 +/- 0.01) x 10( -5) s(-1)] at 25 degrees C, and its small activation enthalpy (Delta H double dagger = 92.6 +/- 1.9 kJ mol(-1)) and large negative activatio n entropy (Delta S double dagger = -28.2 +/- 6.2 J mol(-1) K-1) sugges t an associative mechanism. The reaction is 4 orders of magnitude fast er than the isomerization of (OC)(5)W[eta(1)-PPh2CH2CH2P(p-tol)(2)] (3 ) to (OC)(5)W[eta(1)-P(p-tol)(2)CH2CH2PPh2] (4). Exchange of coordinat ed and dangling phosphines is faster than chelation for each of the fo ur complexes, but chelation of 1 or 2 is much faster than chelation of 3 or 4. It appears that the second dangling phosphine arm present in 1 and 2 accelerates the exchange of all metal-attached ligands. Also o bserved is long-range phosphorus-carbon coupling, possibly enhanced by a ''through-space'' interaction, between the short-armed dangling pho sphine and the equatorial carbonyl groups ((4)J(PC) = 3.89 Hz).