Formation and isomerization of cis, cis-Ir(H)(2)(-equivalent to-Ph)(CO)(PPh3)(2) and cis, cis-Ir(H)(-equivalent to-Ph)(2)(CO)(PPh3)(2) in oxidative addition of hydrogen and phenylacetylene to trans-Ir(CO)(-equivalent to-Ph)(PPh3)(2)

Oxidative addition of H-R (H-equivalent to-Ph and H-2) to trans-Ir(-equival ent to-Ph)(CO)(PPh3)(2) (2) gives the initial products, cis, cis-Ir(H)-(-eq uivalent to-Ph)(2)(CO)(PPh3)(2) (3a) and cis, cis-Ir(H)(2)(-equivalent to-P h)(CO)(PPh3)(2) (3b), respectively, Both cis-bis(PPh3) complexes, 3a and 3b undergo isomerization to give the trans-bis(PPh3) complexes, trans, trans- Ir(H)(-equivalent to-Ph)(2)(CO)(PPh3)(2) (4a) and cis, trans-Ir(H)(2)- (-eq uivalent to-Ph)(CO)(PPh3)(2) (4b). The isomerization, 3b --> 4b is first or der with respect to 3b with k(1) = 6.37 x 10(-4) s(-1) at 25 degrees C unde r N-2 in CDCl3. The reaction rate (k(1)) seems independent of the concentra tion of H-2. A large negative entropy of activation (Delta S-not equal = -2 4.9+/-5.7 cal deg(-1) mol(-1)) and a relatively small enthalpy of activatio n (Delta H-not equal = 14.5+/-3.3 kcalmol(-1)) were obtained in the tempera ture range 15 similar to 35 degrees C for the isomerization, 3b --> 4b unde r 1 atm of H-2. (C) 1999 Elsevier Science Ltd. All rights reserved.