Complex kinetics of "simple" substitution reactions of Os-3(CO)(9)(mu-C4Ph4) with smaller P-donor nucleophiles

The osmacyclopentadiene ring in the triangular Os-3 cluster Os-3(CO)(9)(mu- C4Ph4) bridges two of the Os atoms and is found to activate associative att ack at the third Os atom (which is in an Os(CO)(4) moiety) by a factor of s imilar to 10(9) compared with reactions of the parent cluster Os-3-(CO)(12) . The overall reactions in heptane of 17 P-donor nucleophiles with Tolman c one angles theta less than or equal to 143 degrees lead to substitution at the Os(CO)(4) center in three rapid but kinetically observable stages: (i) reversible attack by a nucleophile to form an adduct in which an Os-Os bond . in the cluster has been broken, (ii) loss of CO and formation of a new cl osed Os-3 cluster, and, usually, (iii) isomerization of the initially forme d cluster to form the final substituted product. The dependence of the rate s on the sigma-basicity and size of the nucleophiles shows that the standar d reactivity toward adduct formation is very high, only high nuclearity clu sters that contain encapsulated C atoms being known to react faster. Nucleo philes with theta less than or equal to 120 +/- 4 degrees react at rates th at are independent of their size and that increase substantially with their sigma-basicity. When the cone angles are larger, steric retardation is obs erved, Equilibrium constants for adduct formation by 14 of the nucleophiles were obtained from the rates of adduct formation and loss of L from the Os -3(CO)(9)L(mu-C4Ph4) adducts and by two other methods. Loss of CO or L from the adducts becomes slower the greater the net donicity of the ligands but is essentially independent of ligand size. Rates of isomerization of the i nitial product are not very precise and are not appreciably sensitive to th e nature of the ligands. Activation parameters were obtained for reactions involving five of the nucleophiles, and these, together with the magnitudes of the electronic and steric effects, can lead to estimates of the contrib utions of bond making and bond breaking in the transition stales.