FORMATION OF THE OXANICKELACYCLOPENTENE COMPLEX FROM NICKEL(0), CARBON-DIOXIDE, AND ALKYNE - AN AB-INITIO MO SD-CI STUDY

An ab initio MO/SD-CI study was carried out on the formation reaction of oxanickelacyclopentene, (H3P)Ni-CH=CH-CO(O) 1, from Ni(PH3), CO2, C2H2. Because C2H2 coordinates to Ni(PH3) more strongly than does CO2 by ca. 11 kcal mol-1 at the SD-CI level, coordination Of C2H2 to Ni(PH3) takes place first. The resultant Ni(PH3)(C2H2) reacts with CO2 to yield 1 with no activation barrier and a significant exo-thermicity of 68 kcal mol-1 at the HF level, but a moderate barrier of 30 kcal mol-1 and exo-thermicity of 17 kcal mol-1 at the SD-CI level. 1 takes a three-coordinate T-shaped structure, due to the low-spin d8 electron configuration of Ni(II). Ni(PH3) stabilizes the transition state through a charge-transfer interaction from the occupied d orbital of Ni to the unoccupied pi* orbitals of CO2 and C2H2. The electron re-distribution during the reaction is discussed, based on orbital mixing among the d orbital of Ni and the pi and pi* orbitals of C2H2 and CO2.