INFLUENCE OF PI-CONFLICT ON STRUCTURE AND REACTIVITY - COMPARATIVE-STUDY OF ETA(2)-NITRILES AND ETA(2)-ALKYNES AS 4-ELECTRON DONOR LIGANDS IN TUNGSTEN(II) FLUORIDE CARBONYL SYSTEMS

This work describes the synthesis, structures, and reactivity studies of a series of novel eta(2)-nitrile and eta(2)-alkyne complexes in whi ch the sidebound nitrile and alkyne ligands serve as four-electron don or ligands. In these tungsten(II) carbonyl fluoride complexes, both st rong pi-acceptor and pi-donor ligands are extant and therefore pi effe cts dominate the structure and reactivity manifested by these systems. The alkyne and nitrile ligands are oriented cis and parallel to the p i acid carbonyl ligand in these pseudooctahedral complexes. From the s pectroscopic and crystallographic data it is evident that the eta(2)-n itrile ligand functions as a better pi-acid/poorer pi-base than the et a(2)-alkyne ligand. This results in greater electron density at the me tal center for the alkyne complexes compared to the nitrile complexes and has a dramatic influence on the coordination geometry of the flexi ble eta(3)-[C,N,N'] chelate ligand system. For the nitrile complexes, the nitrile is trans to the fluoride and the tridentate ligand adopts a meridional geometry. To avoid pi-conflicts with the fluoride ligand at the basic metal center, the tridentate ligand takes on a facial geo metry in the alkyne complexes with the alkyne cis to the fluoride liga nd. Due to steric constraints, the meridional geometry is observed for compounds containing a rigid eta(3)-[C,N,N'] imine chelate ligand. In contrast to the eta(2)-(4e)-alkyne complexes, the eta(2)-(4e)-nitrile ligand can be displaced by 2 equiv of carbon monoxide.