PI-ACIDITY OF THIOETHERS AND SELENOETHERS - TRUTH OR FICTION - A COMPARATIVE DENSITY FUNCTIONAL-STUDY

Using density functional theory (DFT) within the local density approximation (LDA), the electronic and geometrical structure of octahedral (OC)5Cr-L with L - CO, NR3, PR3, AsR3, SR2, SeR2 (N, P, As, R = F, H; S, Se, R = F, H, Me), and OR2 (R - H, Me) were studied. All structures have been optimized in C(s) symmetry. The resulting geometries are rationalized in terms of orbital interactions. The metal-ligand bond energy has been determined for all systems in calculations where nonlocal corrections were added to the LDA energy expression. It was possible to break down the total metal-ligand bond energy into contributions stemming from sigma-donation and pi-back-bonding; hence, it is possible to rank thioethers and selenoethers in terms of a-donor and pi-acceptor ability. In general, thio- and selenoethers are rather poor pi-acceptors and weak sigma-donors. The methyl-substituted chalcogenoethers are better sigma-donors than the chalcogenhydrides. It is found that the fluoro-substituted chalcogen systems exhibit good pi-acceptor abilities, comparable to those found for PF3. Surprisingly, the fluorine substitution does not significantly decrease the sigma-donor ability. The Cr-E bond energies increase in the order Cr-EH2 < Cr-EMe2 < Cr-EF2 (E = O, S, Se).