DISSOCIATION-ENERGIES, VIBRATIONAL FREQUENCIES, AND C-13 NMR CHEMICAL-SHIFTS OF THE 18-ELECTRON SPECIES [M(CO)(6)](N) (M = HF-IR, MO, TC, RU, CR, MN, FE) - A DENSITY-FUNCTIONAL STUDY

Density functional theory has been used to calculate dissociation ener gies; vibrational frequencies, and C-13 NMR chemical shifts of the fol lowing isoelectronic metal hexacarbonyls: [Hf(CO)(6)](2-), [Ta(CO)(6)] (-), W(CO)(6), [Re(CO)(6)](+), [Os(CO)(6)](2+), [Ir(CO)(6)](3+); Mo(CO )(6), [Tc(CO)(6)](+), [Ru(CO)(6)](2+); and Cr(CO)(6), [Mn(CO)(6)](+), [Fe(CO)(6)](2+). The first CO ligand dissociation energy Delta H follo ws the ordering Ir > Re similar to Os > Hf similar to Ta similar to W through the third transition series. A decomposition of Delta H into c ontributions from the CO to metal sigma-donation and metal to CO pi-ba ck-donation reveals that this trend is the result of a stronger a-dona tion in the more oxidized systems. An increase in Delta H toward highe r oxidation state is also apparent for the limited sample of 3d and 4d systems. Within a triad, the 4d metal forms the weakest M-CO bond. Th e calculated CO stretching frequencies are in good agreement with expe riment. Further, CO stretching frequencies, optimized R(C-O) distances , and the calculated contribution to Delta H from the pi-back-donation all reveal the expected decline in pi-back-donation toward the more p ositively charged systems. Both experimental and calculated C-13 NMR c hemical shifts diminish with increasing oxidation state. It was shown that the stretch of CO on coordination and pi-back-donation have posit ive (paramagnetic) contributions to the chemical shift, delta, whereas sigma-donation has a negative (paramagnetic) contribution to delta. A ll factors contribute to the decline in delta with increasing oxidatio n state, although pi-back-donation is predominant.