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 energies; vibrational frequencies, and C-13 NMR chemical shifts of the following 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 follows 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 contributions from the CO to metal sigma-donation and metal to CO pi-back-donation reveals that this trend is the result of a stronger a-donation in the more oxidized systems. An increase in Delta H toward higher 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. The calculated CO stretching frequencies are in good agreement with experiment. 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 positively charged systems. Both experimental and calculated C-13 NMR chemical shifts diminish with increasing oxidation state. It was shown that the stretch of CO on coordination and pi-back-donation have positive (paramagnetic) contributions to the chemical shift, delta, whereas sigma-donation has a negative (paramagnetic) contribution to delta. All factors contribute to the decline in delta with increasing oxidation state, although pi-back-donation is predominant.