| Title
| - Dissociation Energies, Vibrational Frequencies, and 13C NMR Chemical Shifts of the18-Electron Species [M(CO)6]n (M = Hf−Ir, Mo, Tc, Ru, Cr, Mn, Fe). A DensityFunctional Study
|
| Abstract
| - Density functional theory has been used to calculate dissociationenergies, vibrational frequencies, and 13C NMRchemical 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+; andCr(CO)6, [Mn(CO)6]+,[Fe(CO)6]2+.The first CO ligand dissociation energy ΔH followsthe ordering Ir > Re ∼ Os > Hf ∼ Ta ∼ W through thethirdtransition series. A decomposition of ΔH intocontributions from the CO to metal σ-donation and metal toCOπ-back-donation reveals that this trend is the result of a strongerσ-donation in the more oxidized systems. Anincrease in ΔH toward higher oxidation state is alsoapparent for the limited sample of 3d and 4d systems.Withina triad, the 4d metal forms the weakest M−CO bond. Thecalculated CO stretching frequencies are in goodagreement with experiment. Further, CO stretching frequencies,optimized R(C−O) distances, and thecalculatedcontribution to ΔH from the π-back-donation all revealthe expected decline in π-back-donation toward the morepositively charged systems. Both experimental and calculated13C NMR chemical shifts diminish withincreasingoxidation state. It was shown that the stretch of CO oncoordination and π-back-donation have positive(paramagnetic) contributions to the chemical shift, δ, whereasσ-donation has a negative (paramagnetic) contributionto δ. All factors contribute to the decline in δ withincreasing oxidation state, although π-back-donation ispredominant.
- Density functional theoryhas been used to calculate dissociation energies, ΔH, vibrationalfrequencies, and 13C NMR chemical shifts of the followingisoelectronic 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+; andCr(CO)6, [Mn(CO)6]+,[Fe(CO)6]2+. ΔHfollows the ordering Ir > Re ∼ Os > Hf ∼ Ta ∼ W through the 5dsystems. It is shown that there are several factors contributingto the decline in 13C NMR chemical shifts with increasingoxidation state, although π-back-donation ispredominant.
|
