Structure and bonding of the isoelectronic hexacarbonyls [Hf(CO)₆]^2-, [Ta(CO)₆]^-, W(CO)₆, [Re(CO)₆]⁺, [Os(CO)₆]²⁺, and [Ir(CO)₆]³⁺: A theoretical study

Equilibrium geometries, vibrational frequencies, and metal-CO bond dissociation energies of the title compounds have been calculated using quantum-chemical methods at the DFT (B3LYP and BP86) and CCSD(T)//MP2 levels of theory, utilizing relativistic effective core potentials for the metals. The nature of the metal-CO interactions has been analyzed using the CDA method. The theoretically predicted geometries and vibrational spectra at B3LYP, BP86, and MP2 are in good agreement with experimental data. The calculated C-O bond lengths show a regular decrease and the C-O stretching frequencies increase from [Hf(CO)₆]^2- to [Ir(CO)₆]³⁺. The calculated first dissociation energies (FDE) of one CO show the trend [Ir(CO)₆]³⁺ > [Os(CO)₆]²⁺ > [Hf(CO)₆]_2- > [Re(CO)₆]⁺ > W(CO)₆ ≈ [Ta(CO)₆]^-, which does not correlate with the C-O bond length. A remarkable result of the calculations is that the highest FDE is predicted for [Ir(CO)₆]³⁺, which has very little IR→CO π-back-donation. The high FDEs of [Ir(CO)₆]³⁺ and [Os(CO)₆]²⁺ are explained by the strong OC→metal σ-donation, which leads to a large charge transfer from the six CO ligands to the metal. B3LYP and BP86 give bond energies similar to those of CCSD(T) at MP2-optimized geometries. The CDA method shows a regular decrease of metal→CO π-back-donation from [Hf(CO)₆]^2- to [Ir(CO)₆]³⁺. Optimization of the C-O bond length as a function of the Hf-CO distance of [Hf(CO)₆]^2- gives a smooth curve from the equilibrium value, which is longer than in free CO, to the value of free CO. The corresponding curves for single CO dissociation from W(CO)₆ and Cr(CO)₆ have a turning point where the C-O distance is shorter than in free CO. The C-O bond of [Ir(CO)₆]³⁺, which is shorter than in free CO, becomes even slightly shorter when the Ir-CO distance is stretched by up to ∼0.25 Å, before it becomes longer and approaches the value of free CO. The CDA results show that the change in the C-O bond length can be explained by the M→CO π-back-donation and M↔CO repulsive polarization. The repulsive polarization seems to be more important than the π-back-donation for Ir(CO)₆³⁺. The model of metal-CO interactions which is used by Strauss to distinguish between classical and nonclassical carbonyls is supported by the present study.