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-) > [ReCO)(6)](+) > W(CO)(6) approximate to [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 pi-back-donation, The high FDEs of [Ir(CO)(6)](3+) and [Os(CO)(6)](2+) are explained by the strong OC-->metal sigma-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 pi-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 similar to 0.25 Angstrom, 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 pi-back-donation and M<->CO repulsive polarization, The repulsive polarization seems to be more important than the pi-back-donation for Ir(CO)(6)(3+). The model of metal-CO interactions which is used by Strauss to distinguish between classical and nonclassical carbonyls is supported by the present study.