STRUCTURE AND BONDING OF THE ISOELECTRONIC HEXACARBONYLS [HF(CO)(6)](2-), [TA(CO)(6)](-), W(CO)(6), [RE(CO)(6)](- A THEORETICAL-STUDY(), [OS(CO)(6)](2+), AND [IR(CO)(6)](3+) )

Equilibrium geometries, vibrational frequencies, and metal-CO bond dis sociation energies of the title compounds have been calculated using q uantum-chemical methods at the DFT (B3LYP and BP86) and CCSD(T)/MP2 le vels of theory, utilizing relativistic effective core potentials for t he metals. The nature of the metal-CO interactions has been analyzed u sing the CDA method, The theoretically predicted geometries and vibrat ional spectra at B3LYP, BP86, and MP2 are in good agreement with exper imental data, The calculated C-O bond lengths show a regular decrease and the C-O stretching frequencies increase from [Hf(CO)(6)](2-) to [I r(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 cal culations is that the highest FDE is predicted for [Ir(CO)(6)](3+), wh ich 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 si gma-donation, which leads to a large charge transfer from the six CO l igands to the metal. B3LYP and BP86 give bond energies similar to thos e of CCSD(T) at MP2-optimized geometries, The CDA method shows a regul ar 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 equil ibrium 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) an d Cr(CO)(6) have a turning point where the C-O distance is shorter tha n in free CO. The C-O bond of [Ir(CO)(6)](3+), which is Shorter than i n free CO, becomes even slightly shorter when the Ir-CO distance is st retched by up to similar to 0.25 Angstrom, before it becomes longer an d approaches the value of free CO. The CDA results show that the chang e in the C-O bond length can be explained by the M-->CO pi-back-donati on 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 distinguis h between classical and nonclassical carbonyls is supported by the pre sent study.