Filling a Gap: The Coordinatively Saturated Group 4 Carbonyl Complexes TM(CO)(8) (TM=Zr, Hf) and Ti(CO)(7)

Homoleptic Group 4 metal carbonyl cation and neutral complexes were prepared in the gas phase and/or in solid neon matrix. Infrared spectroscopy studies reveal that both zirconium and hafnium form eight‐coordinate carbonyl neutral and cation complexes. In contrast, titanium forms only the six‐coordinate Ti(CO)(6) (+) and seven‐coordinate Ti(CO)(7). Titanium octacarbonyl Ti(CO)(8) is unstable as a result of steric repulsion between the CO ligands. The 20‐electron Zr(CO)(8) and Hf(CO)(8) complexes represent the first experimentally observed homoleptic octacarbonyl neutral complexes of transition metals. The molecules still fulfill the 18‐electron rule, because one doubly occupied valence orbital does not mix with any of the metal valence atomic orbitals. Zr(CO)(8) and Hf(CO)(8) are stable against the loss of one CO because the CO ligands encounter less steric repulsion than Zr(CO)(7) and Hf(CO)(7). The heptacarbonyl complexes have shorter metal−CO bonds than that of the octacarbonyl complexes due to stronger electrostatic and covalent bonding, but the significantly smaller repulsive Pauli term makes the octacarbonyl complexes stable.