The critical re-evaluation of the aromatic/antiaromatic nature of Ti-3(CO)(3): a missed opportunity?

The nature of bonding and aromaticity of Ti-3(CO)(3), a mill-shaped metal-carbonyl complex, is studied carefully. A unique bonding mechanism between metal and carbonyl groups is found in this species. Ti-3(CO)(3) is an example of a metal-carbonyl complex with prominent metal to carbonyl donation. Moreover, it is proven that not only is Ti-3(CO)(3) not an antiaromatic complex but also it is the first synthesized example of d-block, sigma+pi aromatic species. A quick survey among the first row of transition metals in the periodic table shows that other local minima with similar structures and aromaticity are present and Ti-3(CO)(3) is the first synthesized species of an unknown family.