tricarbonyl

Tricarbonyl refers to a chemical species in which three carbonyl groups (CO) are bound to a central metal atom or are present as a three-CO fragment within a larger complex. In organometallic chemistry, tricarbonyl often denotes the fragment M(CO)3, where M is a transition metal such as iron, manganese, or chromium, as well as the various coordination complexes that contain three CO ligands attached to a metal center.

In terms of structure, the three carbonyl ligands usually form a facial arrangement around the metal in octahedral coordination when three additional ligands complete the coordination sphere. Carbon monoxide ligands are strong-field ligands and participate in back-donation from the metal to the π* orbitals of CO, influencing the electronic properties, reactivity, and stability of the complex. The term tricarbonyl is used both for the isolated M(CO)3 fragments and for complete complexes that retain three CO ligands.

Preparation of tricarbonyl complexes commonly involves modification of higher carbonyl precursors, such as photochemical or thermal loss of CO from species like M(CO)5L or M(CO)6, followed by coordination of other ligands to form stable M(CO)3Lx complexes. They may also be synthesized directly from metal precursors under carbon monoxide atmosphere under appropriate conditions. Well-known examples include tricarbonyl(η4-1,5-cyclooctadiene)iron, Fe(CO)3(η4-COD), and related Mn(CO)3 and Cr(CO)3 complexes with η6-arene ligands.

Applications of tricarbonyl complexes include serving as versatile precursors in organometallic synthesis, as catalysts or catalyst precursors in various transformations, and as models for studying metal–CO bonding. Safety considerations center on carbon monoxide, a toxic gas that requires appropriate handling and ventilation. See also carbonyl complexes and metal carbonyl chemistry.