A complex of a transition metal combined with carbon monoxide (CO). In a metal carbonyl, the CO groups form sigma bonds to the metal through lone pairs of electrons on carbon; the metal, in turn, donates electrons to antibonding pi orbitals on CO. In so doing, the metal usually attains the electronic configuration of an inert gas (the 18-electron rule). Thus elemental chromium (Cr⁰), with six valence electrons, combines with six molecules of CO, each donating two electrons, to afford chromium hexacarbonyl (Cr(CO)₆), in which Cr has 18 valence electrons and is isoelectronic to krypton (Kr).