A metal attached to Carbonyl(-CO) ligand(s)

The term carbonyl can also refer to carbon monoxide as a ligand in an inorganic or organometallic complex (a metal carbonyl, e.g. nickel carbonyl). A carbonyl group characterizes the following types of compounds.

carbonyl in electron acceptor ligand

it has empty orbital which can accept electron as well as filled orbital which can donate electrons .

when carbonyl binds with metal it donates the electrons ,metal which is in low oxidation state now has excess electrons and it becomes stable if it back donate some electrons to the carbonyl in its anti bonding orbital forming a pi bond

A material that spontaneously ignites in air below 113°C (45°C), such as fine metal powder, alkali metal, partially or fully alkylated metal or nonmetal hydride, and metal carbonyl

Carbonyl