Coordination entities are classified based on the number of central metal atoms they contain.

1. Mononuclear: Entities containing a single central metal atom.
2. Polynuclear (Dinuclear, Trinuclear, etc.): Entities containing more than one metal atom.

$Fe(CO)_5$ (Pentacarbonyliron(0)) contains only one iron atom bonded to five carbonyl ligands. Therefore, it is a mononuclear carbonyl. In contrast, compounds like $Mn_2(CO)_{10}$ or $Co_2(CO)_8$ are dinuclear as they contain two metal atoms bridged or bonded directly .