Although the nuclearity of binary metal carbonyl clusters is usually six or fewer, carbido clusters often have higher nuclearities. Metal carbonyls of the iron and cobalt triads are well known to form carbido derivatives. Examples include [RhC(CO)] and [RuC(CO)]. Carbonyl carbides exist not only with fully encapsulated carbon (e.g., [FeC(CO)]) but also with exposed carbon centres as in FeC(CO) and FeC(CO).