text: Bridging carbido ligands can be subdivided into three classes:
  *cumulenic ,
  *metallocarbyne , and
  *polar covalent .
  Cumulenic compounds generally bridge two metal atoms of the same element and are symmetrical. However, there are exceptions to this. 
  In contrast, metallocarbyne compounds are generally constitutionally heterobimetallic, with complexes containing varying coordination geometries being common. These moieties have been able to serve as precursors to elaborate molecular scaffolds such as porphyrin derivatives.
  The polar covalent class is distinguished from metallocarbynes by a very fine line. This carbide-metal interaction is considered labile in nature. Carbon here can be understood fundamentally as being similar to CO ligands, that is, dative (L-type). Although, this class has also been described to some extent being analogous to the behavior of Lewis acid adduct-forming terminal nitrido and oxo complexes e.g. (PMePh)Cl-Re≡N-BCl and tBu(CH)(Br)W=O-AlBr.