Bridging-methylene ligands

Like CO, carbenes can act as bridging ligands, usually accompanied by a metal-metal bond (11). In bridging, carbenes lose some of their unsaturation, and therefore the very high reactivity of their mononuclear analogs. Fischer methylenes are very reactive and barely isolable, while bridging methylenes are well known and relatively stable. 

The pyrophosphite bridged lantem-t3q)e platinum(II) complexes are known to give strong emissions. Analogous complexes with methylene-bis(phosphonite) as a bridging ligand are also emitters. Their electronic structures and associated reactivities have been extensively studied and are summarized in a review article (8). Some further information... 

Like CO, CR2 can act not only as a terminal but also as a bridging ligand. On the traditional model, when CO or CR2 bridge, a metal-metal bond is usually present (11.12 and 11.13). In bridging, the carbene carbon moves from tricoordinate sp toward tetracoordinate sp. Fischer methylenes are rare, while the bridged form is better known and less reactive. Bridging carbenes can be made from diazomethane (Eq. 11.35). 

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