Greed is Good - Polydentate Ligands

Modes through which bridging between two different metals can arise, illustrated for simple N-donor ligands. 

The former leads to greater distance between the like-charged metal centres, and is thus likely to be a less demanding process. The latter brings the metal ions much closer together but can only occur, of course, with donor groups that have the potential to provide at least two lone pairs, and that are usually anionic so as to help two like-charged metal centres approach each other closely. Systems where several metal ions are bound in close proximity employ the same basic concepts discussed here. 

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