The numbers of electrons "donated" by some ligands depends on the geometry of the metal-ligand ensemble.  An example of this complication is the M–NO entity. When this grouping is linear, the NO ligand is considered to be a three-electron ligand.  When the M–NO subunit is strongly bent at N, the NO is treated as a pseudohalide and is thus a one electron (in the neutral counting approach).  The situation is not very different from the η versus the η allyl.  Another unusual ligand from the electron counting perspective is sulfur dioxide.