text: Sulfur dioxide forms complexes with many transition metals.  Most numerous are complexes with metals i in oxidation state 0 or +1.
  In most cases SO binds in monodentate fashion, attaching to the metal through sulfur.  Such complexes are further subdivided according to the planarity or pyramidalization at sulfur.   The various bonding modes are:
  *η-SO, planar (meaning that the MSO subunit forms a plane). In such complexes, SO is classified as a 2e donor complemented by pi-back bonding into the empty p orbital localized on sulfur.
  *η-SO, pyramidal (meaning that the MSO subunit is pyramidal at sulfur). In such complexes, SO is classified as a pure Lewis acid.  The structure is similar to that for conventional Lewis base adducts of SO.
  *η-SO.  Both S and one O centre are attached to the metal.  The MSO subunit is pyramidal at sulfur.  This bonding mode is more common for early metals, which are typically strongly pi-donating. 
  *η-SO, O-bonded.  In such cases, SO attaches to a metal via one of its two oxygen centres. Such complexes are prevalent for hard metal cations such as Na and Al. In these compounds the M-O interaction is usually weak.
  More exotic bonding modes are known for clusters.