text: When tin is combined with an alkali or alkaline earth metal some of the compounds formed have ionic structures containing monatomic or polyatomic tin anions (Zintl ions), such as Sn in MgSn or  in KSn.
  Even with these metals not all of the compounds formed can be considered to be ionic with localised bonding, for example  SrSn, a metallic compound, contains {Sn} square pyramidal units.