sentences:  It is now recognised that some Zintl phases contain Zintl clusters and that this accounts for the variable stoichiometries.The bonding in many of these clusters cannot be accounted for by classical octet rule involving covalent, 2-centre, 2-electron bonds, as implied by the Zintl rule.The reaction of Ge, Sn or Pb and Na in liquid NH3 in the presence of ethylene diamine (en) gives the Zintl cluster Na4en7Sn9.