Synthesis and characterization of five-coordinate platinum(II) complexes [Pt(2,9-dimethyl-1,10-phenanthroline)(SnRnX3-n)X(olefin)](X=Cl or Br; R=Ph or Me). Molecular structure of [Pt(2,9-dimethyl-1,10-phenanthroline) (SnPh2Cl)Cl (ethylene)]