Rational synthesis of metalla–sulphur–nitrogen complexes: use of [Me2SnS2N2]2in the preparation of complexes of the type [M(S2N2H)(PR3)2]X(M = Pt or Pd). X-Ray structures of [Pt(S2N2H)(PEt3)2][Me2SnCl3],[Pt(S2N2H)(PBun3)2]2[PF6]Cl, and [{Co(S2N2H)2}2{Ph2P(O)CH2CH2P(O)Ph2}]