Platinum metal complexes of potentially chelating alkene–sulphur and alkene–selenium ligands. The synthesis by chalcogen dealkylation and X-ray structures of the dimeric complexes [{PtI(SCH2CH2CMeCH2)}2] and [{PtI(PPh3)(SCH2CH2CMeCH2)}2], and a dynamic nuclear magnetic resonance study of [{PtI(L)(SCH2CH2CMeCH2)}2][L = PPh3, PPh2Me, or As(CH2SiMe3)3]