Platinum metal complexes of potentially chelating alkene thioether and selenoether ligands: the synthesis and dynamic nuclear magnetic resonance study of [MX 2 {E[(CH 2 ) n CRCR 2 ] 2 }](M = Pt or Pd; X = Cl, Br, or I; E = S or Se; n= 2 or 3; R = H or Me) and the X-ray crystal structure of cis-di-iodo-(5-thianona-1,8-diene)platinum( II ), [PtI 2 {S[(CH 2 ) 2 CHCH 2 ] 2 }]