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Trimethylplatinum(iv) halides [ (PtXMe,),] react w i t h equimolar amounts of the ligand (MeS),CHCH (SMe),( L) t o give chelate complexes of type [PtXMe,L] as major products and dinuclear complexes [ (PtXMe,),L] as minor components. The chelate complexes in CDCI, solution exist as cis and trans configurational isomers, with the trans predominating. A t temperatures below ca. -3 0 "C, pyramidal inversion of the co-ordinated S atoms is slow o n the I H and lg5Pt time-scales and six invertomer species 2,3,and 4 and cis-I /meso-I and D L ) have been detected, identified, and characterised by 'H and lg5Pt n.m.r. spectroscopy. The pyramidal inversion rates and energies of the co-ordinated S atom pairs in all six invertomers have been quantitatively measured by a combination of 'H bandshape analysis and lg5Pt two-dimensional EXSY experiments. The resulting AG' (298.1 5 K) values lie in the range 54-65 kJ mot-' and are governed primarily b y the orientation of the contiguous unco-ordinated S-methyl group. The X-ray crystal structure of [ Ptl Me,(( MeS),CHCH (SMe),}] depicts the ligand with its co-ordinated S-methyls trans t o each other and trans t o their contiguous unco-ordinated S-methyls. This structure corresponds t o the most populous CDCI, solution invertomer ( t r a n s / ~~-2 ) .Bridge-cleavage reactions of trimethylplatinum(1v) halide tetramers [(PtXMe,),] (X = C1, Br, or I) with dithio-and diseleno-ethers, MeE(CH,),EMe (E = S or Se), have been extensively studied.' When n = 2 or 3, mononuclear chelate complexes of formula [PtXMe,{ MeE(CH,),EMe)] are formed,"" whereas when M = 0 or 1, halogen-bridged dinuclear complexes of type [(PtXMe,),(MeE(CH,),EMe)] are the exclusive product^.^ ' Thus, the principal factor determining

The complexes [PtXMe,{(MeS),CHCH(SMe),}J (X = CI, Br, or I) in C,D,NO, solution exist as trans and cis-I isomers which, in the temperature range 333-423 K, exhibit novel fluxionality. This involves a trans c cis-1 interconversion which proceeds via a mechanism which exchanges all co-ordinated and unco-ordinated S-methyl environments and all Pt-methyl environments. The dynamics of this process were measured b y 'H n.m.r. band-shape analysis and two-dimensional exchange spectroscopy. Both methods yielded energy data in good agreement, AG' (363 K) values being in the range 88.7-91.2 kJ m0I-l for the three halogen complexes. Since AGI values based on S-methyl and Pt-methyl exchanges were identical within experimental error a single concerted mechanism is operating. This is, in essence, a I ,3-metal-pivot process resulting from a

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ChemInform Abstract The Pt(IV) halides (I) react with equimolar amounts of the title ligand (II) to give the chelate complexes (III) as major products and the ligand-bridged dinuclear complexes (IV) as minor components. In CDCl3 solution, the chelate complexes (III) exist as cis and trans configurational isomers, with trans predominating. At temp. below ca. -30 rc C, pyramidal inversion of the coordinated S atoms is low on the 1H and 195Pt time-scales and six invertomer species (viz. trans/DL-1,2,3, and 4 and cis-1/meso-1 and DL) are detected, identified, and characterized by 1H and 195Pt NMR spectroscopy. The pyramidal inversion rates and energies of the coordinated S atom pairs in all six invertomers are quantitatively measured. The X-ray structure of the title complex (III) (X: I) (space group P21/n, Z=4) depicts the ligand with its coordinated S-methyls trans to each other and trans to their contiguous uncoordinated S-methyls. This structure corresponds to the most populous CDCl3 solution invertomer (trans/DL-2).