A pair of internally diastereomeric platinum(R) complex cations containing the orthometalated (S)-(1-(dimethylamino)ethyl)naphthalene and the racemic form of the sulfinyl-substituted ligand Ph2ECH$H#(O)Me ((E-S(O), where E = As, P) has been prepared via two routes originating from (S)-(+)bis(m-chloro)bis[ l-[ 1-(dimethylamino)ethyl]-2-naphthalenyl-CJVldiplatinum(I~. The crystal structure of the arsenic analogue (crystal data: triclinic, P 1, a = 7.950(2) A, b = 17.449(3) A, c = 21.981(4) A, a = 82.67(2)' /I = 87.06(2)", 7= 82.87(2)', Z = 4 and R = 0.0409) shows the ligand being coordinated to platinum(D) in both diasteteomers as a bidentate chelate exclusively via arsenic and sulfur donor atoms. Proton NMR studies also indicate the retention of this As-S coordination in solution. A similar bonding mode was observed for the analogous phosphine complex. The E-S chelation appears to be governed by the electronic properties of platinum. In contrast to the previously documented palladium(R) analogues, the E-O chelation is not observed in the present series of platinum(R) compounds although there will be less severe ligand-ligand interactions if the E-S(O) ligands form six-membered chelate rings in these square-planar systems. The study of ligand-based stereoelectronic effects in transition metal complexes containing s&oxides is an interesting yet infrequently addressed topic. Two of the few notable representations of monodentate sulfoxide complexes come from [Pd(R2S0)4]2+ complexes where steric and electronic factors interweave to produce a variety of complexes that displayed cis-rruns isomerism and the ambidentate nature of the sulfoxide moiety,l-3 and the [MCl(Ph2PCH2CH2PPhZ)(dmso)]+ (M = Pd, Pt) complexes where the sulfinyl-0 coordination of the dmso ligand was brought about by the steric imposition of the diphosphine ligand.4 In contrast, there is a dearth of such investigations for complexes with hetero-ligands incorporating a sulfoxide and a soft arsenic or phosphorus atom. 5~5 Recent complexations of the novel PhZECH2CH2S(O)Me (E = As or P, E-S(O) for short) ligands in our laboratories have nevertheless uncovered an abundant coordination chemistry that was undoubtedly influenced by stereoelectronic factors. Indeed, while the E-S(O) ligands coordinate to gold(I) in a unidentate manner through their E donors in the linear [AuCl(E-S(O))] complexes.7 an E-S coordination mode was observed in the square-planar cis-[MClz(E-S(O))] (M = Pd. Pt) complexes.8 Interestingly, X-ray structural and solution NMR spectroscopic studies revealed an unexpected E-O coordination mode when these ligands were reacted with an optically active ortho-metalated [l-[l-(dimethylamino)ethyl]-2-naphthalenyl-C~palla~um(II) resolving unit.9 Intrigued by this observation, we