Heteropolytopic arsanylthiolato ligands 1-AsPh2-2-SHC6H4. (AsSH), PhAs(2-SHC6H4)(2) (AsS2H2), and As(2-SHC6H4)(3) (AsS3H3) have been prepared by lithiation-electrophilic substitution procedures. The 2:1 reaction of AsSH with NiCl2 center dot 6H(2)O, Na-2[PdCl4], and [PtI2(cod)] (cod = 1,5-cyclooctadiene) in the presence of NEt3 afforded the square-planar complexes trans-[Ni{(AsS)-kappa S-2,As}(2)] (1), cis-[Pd{(AsS)-kappa S-2,As}(2)] (2), trans-[Pd{(AsS)-kappa S-2,As}(2)] (3), and cis-[Pt{(AsS)-kappa S-2,As}(2)] (4). In the cases of nickel and platinum, only one isomer was isolated. With palladium, initially the cis isomer 2 is formed and undergoes slow isomerization to the trans isomer 3 in solution. Small amounts of the trinuclear complex [{PtI(1-AsPh2-p-2-S-C6H4-kappa S-2,As)}(3)] (5) are also formed besides the mononuclear platinum bis-chelate complex 4. Density functional theory calculations support a dissociative mechanism for the isomerization of the palladium(II) complexes.