Unbridged Pt-III dimers, cis-and trans-[PtCl3L2](2) (L = (E)-HN=C(OMe)Me, cis-2 and trans-2), are formed, as transient species, in the oxidation with excess chlorine of the Pt-II compounds cis- and trans-[PtCl2L2] (cis-1 and trans-1) to the corresponding Pt-IV species, cis-and trans-[PtCl4L2] (cis-3 and trans-3). Complexes cis-2 and trans-2 are rather stable at low temperature (<0 and -20 degrees C for cis-2 and trans-2, respectively), while at higher temperature (20 degrees C) they readily oxidize to Pt-IV (tau(1/2) = 80 and 5 min for cis-2 and trans-2, respectively). The structures of the Pt-III species have been solved by single-crystal X-ray diffraction. The compounds are neutral dimers composed of two square-planar PtCl2L2 units perpendicularly connected by a Pt-Pt bond and capped by axially coordinated chloride ligands. Bond distances and angles within the equatorial PtCl2L2 units are normal. The axial Pt-Cl bond lengths are ca. 0.14 Angstrom longer than the equatorial ones, indicating a strong trans influence of the intermetallic bond. The intermetallic bond distances are typical of either unbridged or pyrophosphite four-bridged Pt-III dimers, while they are much longer than those found in other Pt-III complexes with either two (>0.10 Angstrom) or four bridging ligands (>0.20 Angstrom). In cis-2, the iminoether ligands are in equivalent pairs, and a 90 degrees rotation of one platinum subunit with respect to the other about the Pt-Pt vector interchanges the Ligand positions and converts the complex into its enantiomeric form. Both enantiomers are found in the crystal and in solution, where a Delta G* of 16.9 +/-0.2 kcal mol(-1) was calculated for the interconversion process (H-1 NMR). The formation of intermediate Pt-III dimers appears to be a relatively common although overlooked, feature of platinum chemistry. The oxidation of Pt-II to Pt-IV requires the removal of two electrons by the oxidant (an acid), assisted by a ligand (a base) entering the coordination sphere from the opposite side. A second Pt-II unit can act as a base contributing two d(z)(2) electrons and binding face-to-face from the axial direction. The resulting dimer completes the coordination sphere of the adjoined platinum unit by binding an extra ligand.