The reaction of 2 equiv of LiSeCC-n-C5H11 (1) with cis-PtCl2(Ph3P)(2) (2) gives a mixture of the cis and trans isomers of Pt(Ph3P)(2)(SeCC-n-C5H11)(2) (3), which slowly isomerizes in CH2Cl2 to the preferred trans form trans-3. The closely related cis-[Pt(dPPf)(2)(SeCC-n-C5H11)(2)] (4) (dppf = bis(diphenylphosphino)ferrocene) was prepared by a similar metathetical reaction using the platinum chloride complex of the chelating dppf to impose the cis geometry. The structures of the cis and trans complexes have been investigated in solution by heteronuclear NMR ((31)p, Se-77, and Pt-195) and, in the cases of trans-3 and 4, characterized in the solid state by single-crystal X-ray diffraction. Changing the coordination geometry from cis to trans induces significant changes in the structural and spectroscopic parameters, which do not comply with the previously anticipated donor-acceptor properties of selenolate ligands.