Synthesis and Characterization of Trigonal-Bipyramidal Platinum(II) Olefin Complexes with Chalcogenide Ligands in Axial Positions. X-ray Molecular Structures of [Pt(SMe)2(dmphen)(diphenyl fumarate)], Its Cationic Dipositive Derivative [Pt(SMe2)2(dmphen)(diphenyl fumarate)][BF4]2, and Free Diphenyl Fumarate