This review covers over two hundred Pt(II) complexes with a PtP2XY inner coordination sphere, in which the P-donor ligands are organomonophosphines. These complexes can be divided into six groups: PtP2HX (X = O, N, B, Cl, S, Br, Se, Si, or I); PtP2OX (X = N, Cl, S, or Se); PtP2NX (X = CN, Cl, B, S, Br, Se, or Te); PtP2BX (X = F, Cl, S, Br, or I); PtP2ClX (X = S, Se, Si, As, or Te); and PtP2SiX (X = Br or Te). The complexes crystallize in several crystal systems: hexagonal (×1), tetragonal (×1), orthorhombic (×22), triclinic (×78), and monoclinic (×130). There are complexes with cis-configuration and trans-configuration, the latter by far prevails. Besides monodentate ligands, there are also heterobidentate ligands with: O/N, O/S, O/Se, N/S, N/Se, and N/Te, donor sites. These chelating ligands form a wide variety of metallocyclic, four-, five-, and six-membered rings, and the effects of both steric and electronic factors can be seen from the values of the L–Pt–L bite angles. The structural parameters are analyzed and discussed with particular attention to trans-influences.