[PtX 2(Me 2-phen)] complexes (X Cl ( 1), Br ( 2), I ( 3); Me 2-phen=2,9-dimethyl1,10-penanthroline) react with dimethylsulfoxide, dimethylsulfide and nitrosobenzene to give the addition products [PtX 2(Me 2-phen)(L)] (L DMS, 1–3a; DMSO, 1–3b; PhNO, 1–3c) containing mono-coordinated Me 2-phen. The equilibrium constants for the addition reactions ( K f) have been determined by 1H NMR spectroscopy and found to be strongly dependent upon the bulkiness of the halogen atoms; log K f increases linearly with the van der Waals radius of the halogens. The crystal structures of the complexes [PtBr 2(Me 2-phen ( 2) and [PtI 2(Me 2-phen)] ( 3) have been determined by X-ray diffraction methods. Crystal data for 2: monoclinic, space group P2 1/ n, a = 12.155(4), b = 7.933(2), c = 14.522(2) A ̊ , β = 93.74(2)°, Z = 4, R = 0.046 . Crystal data for 3: monoclinic, space group P2 1/ n, a = 12.542(2), b = 7.957(1), c = 14.976(3) A ̊ , β = 94.49(3)°, Z = 4, R = 0.036 . The distortions in the square-planar coordination geometry are compared to those found for the analogous chloro species and correlated to differences in reactivity.