STERIC CONSTRAINTS AND ADDITION-REACTIONS IN PLATINUM(II) COMPLEXES CONTAINING 2,9-DIMETHYL-1,10-PHENANTHROLINE (ME(2)-PHEN), X-RAY CRYSTAL-STRUCTURES OF [PTBR2(ME(2)-PHEN)] AND [PTI2(ME(2)-PHEN)]

[PtX(2)(Me(2)-phen)] complexes (X = Cl (1), Br (2), I (3); Me(2)-phen = 2,9-dimethyl-1,10-phenanthroline) react with dimethyl-sulfoxide 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 H-1 NMR spectroscopy and found to be strongly dependent upon the bulkiness of the halogen atoms; log KI increases linearly with the van der Waals radius of the halogens. The crystal structures of the complexes [PtBr2(Me(2)-phen)] (2) and [PtI2(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) Angstrom, beta = 93.74(2)degrees, 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) Angstrom, beta = 94.49(3)degrees, 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.