In contrast to the anion radicals of 1,10-phenanthroline (phen) or its 4,7-dimethyl derivative (dmphen) with their B-2(1) ground state, the 3,4,7,8-tetramethyl derivative tmphen(.-) has a (2)A(2) ground state as evident from EPR/ENDOR measurements; The small eta spin population at the potentially metal-coordinating nitrogen centres in the (2)A(2) species as opposed to the B-2(1) State causes distinct spectroscopic effects in corresponding diorganoplatinum(II) complexes. The neutral compound (phen)PtMes(2), Mes = mesityl, has been structurally characterized as a square planar Pt-II complex with tightly protected axial sites: monoclinic, space group P2(1)/c, Z = 4, a = 9.378(1), b = 15.791(2), c = 16.581(1) Angstrom, beta = 103.18(1)degrees. All complexes ((NN)-N-Lambda)PtMes(2), (NN)-N-Lambda = bpy (2,2'-bipyridine), phen, tmphen, dppz (dipyrido[3,2-a :2',3'-c]phenazine) and pdo (1,10-phenanthroline-5,6-dione) are oxidized to EPR-silent platinum(In) species and reduced to anion radical complexes. The latter clearly contain Pt-II and heterocyclic anion radical ligands; however, they exhibit a trichotomous EPR behaviour. Whereas the radical complex [(phen)PtMes(2)](.-) shows a rather low g(iso)-value of 1.983 and distinct anistropy in frozen solution like the analogous complex of bpy(.-), the formally related [(tmphen)PtMes(2)](.-) has an isotropic g-factor (2.0030) much closer to the free-electron value and a distinctly smaller g anisotropy. The phenanthroline-derived radical complexes [(pdo)PtMes(2)](.-) and [(dppz)Pt-Mes(2)](.-) exhibit virtually no contribution from the coordinated heavy metal to the singly occupied MO (b,) which is mainly centred at the heteroatoms outside the phenanthroline pi system.
