The ionic methylplatinum(II) complexes [Pt(Me)(L)(dmphen)]X (dmphen = 2,9-dimethyl-1,10-phenanthroline, L = Me2SO, X = PF6-1a, BF4- 1b, CF3SO3- 1c, ClO4- 1d, B(C6H5)(4)(-) 1e, [B(3,5-(CF3)(2)C6H3)(4)](-) 1f, L = n-Bu2SO, X = CF3SO3- 1g; L = PPh3, X = PF(6)(-)2a, BF4- 2b, CF3SO3- 2c, ClO4- 2d, B(C6H5)(4)(-) 2e, [B(3,5 (CF3)(2)C6H3)(4)](-) 2f; X = CF3SO3-, L = CyNH2 3a, i-PrNH2 3b, 2,6-Me(2)py 3c, EtNH2 3d, AsPh3 3e, dimethylthiourea (Me(2)th) 3f and the uncharged [Pt(Me)(X)(dmphen)] (X = SCN- 4a, SeCN- 4b) complexes have been synthesized and fully characterized. In chloroform, as well as in acetone or methanol, complexes 1a-1g, 2a-2h (X = Cl- g, NO2-h, formed "in situ"), acid 3e show dynamic behavior due to the oscillation of the symmetric chelating ligand dmphen between nonequivalent bidentate modes. All the other compounds feature a static structure in solution. The crystal structure of 2a shows a tetrahedral distortion of the square planar coordination geometry, a loss of planarity of the dmphen ligand, and, most notably, a rotation of the dmphen moiety, around the N1-N2 vector, to form a dihedral angle of 42.64(8)degrees with the mean coordination plane. The hexafluorophosphate ion lies on the side of the phenanthroline ligand. The interionic structures of 2a, 2b, and 2f were investigated in CDCl3 at low temperature by H-1-NOESY and F-19{H-1}-HOESY NMR spectroscopies. Whereas PF6- (2a) and BF4-(2b) show strong contacts with the cation [Pt(Me)(PPh3)(dmphen)If, being located preferentially on the side of the phenanthroline ligand, the [B(3,5-(CF3)(2)C6H3)(4)](-) (2f) ion does not form a tight ion pair. The dynamic process was studied by variable-temperature NMR spectroscopy for la-lf and 2a-2h in CDCl3. The activation energies Delta (double dagger)G(298) for the sulfoxide complexes la-lf are lower than those of the corresponding phosphine complexes 2a-2f by approximate to 10 kJ mol(-1). The nature of the counteranion exerts a tangible influence on the fluxionality of dmphen in both series of complexes 1 and 2. The sequence of energies observed for 2a-2h encompasses an overall difference of about 16 kJ mol(-1), increasing in the order Cl-approximate to NO2- much less than CF3SO3- < ClO4- < B(C6H5)(4)(-) < BF4- approximate to PF6- < B(3,5-(CF3)(2)C6H3)(4)(-). Acetone and methanol have an accelerating effect on the flipping. Concentration-dependent measurements, carried out in CDCl3 for 2a with n-Bu4NPF6 and the ligands dmphen, n-Bu2SO, sec-Bu2SO, and sec-Bu2S showed that the rate of the fluxional motion is unaffected by added n-Bu4NPF6, whereas in the other cases this increases linearly with increasing ligand concentration, according to a pattern of behavior typical of substitution reactions. Dissociative and associative mechanisms can be envisaged for the observed process of flipping. Dissociation can be prevalent within the ion pair formed by a "noncoordinating" anion with the metallic cationic complex in chloroform. Among the possible associative mechanisms, promoted by polar solvents or by relatively strong nucleophiles, a consecutive displacement mechanism is preferred to intramolecular rearrangements of five-coordinate intermediates.