A dynamic H-1 NMR study has been carried out on the fluxional motion of the symmetric chelating ligand 2,9dimethyl-1,10-phenanthroline (Me-2-phen) between nonequivalent exchanging sites in a variety of square-planar complexes of the type [Pt(Me)(Me-2-phen)(PR3)]BArf, 1-14, (BArf = B[3,5-(CF3)(2)C6H3](4)). In these compounds, the P-donor ligands PR3 encompass a wide range of steric and electronic characteristics [PR3 = P(4-XC6H4)(3), X = H 1, F, 2, Cl 3, CF3 4, MeO 5, Me 6; PR3 = PMe(C6H5)(2) 7, PMe2(C6H5) 8, PMe3 9, PEt3 10, P(i-Pr)(3) 11, PCY(C6H5)(2) 12, PCy2(C6H5) 13, PCy3 14]. All complexes have been synthesized and fully characterized through elemental analysis, H-1 and P-31{H-1} NMR. X-ray crystal structures are reported for the compounds 8, 11, 14, and for [Pt(Me)(phen)(P(C6H5)(3))]PF6 (15), all but the last showing loss of planarity and a significant rotation of the Me2-phen moiety around the N1-N2 vector. Steric congestion brought about by the P-donor ligands is responsible for tetrahedral distortion of the coordination plane and significant lengthening of the Pt-N-2 (cis to phosphane) bond distances. Application of standard quantitative analysis of ligand effects (QALE) methodology enabled a quantitative separation of steric and electronic contributions of P-donor ligands to the values of the platinum-phosphorus (1)J(PtP) coupling constants and of the free activation energies DeltaG(double dagger) of the fluxional motion of Me2-phen in 1-14. The steric profiles for both (1)J(PtP) and DeltaG(double dagger) show the onset of steric thresholds (at cone angle values of 150degrees and 148degrees, respectively), that are associated with an overload of steric congestion already evidenced by the crystal structures of 11 and 14. The sharp increase of the fluxional rate of Me-2-phen can be assumed as a perceptive kinetic tool for revealing ground-state destabilization produced by the P-donor ligands. The mechanism involves initial breaking of a metal-nitrogen bond, fast interconversion between two 14-electron three-coordinate T-shaped intermediates containing)eta(1)-coordinated Me-2-phen, and final ring closure. By use of the results from QALE regression analysis, a free-energy surface has been constructed that represents the way in which any single P-donor ligand can affect the energy of the transition state in the absence of aryl or pi-acidity effects.