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Specific force field parameters have been derived through an optimisation procedure based on crystal structure data to describe the geometry of PdII-phenanthroline complexes. The X-ray structure of [Pd(CH3)(3-sBu-phen)2][OTf] is also reported. Molecular mechanics calculations show that intramolecular steric interactions yield severe distortions from a square-planar geometry of the metal atom, both in bischelated [Pd(3-R-phen)2]2+ and monochelated [Pd(CH3)(3-R-phen)2]+ complexes. A complete stereochemical analysis of the possible diastereoisomers of this last complex (R = sBu) allows the rationalisation of its fluxional behaviour observed in solution. Complexes like [Pd(H)(3-tmp-phen)2]+ and [Pd{CH(C6H5)CH2C(O)OCH3}(3-tmp-phen)(CH3OH)]+ (tmp = 1,2,2-trimethylpropyl) have also been investigated as species reasonably involved in the copolymerisation reactions. It is suggested that the remarkable increase of the copolymer molecular weight obtained with the 3-tmp-phen ligand derives from the steric destabilisation of an intermediate species postulated for the termination reaction. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)