The coordination chemistry of the four phosphines, P{C₆H₃(o-CH₃)(p-Z)}₃ where Z = H (1a) or OMe (1b) and P{C₆H₃(o-CHMe₂)(p-Z)}₃ Z = H (1c) or OMe (1d) with platinum(II) and palladium(II) is reported. Mononuclear complexes trans-[PdCl₂L₂] (L = 1a,b) and trans-[PtCl₂L₂] (L = 1a–c) have been prepared and the crystal structures of trans-[PdCl₂(1b)₂] and trans-[PtCl₂(1c)₂] as their dichloromethane solvates have been determined. The structures show that in these complexes, the ligands adopt g⁺g⁺a conformations. Examination of the Cambridge Structural Database confirms this to be one of only two conformer types that tri-o-tolylphosphines adopt and the only viable conformer in 4 and 6 coordinate complexes. The binuclear complexes trans-[Pd₂Cl₄L₂] (L = 1c,d) are formed even when an excess of the bulky 1c,d is used in the synthesis and the crystal structure of trans-[Pd₂Cl₄(1c)₂] as its chloroform solvate is reported. Reaction of [PtCl₂(NCBu^t)₂] with 1b–d in refluxing toluene gave the cycloplatinated species [Pt₂Cl₂(L − H)₂] where L − H is phosphine 1b–d deprotonated at one of the ortho-methyl carbon atoms. Variable temperature ³¹P and ¹H NMR spectroscopy reveals that all the complexes reported are fluxional. The processes are analysed in terms of restricted P–C and P–M rotations that give rise to diastereoisomeric rotamers because of the helically chiral orientations of the aryl substituents. For the complexes of the bulky ligands 1c,d, rotation about the P–C bond is slow on the NMR timescale even up to 75 °C. The crystal structure of the cyclometallated complex [Pt₂Cl₂(1d − H)₂] has been determined.