A series of new, relatively hydrophilic, N,N-dialkyl-N′-(2,2-dimethylpropyl)thioureas (HLⁿ) (n = 1, N,N-dimethyl-; n = 2, N-pyrrolidyl-; n = 3, N-piperidyl-) and their corresponding cis-[M(Lⁿ-S,O)₂] [for M = Pt(II), Pd(II); n = 1–3] and fac-[Rh(Lⁿ-S,O)₃] (n = 2, 3) complexes have been prepared and characterised. The crystal and molecular structures of fac-tris(N-pyrrolidyl-N′-(2,2-dimethylpropyl)thioureato)rhodium(III) and cis-bis(N-piperidyl-N′-(2,2-dimethylpropyl)thioureato)platinum(II) are reported, as representative examples. The favourable physiochemical properties and ready formation of stable mono-isomeric cis-[Pt(Lⁿ-S,O)₂] and cis-[Pd(Lⁿ-S,O)₂] complexes with these ligands makes these compounds suitable for the reversed-phase high performance liquid chromatography (RP-HPLC) determination of traces of these metal ions in hydrochloric acid, following in situ complex formation in a homogeneous acetonitrile–hydrochloric acid phase and subsequent salt (NaCl) induced phase separation. A study of the effect of ligand structure on the separation behaviour of cis-[M(Lⁿ-S,O)₂] of Pd(II) and Pt(II) complexes has been undertaken, showing that the nature of the ligand has a significant influence on the separation characteristics of these complexes.