The reaction of trans-[Ru III (AsPh 3 ) 2 Cl 3 (CH 3 OH)] (green powder) with 2-thiopyrimidine-1,3, HTPYM, in ethanol, produced red crystals of trans,cis,cis-[Ru II (AsPh 3 ) 2 (N,S-2-thiopyrimidinato) 2 ]. The compound has two TPYM − chelating anions in the equatorial plane, whereas the As atoms occupy the apical positions. It is stable in the solid state but the yellow chloroform solutions turn to green quickly in air atmosphere. The Ru–As, Ru–S and Ru–N bond distances average 2.432(1), 2.440(2) and 2.078(6) Å, respectively. The AsPh 3 ligands assume a semi-trefoil C 1 arrangement and have C–H⋯S intra-molecular hydrogen bond type interactions to TPYM − ligands. These latter ligands are also involved in C–H⋯N and C–H⋯S interactions that pair two thiobase ligands via an unusual way. Density functional computational studies on [Ru(AsH 3 ) 2 (N,S-TPYM) 2 ] model molecules show that the cis,cis,trans isomer is more stable than the trans,cis,cis one by some 5kcalmol −1 .