Molecular structure of dinuclear complexes of platinum(II). Part II. α-and β-Forms of dichloro-di-µ-thiocyanato-bis(tri-n-propylphosphine)-platinum(II)

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Abstract
[α-Pt2Cl2(PPrn 3)2(SCN)2], (m.p. 154°), forms monoclinic crystals, a= 7·514(2), b= 13·583(4), c= 16·197(4)Å, β= 112·38(2)°, space group P21/c. The β-isomer, m.p. 172°, also forms monoclinic crystals, a= 8·084(2), b= 13·409(4), c= 14·368(5)Å, β= 98·29(2)°, space group P21/n. In both cases the unit cell contains two molecules which are centrosymmetric. Refinement of diffractometer data using 1729 and 1348 independent structure amplitudes led to R 0·054 and 0·038 for the α- and β-isomers respectively. In both isomers the molecule has an approximately planar eight-membered {–Pt–S–C–N–Pt–S–C–N–} ring; in the α-isomer (I) the phosphine groups are trans to the nitrogen atoms of the (SCN) groups, and in the β-isomer (II) they are trans to the sulphur. This principal plane also contains the chlorine and phosphorus atoms, and two α-carbon atoms. There is no evidence of steric strain except in the Pt–NC angles (166° instead of 180°). The lengths of the Pt–S and Pt–N bonds cis and trans to the phosphine group show that this group exerts a static trans-weakening effect which can be explained without invoking π-bonding : Pt–S 2·327(5)trans to Cl, 2·408(4)trans to P, and Pt–N 1·97(1)trans to Cl, 2·08(1)Åtrans to P. The sulphur atom has a similar but smaller trans-effect on the Pt–P and Pt–Cl bonds. There is no satisfactory evidence for a delocalised π-bond system extending over the molecular plane, and the planar configuration is attributed to normal steric interactions.

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