Crystal and molecular structures of di-µ-bromo-dibromo-trans-bis-(dimethyl sulphide)-dipalladium(II), Pd2Br4(SMe2)2, and tetrabromo-µ-bis(diethyl sulphide)-diplatinum(II), Pt2Br4(SEt2)2

Abstract

The crystal structures of Pd₂Br₄(SMe₂)₂ and Pt₂Br₄(SEt₂)₂ have been determined by single-crystal X-ray analysis. Both crystals are monoclinic, space-group P2₁/n which, with Z= 2, requires both molecules to posses a centre of symmetry. In fact both molecules have a planar skeleton comprising two metal atoms held apart by bridging groups, but whereas in Pd₂Br₄(SMe₂)₂ the bridging groups are halogen, with the sulphur atoms in a trans terminal configuration, in Pt₂Br₄(SEt₂)₂ the metal atoms are symmetrically bridged by the sulphur atoms, and all the halogens are terminal. These structures were determined to corroborate evidence from infrared spectral studies that, despite similar modes of preparation, these crystals possess fundamentally different structures.

In Pd₂Br₄(SMe₂)₂, a= 6·00, b= 10·49, c= 11·47 Å, β= 107° 35′ . The Pd–Br terminal distance is 2·404(4) significantly shorter than the (weighted mean) Pd–Br bridging distance of 2·440(10)Å. The Pd–S terminal distance is 2·30(2), significantly longer than the (weighted mean) Pt–S bridging distance in Pt₂Br₄(SEt₂)₂ of 2·22(1)Å. The carbon atoms of the methyl groups attached to the sulphur atoms are not symmetrically related to the plane of the molecule; one S–C bond lies close to the plane, the other nearly perpendicular to it.

In Pt₂Br₄(SEt₂)₂, a= 7·36, b= 13·45, c= 9·20 Å, β= 104° 05′ . The (weighted mean) Pt–Br terminal distance of 2·385(8)Å is closely similar to the Pd–Br distance in Pd₂Br₄(SMe₂)₂, while the Pt–S distance is significantly shorter. The sulphur-carbon bonds are here symmetrically related to the plane of the molecule.