Tetrakis(thiourea-κS)palladium(II) dithiocyanate

The title compound, [Pd(CH4N2S)4](SCN)2, consists of complex [Pd(TU)4]2+ [TU = thiourea, SC(NH2)2] cations and thiocyanate counter-anions. The PdII cation is situated on an inversion centre and exhibits an almost square-planar coordination by the S atoms of the TU ligands. The complex cations are connected through the thiocyanate ions via N—H⋯N [2.922 (3)–3.056 (3) Å] and N—H⋯S [3.369 (2)–3.645 (2) Å] hydrogen bonds.

M. K. Rauf is grateful to the Higher Education Commission of Pakistan for financial support for a PhD programme.
possessing thioamido binding sites (Akrivos, 2001;Raper, 1996;Cusumano et al., 2005). The ability of TU to form stable adducts with a variety of transition metals, e.g. Cu, Ag, Au and Pt, is well established. The crystal structures of several such complexes have been determined (Bott et al., 1998;Gale et al., 2006). These studies demonstrate that TU can act both as a terminal ligand in monomeric complexes (Hunt et al., 1979), or as a bridging ligand in polymeric complexes (Taylor et al., 1974). In order to investigate other transition metal complexes of thiourea, we report here the crystal structure of a monomeric complex, viz. [Pd(SC(NH 2 ) 2 ) 4 ](SCN) 2 , (I).
The crystal structure of (I) is composed of complex [Pd(TU) 4 ] +2 cations and thiocyanate counter anions. The Pd 2+ ion is situated on an inversion centre and, as expected for a d 8 system, has an almost square planar environment with cis angles (S-Pd-S) ranging from 87.87 (2) to 92.13 (2)°, and trans angles (S-Pd-S) of 180.0°. The TU ligands are coordinated to Pd II at almost equal distances. The Pd-S bond lengths of 2.3302 (8) and 2.3448 (7) Å (Table 1) are comparable to those of similar compounds reported in the literature (Gale et al., 2006). In the cationic complex, TU ligands behave as S-donors and all four ligands are binding in a terminal mode. Therefore no bridging of metal centers are found as it is observed in some other metal-thiourea compounds, for example, [Cu 4 (TU) 7 (SO 4 ) 2 ]NO 3 (Bott et al., 1998) and [Ag 2 (TU) 6 ](ClO 4 ) 2 (Dupa & Krebs, 1973). The C-S and C-N bond lengths of 1.723 (2) Å and 1.326 (3) Å, respectively, agree with those of coordinated thiourea molecules reported in the Cambridge Crystallographic database (Allen, 2002). In the crystal structure, the building units are connected via hydrogen bonds of the type N-H···N [2.922 (3)-3.058 (3) Å] and N-H···S [3.370 (2)-3.646 (2) Å] (see Table 2).

Experimental
Crystals of (I) were obtained by adding 4 equivalents of thiourea in 15 ml methanol to a solution of K 2 [PdCl 2 ] (0.326 g) in 15 ml of water and stirring for one h. The resulting orange solution was kept after filtration at room temperature for three d. Orange crystals of (I) were obtained on slow evaporation. The counter anion SCNhas apparently been introduced due to impurities (presumably KSCN), that were present in thiourea.

Refinement
The H atoms attached to the N atoms were placed in idealized positions and refined with a N-H distance of 0.88 Å and U iso (H) = 1.2U eq (N).  Fig. 1. Molecular structure of (I) showing the atom-labelling scheme and displacement ellipsoids at the 30% probability level. Unlabelled atoms and atoms labelled by superscript i) are related by the symmetry operator i) 1-x, y, z.