{Ba[Au(SCN)2]2}n: a three-dimensional net comprised of monomeric and trimeric gold(I) units

The noteworthy structural feature of the title complex, poly[acetonitriletetra-μ2-thiocyanato-barium(II)digold(I)], {[Au2Ba(SCN)4(CH3CN)]}n, is that the bis(thiocyanato)aurate(I) anion adopts both monomeric and trimeric motifs. The trimer unit has an Au⋯Au distance of 3.1687 (3) Å. In both the monomeric and trimeric units, the AuI atoms are also bonded to two S atoms. Within the trimeric unit, the AuI atoms exist in differing environments; one Au atom has a T-shaped three-coordinate geometry while the other has a square-planar four-coordinate geometry. The AuI atom of the monomer adopts a linear two-coordinate geometry. The extended structure can be described as a three-dimensional coordination polymer consisting of chains of Ba atoms bridged by thiocyanate N atoms. These chains are cross-linked via the gold monomeric and trimeric units.

The noteworthy structural feature of the title complex, poly[acetonitriletetra-2 -thiocyanato-barium(II)digold(I)], {[Au 2 Ba(SCN) 4 (CH 3 CN)]} n , is that the bis(thiocyanato)aurate(I) anion adopts both monomeric and trimeric motifs. The trimer unit has an AuÁ Á ÁAu distance of 3.1687 (3) Å . In both the monomeric and trimeric units, the Au I atoms are also bonded to two S atoms. Within the trimeric unit, the Au I atoms exist in differing environments; one Au atom has a Tshaped three-coordinate geometry while the other has a square-planar four-coordinate geometry. The Au I atom of the monomer adopts a linear two-coordinate geometry. The extended structure can be described as a three-dimensional coordination polymer consisting of chains of Ba atoms bridged by thiocyanate N atoms. These chains are crosslinked via the gold monomeric and trimeric units.
Funding for the diffractometer through NSF-MRI grant CHE-0215950 is gratefully acknowledged. The authors thank Dr Allen G. Oliver (University of Notre Dame) for insightful discussion.

Comment
The propensity for gold complexes to adopt fascinating structures, high stability and unexpected stoichiometries arising from various gold-gold interactions, termed aurophilicty or more generally metallophilicity, ultimately result in intriguing physical properties (Schmidbaur & Schier, 2008;Katz et al., 2008;Puddephatt, 2008).
Over the course of our research on gold(I)-thiocyanate complexes we have observed a variety of interesting bonding motifs and luminescent properties (Coker et al., 2004;Arvapally et al., 2007) attributed to the [Au(SCN) 2 ]anion. The [Au(SCN) 2 ]anion has been observed as a monomer as well as adopting polymeric geometries. For example, in the tetraphenyl arsonium (Schwerdtferger et al., 1990) and phosphonium (Coker, 2003) salts, the [Au(SCN) 2 ]exists as a monomer.
More recently our investigations have turned to the alkaline earth salts of gold(I)-thiocyanate. Our motivation is to further explore the influence of the cation on the highly structurally-versatile behavior of the [Au(SCN) 2 ]anion. In the present work, the structure of the barium salt, (I), is presented.
The geometry of the anion in (I) ( Fig. 1) is such that both monomeric and trimeric gold units are present with Au-Au and Au-S bond distances of 3.1687 (3) Å and 2.294 (2)-2.314 (2) Å, respectively. Within the trimer unit of (I), the gold atoms exist in differing environments; Au1 has a T-shaped three-coordinate geometry while Au2 has a square planar fourcoordinate geometry. The monomeric gold, Au3, adopts a linear two-coordinate geometry. The S-Au1-Au2-S torsion angles, are intermediate between staggered and eclipsed (Table 1) geometries. As commented on by Pathaneni & Desiraju (1993) and further expanded by Anderson et al. (2007), complexes with larger Au-Au distances more frequently adopt an eclipsed conformation (L-Au-Au-L torsions ~ 0 or ±180°) presumably to lessen steric hindrance while the staggered conformation (L-Au-Au-L torsions \sim ±90°) is observed for smaller Au-Au distances. However it should be noted that there is a large spread of intermediates transitioning from eclipsed to staggered conformations with torsion angles in the ±50 to ±140° range.

Experimental
Reaction of barium hydroxide (1 equiv) with ammonium thiocyanate (2 equiv) in water results in the formation of barium thiocyanate with the release of ammonia gas. (I) was prepared following the method described by Coker et al. (2004).
Crystals were obtained from slow diffusion of acetonitrile-diethyl ether solution at -4 °C.

Refinement
The H-atoms were placed in calculated positions (C-H = 0.98 Å) and treated with a riding model. The isotropic displacement parameters were defined as 1.5U eq of the adjacent atom. The largest residual electron-density peaks are located near the gold trimer unit (approx. 1.6 Å from S2 and S3).

Special details
Experimental. A suitable crystal was mounted in a loop with paratone-N and immediately transferred to the goniostat bathed in a cold stream.
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.