Bis(μ-cobaltoceniumselenolate-1:2κ2 Se:Se)bis[bis(cobaltoceniumselenolate-κSe)mercury(II)] tetrakis(hexafluoridophosphate) acetonitrile disolvate

The cation in the title salt has a bitetrahedral {Hg2Se6} core.

The sole comparable compound found in the literature is [Yb(C 4 H 10 O 2 ) 4 ][Hg 2 (C 6 H 5 Se) 6 ] (Romanelli et al., 2008). Here, instead of the cationic cobaltocenium species the selenium atoms are bonded to phenyl residues, which makes the Hg species a dianion. With an average Se-Hg bond length of 2.81 Å in the title compound, bonds are elongated in comparison with the dianion of Romanelli et al. (2.68 Å ).
Since the packing of the molecules (Fig. 2) shows no remarkable hydrogen bonding or -stacking interactions, the cohesion within the crystal structure is dominated by van der Waals forces.

Synthesis and crystallization
In a 50 ml Schlenk flask, 11.1 mg of [(CcSe)(PPh 3 )Au]PF 6 (1 eq., 0.013 mmol) were suspended in 5 ml of dry ortho-dichlorobenzene. Approximately 0.1 ml of liquid mercury was added and the mixture stirred for 48 h. This reaction was originally carried out with the aim of removing selenium from the desired compound. The bright-orange precipitate was filtered off, washed with two portions of 10 ml of diethyl ether and dissolved in 5 ml of acetonitrile. This orange solution was concentrated to about 1 ml. Bright-orange needle-shaped crystals were obtained by diffusion-crystallization with diethyl ether at 253 K. 1 H NMR (300 MHz, CD 3 CN): 5.70 (t, J = 2.0 Hz, 2H), 5.51 (t, J = 2.0 Hz, 2H), 5.46 (s, 5H).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. One of the two PF 6 anions (P2) shows disorder of four fluorine atoms over two sets of sites in a 2:1 ratio for F7, F8, F9, F10 and F7A, F8A, F9A, F10A. Another positional disorder occurs for the complete acetonitrile solvent molecule in a 1:1 ratio. All disordered atoms were refined with anisotropic displacement parameters without further restraints, but with fixed occupation factors.

Figure 2
Molecular packing in the title compound with displacement ellipsoids drawn at the 50% probability level in views along the a (left), b (center) and c (right) axes. Hydrogen atoms, anions and solvent molecules are omitted for clarity.

Figure 1
The molecular entities of the title salt, with displacement ellipsoids drawn at the 50% probability level. Hydrogen atoms and solvent molecules were omitted for clarity. Non-labeled atoms are generated by symmetry operation Àx + 1, Ày + 1, Àz + 1.