Crystal structure of [Co(NH3)6][Co(CO)4]2

The structure of hexaamminecobalt(II) bis[tetracarbonylcobaltate(-I)] contains discrete octahedral [Co(NH3)6]2+ cations and [Co(CO)4]− anions held together by N—H⋯O hydrogen bonds.


Chemical context
The reaction of Co 2 (CO) 8 with bases has already been described in the literature (Hieber et al., 1960). In addition, the reaction of dicobalt octacarbonyl with liquid ammonia has been known for several decades (Behrens & Wakamatsu, 1966). Thereby Co 2 (CO) 8 forms with NH 3 hexaamminecobalt (II) bis[tetracarbonylcobaltate (-I)], [Co(NH 3 ) 6 ]-[Co(CO) 4 ] 2 , which is obtained as orange air-sensitive crystals. During this reaction, CO is released and reacts with ammonia to urea. However, structural data of of the title compound were missing and are presented in this communication.

Structural commentary
The cobalt atom Co1 of the hexaamminecobalt(II) cation occupies Wyckoff position 3a with site symmetry 3.. It is coordinated by six symmetry-related ammine ligands in form of a slightly distorted octahedron. The Co-N distance in the [Co(NH 3 ) 6 ] octahedron is 2.1876 (16) Å which compares well ISSN 2056-9890

Figure 1
The molecular structures of the tetracarbonylcobaltate(ÀI) anion and of the hexaamminecobalt(II) cation of the title compound. Displacement ellipsoids are shown at the 70% probability level. Labelling of symmetryequivalent atoms has been omitted for clarity. with those of other reported hexaamminecobalt(II) structures (Barnet et al., 1966).
The cobalt atom Co2 of the tetracarbonylcobaltate(-I) anion occupies Wyckoff position 6c and exhibits site symmetry 3.. It is coordinated by four carbonyl ligands in a shape close to an ideal tetrahedron. The distances between the Co2 atom and the carbon atoms C1 and C2 of the ligands are 1.7664 (18) and 1.779 (3) Å , respectively. In the literature, distances in the range from 1.77 (2) to 1.82 (2) Å are reported for Co-C in the compound Co 2 (CO) 8 (Sumner et al., 1964). In the carbonyl ligands, the observed distances are in the expected range with 1.153 (2) and 1.140 (4) Å for C1-O1 and C2-O2, respectively. For the compound Co 2 (CO) 8 distances from 1.14 (2) to 1.33 (2) Å were reported (Sumner et al., 1964).
The crystal structure of [Co(NH 3 ) 6 ][Co(CO) 4 ] 2 can be derived from the high-pressure rhombohedral phase of BaC 2 (BaC 2 -HP1, R3m) (Efthimiopoulos et al., 2012). Formally, the Ba sites on Wyckoff position 3a are replaced by the hexaammine cobalt(II) octahedra and the C site on position 6c is replaced by the tetracarbonylcobaltate(-I) tetrahedron.
The molecular components of the title compound are shown in Fig. 1

Supramolecular features
The arrangement of [Co(NH 3 ) 6 ] 2+ octahedra and [Co(CO) 4 ] À tetrahedra in the crystal structure is stabilized by N-HÁ Á ÁO hydrogen bonds with the N1 atom as donor and the oxygen atoms O1 and O2 as acceptors atoms. One of the hydrogen bonds (N-H1C) is forked while, remarkably, in the neigh-bourhood of the hydrogen atom H1B no acceptor atom in the range of the sum of the van der Waals radii is present. Detailed information about hydrogen-bonding distances and angles are given in Table 1.

Synthesis and crystallization
86 mg (29.4 mmol) of Co 2 (CO) 8 were placed in a flame-dried bomb tube under argon. 0.2 ml of liquid ammonia were condensed to the bomb tube. The bomb tube, now containing an orange solution, was flame-sealed and stored at room temperature. The reaction equation is given in Fig. 3. After six months of crystallization time, moisture-and temperaturesensitive, orange single crystals of the title compound were obtained in almost quantitative yield from the still orange solution. After manual separation of the crystals under a lightoptical microscope and evaporation of the solvent only a minute orange residue remained.    (3) 146 (3) Symmetry codes: (i) Àx þ y À 1; Àx À 1; z; (ii) x À y þ 2 3 ; x þ 1 3 ; Àz þ 1 3 ; (iii) x þ 2 3 ; y þ 1 3 ; z þ 1 3 .

Figure 3
Reaction equation for the preparation of the title compound.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All hydrogen atoms of the ammine ligands were located from a difference Fourier map and were refined isotropically without any further restraints.