Bis(μ-diphenylphosphanyl)bis[(trimethylphosphane)cobalt(I)](Co—Co)

The title compound, [Co2{P(C6H5)2}2(C3H9P)4], was obtained by the addition of diphenylphosphane to a solution of Co(CH3)(C3H9P)4. The dinuclear complex molecule exhibits inversion symmetry with the inversion centre located between the two CoI atoms. The short Co—Co distance of 2.3670 (8) Å lies within the range of metal–metal double bonds. As a result of inversion symmetry, the four-membered Co2P2 core is rigorously planar, and the two bridging P(C6H5)2-ligands and the terminal C3H9P ligands are arranged in a pseudo-tetrahedral fashion about the CoI atom.


Comment
The synthesis of bimetallic complexes with bridging anionic PR 2 ligands (R = aryl or alkyl) are long time established (Harley et al., 1983), and numerous variants of synthetic approaches were developed (Jones et al., 1983). During the course of our investigations into the chemistry of cyclometallation of triphenylphosphane derivatives (Winterhalter et al., 2001), we isolated, structurally and spectroscopically characterized a series of ortho-metallated cobalt complexes (Klein et al., 2003). Using previous method allowed to synthesize a related bimetallic cobalt complex via salt metathesis of CoCl(PMe 3 ) 3 with LiPPMe 2 in high yield (Klein et al., 1988).
The molecular structure of the bimetallic title complex, (I), [Co(C 3 H 9 P) 2 (µ 2 -P(C 6 H 5 ) 2 )] 2 , is shown in Fig. 1. The complex exhibits a crystallographically imposed inversion centre in the middle of the molecule. Each Co I atom has a distorted tetrahedral coordination geometry with a short Co-Co distance of 2.3670 (8) Å, slightly longer than found for other Co═Co distances of homobimetallic cobalt complexes. Each set of terminal ligands is trans with respect to the Co═Co bond. Both bridging µ 2 -PPh 2 and PMe 3 -ligands are bent away from the central Co 2 P 2 core with the distortion from idealized tetrahedral geometry being greater for the larger PPh 2 group with 115.13 (3)°.
The crystal packing of compound (I) can be described as being composed of rods of single molecules stacked along [100] and [001], with the Co 2 P 2 cores arranged in alternating directions (Fig. 2).

Experimental
Standard vacuum techniques were used in manipulations of volatile and air sensitive material. Literature methods were applied for the preparation of Co(CH 3 )(PMe 3 ) 4 (Klein & Karsch, 1975). Other chemicals were used as purchased. The title compound bis(µ 2 -diphenylphosphino)tetrakis(trimethylphosphane) dicobalt(I) was synthesized by combining stoichiometric amounts of diphenylphosphane (118 mg, 0.63 mmol) in 20 ml of n-pentane at 203 K with a sample of Co(CH 3 )(PMe 3 ) 4 (240 mg, 0.63 mmol) in 20 ml of n-pentane, effecting a change of color from red to dark brown. After warm-up, the mixture was kept stirring at 293 K for 16 h, and then the volatiles were removed in vacuo to give a dark brown, waxy solid. This was dissolved in a mixture of 10 ml of n-pentane / diethyl ether (3:1) and crystallized at 253 K to give brown rhombic crystals, which were suitable for X-ray diffraction. Isolated yield 145 mg (58%); m. p. 391-393 K PPh 2 ). Anal. Calcd. for C 36 H 56 Co 2 P 6 (792.5): C, 54.56; H, 7.12; P, 23.45: Found: C, 54.88; H, 6.72; P 23.98%.

Refinement
H atoms were fixed geometrically and treated as riding on their parent atoms with C-H = 0.93 Å (aromatic) and 0.96 Å (methyl), and with U iso (H) = 1.2 (1.5 for methyl groups) times U eq (C).

Figure 1
The molecular structure of compound (I), with atom labels and thermal displacement parameters of the atoms drawn at the 50% probability level for non-H atoms; H atoms were omitted for clarity. Symmetry code (i) -x, 1 -y, -z indicates symmetry-related atoms generated by a crystallographic inversion centre.