Synthesis and X-ray characterization of the phosphido - Carbonyl cluster anions [CO 9(μ 8-P)(CO) 21] 2- and [CO 10(μ 8-P)(CO) 22] 3

The [Co9P(CO)21]2- anion has been isolated from the products of the reaction between Na[Co(CO)4] and PCl5 in tetrahydrofuran at reflux. The structure of the cluster anion [Co9P(CO)21]2- in its tetraphenylphosphonium salt has been elucidated by X-ray analysis. The crystals are monoclinic, space group P21n, a = 12.528(3), b = 14.711(5), c = 19.312(6) Å, β = 93.68(2)°, Z = 2. Final R = 0.065 for 2300 unique reflections having I > 3σ(I). The anion, which is disordered about an inversion center, consists of a monocapped square antiprismatic cluster containing an interstitial phosphide and surrounded by 13 terminal and 8 edge-bridging carbonyl ligands. Average values are: Co-Co 2.685] Å, and Co-P 2.256] Å. The [Co10P(CO)22]3- anion has been obtained by condensation of the [Co9P(CO)21]2- anion with [Co(CO)4]- in tetrahydrofuran at reflux. While the [Co9P(CO)21]2- anion is stable under CO, the [Co10P(CO)22]3- anion is decomposed to [Co9P(CO)21]2- and [Co(CO)4]-. The benzyltri-methylammonium salt of the [Co10P(CO)22]3- anion has been studied by X-ray analysis. It gives triclinic crystals, space group P, a = 11.452(3), b = 23.510(6), c = 25.606(4) Å, α = 112.46(1) β, p = 95.79(1), γ = 73.548(2)°, Z = 4. Final R = 0.041 for 8600 unique reflections having I > 3σ(). There are two independent trianions in the asymmetric unit, both showing similar geometries, consisting of bicapped square antiprismatic clusters with a central P atom, each bearing 10 terminal and 12 edge-bridging carbonyl ligands, 8 of which, bound to the capping metals, are markedly asymmetric. Average values are: Co-Co 2.678 Å, and Co-P 2.262 Å. Electrochemistry shows that [Co9P(CO)21]2- and [Co10P(CO)22]3- in acetonitrile solution undergo either a one-electron oxidation or a two-electron reduction. This latter process appears as a single step in the case of the dianion and as two separated one-electron steps in the case of the trianion. All the processes are accompanied by slow chemical complications, thus testifying that no stable redox congeners exist for these phosphide clusters.