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

The [Co9P(CO)(21)](2-) anion has been isolated from the products of the rea ction between Na[CO(CO)(4)] and PCl5 in tetrahydrofuran at reflux. The stru cture of the cluster anion [Co9P(CO)(21)](2-) in its tetraphenylphosphonium salt has been elucidated by X-ray analysis. The crystals are monoclinic, s pace group P2(1)/n, a = 12.528(3), b = 14.711(5), c = 19.312(6) Angstrom, b eta = 93.68(2)degrees, Z = 2. Final R = 0.065 for 2300 unique reflections h aving I > 3 sigma (I). The anion, which is disordered about an inversion ce nter, consists of a monocapped square antiprismatic cluster containing an i nterstitial phosphide and surrounded by 13 terminal and 8 edge-bridging car bonyl ligands. Average values are: Co-Co 2.685 Angstrom, and Co-P 2.256.Ang strom. The [Co10P(CO)(22)](3-) anion has been obtained by condensation of t he [Co9P(CO)(21)](2-) anion with [Co(CO)(4)](-) in tetrahydrofuran at reflu x. 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 be nzyltrimethylammonium salt of the [Co10P(Co)(22)](3-) anion has been studie d by X-ray analysis. It gives triclinic crystals, space group P (1) over ba r, a = 11.452(3), b = 23.510(6), c = 25.606(4) Angstrom, alpha = 112.46(1), beta = 95.79(1), gamma = 73.548(2)degrees, Z = 4. Final R = 0.041 for 8600 unique reflections having I > 3 sigma (I). There are two independent trian ions 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 th e capping metals, are markedly asymmetric. Average values are: Co-Co 2.678 Angstrom, and Co-P 2.262 Angstrom. Electrochemistry shows that [Co9P(CO)(21 )](2-) and [Co10P(Co)(22)](3-) in acetonitrile solution undergo either a on e-electron oxidation or a two-electron reduction. This latter process appea rs as a single step in the case of the dianion and as two separated one-ele ctron steps in the case of the trianion. All the processes are accompanied by slow chemical complications, thus testifying that no stable redox congen ers exist for these phosphide clusters.