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 P2(1)/n, a = 12.528(3), b = 14.711(5), c = 19.312(6) Angstrom, beta = 93.68(2)degrees, Z = 2. Final R = 0.065 for 2300 unique reflections having I > 3 sigma (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 Angstrom, and Co-P 2.256.Angstrom. 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 benzyltrimethylammonium salt of the [Co10P(Co)(22)](3-) anion has been studied by X-ray analysis. It gives triclinic crystals, space group P (1) over bar, 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 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 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 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.