Rare mono- and diorganopentaphosphido cobalt complexes are accessible by P–P condensation using the unprecedented, reactive cobalt-gallium tetraphosphido complex [K(dme)2{(MesBIAN)Co(μ-η4:η2-P4)Ga(nacnac)}] (2). Compound 2 was prepared in good yield by reaction of [K(Et2O){(MesBIAN)Co(η4-1,5-cod)}] [1, BIAN = bis(mesitylimino)acenaphthene diimine, cod = 1,5-cyclooctadiene] with [Ga(nacnac)(η2-P4)] (nacnac = CH[CMeN(2,6-iPr2C6H3)]2). Reactions with R2PCl (R = iPr, tBu, and Cy) selectively afford [(MesBIAN)Co(cyclo-P5R2)] (3a–c), which feature η4-coordinated 1,1-diorganopentaphosphido ligands. The mechanism of formation of these species has been studied by 31P{1H} NMR spectroscopy and DFT calculations. In the case of 3a (R = iPr), it was possible to identify the intermediate [(MesBIAN)Co(μ-η4:η2-P5iPr2)Ga(nacnac)] (4) by single-crystal X-ray diffraction. A related, monosubstituted organopentaphosphido cobalt complex [(MesBIAN)Co(μ-η4:η1-P5tBu)GaCl(nacnac)] (5) was isolated by reacting dichloroalkylphosphane tBuPCl2 with 2. Heterobimetallic complexes such as 2 thus may enable the targeted construction of a range of new metal-coordinated polyphosphorus frameworks by P–P condensation.