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-η 4:η 2-P 4)Ga(nacnac)}] ( 2). Compound 2 was prepared in good yield by reaction of [K(Et 2O){( MesBIAN)Co(η 4-1,5-cod)}] [ 1, BIAN = bis(mesitylimino)acenaphthene diimine, cod = 1,5-cyclooctadiene] with [Ga(nacnac)(η 2-P 4)] (nacnac = CH[CMeN(2,6-iPr 2C 6H 3)] 2). Reactions with R 2PCl (R = iPr, tBu, and Cy) selectively afford [( MesBIAN)Co( cyclo-P 5R 2)] ( 3a-c), which feature η 4-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-η 4:η 2-P 5iPr 2)Ga(nacnac)] ( 4) by single-crystal X-ray diffraction. A related, monosubstituted organopentaphosphido cobalt complex [( MesBIAN)Co(μ-η 4:η 1-P 5 tBu)GaCl(nacnac)] ( 5) was isolated by reacting dichloroalkylphosphane tBuPCl 2 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.