The reaction between diphosphorus derivatives [(^ClIm^Dipp)P₂(Dipp)]OTf (1[OTf]) and [(^ClIm^Dipp)P₂(Dipp)Cl] (1[Cl]) with the cyclotetraphosphido cobalt complex [K(18c-6)][(PHDI)Co(η⁴-cyclo-P₄)] (2) leads to the formation of complex [(PHDI)Co{η⁴-cyclo-P₆(Dipp)(^ClIm^Dipp)}] (3), which features an unusual hexaphosphido ligand [^ClIm^Dipp=4,5-dichloro-1,3-bis(2,6-diisopropylphenyl)imidazol-2-yl, Dipp=2,6-diisopropylphenyl, 18c-6=18-crown-6, PHDI=bis(2,6-diisopropylphenyl)phenanthrene-9,10-diimine]. Complex 3 was obtained as a crystalline material with a moderate yield at low temperature. Upon exposure to ambient temperature, compound 3 slowly transforms into two other compounds, [K(18c-6)][(PHDI)Co(η⁴-P₇Dipp)] (4) and [(PHDI)Co{cyclo-P₅(^ClIm^Dipp)}] (5). The novel complexes 3–5 were characterized using multinuclear NMR spectroscopy and single-crystal X-ray diffraction. To shed light on the formation of these compounds, a proposed mechanism based on ³¹P NMR monitoring studies is presented.