Despite the accessibility of numerous transition metal polyphosphido complexes through transition-metal-mediated activation of white phosphorus, the targeted functionalization of P_n ligands to obtain functional monophosphorus species remains challenging. In this study, we introduce a new [3+1] fragmentation procedure for cyclo-P₄ ligands, leading to the discovery of acylcyanophosphanides and -phosphines. Treatment of the complex [K(18c-6)][(Ar*BIAN)Co(η⁴-P₄)] ([K(18c-6)]3, 18c-6=[18]crown-6, Ar*=2,6-dibenzhydryl-4-isopropylphenyl, BIAN=1,2-bis(arylimino)acenaphthene diimine) with acyl chlorides results in the formation of acylated tetraphosphido complexes [(Ar*BIAN)Co(η⁴-P₄C(O)R)] (R=tBu, Cy, 1-Ad, Ph; 4 a–d). Subsequent reactions of 4 a–d with cyanide salts yield acylated cyanophosphanides [RC(O)PCN]⁻ (9 a–d⁻) and the cyclo-P₃ cobaltate anion [(Ar*BIAN)Co(η³-P₃)(CN)]⁻ (8⁻). Further reactions of 4 a–d with trimethylsilyl cyanide (Me₃SiCN) and isocyanides provide insight into a plausible mechanism of this [3+1] fragmentation reaction, as these reagents partially displace the P₄C(O)R ligand from the cobalt center. Several potential intermediates of the [3+1] fragmentation were characterized. Additionally, the introduction of a second acyl substituent was achieved by treating [K(18c-6)]9b with CyC(O)Cl, resulting in the first bis(acyl)monocyanophosphine (CyC(O))₂PCN (10).