A series of η1-α-phosphinocarbene complexes of manganese, Cp(CO)2Mn═C(R)PR′R′′ (3; 3a: R = Ph, R′ = H, R′′ = Mes; 3b: R = Me, R′ = H, R′′ = Mes; 3c: R = Ph, R′ = R′′ = Ph; 3d: R = Ph, R′ = R′′ = N(i-Pr)2; 3e: R = Ph, R′ = Me, R′′ = Mes), was generated at low temperature upon nucleophilic addition of the primary phosphine H2PMes and secondary phosphines HPPh2, HP(N(i-Pr)2)2, or (±)-HPMeMes to the cationic carbyne complexes Cp(CO)2Mn+≡C−R ([1]+; [1a]+: R = Ph; [1b]+: R = Me), followed by deprotonation of the resulting α-phosphoniocarbene intermediates Cp(CO)2Mn═C(R)P+(H)R′R′′ ([2a−e]+). The η1-α-phosphinocarbene complexes 3c−e derived from secondary phosphines were found to be highly thermolabile, giving upon intramolecular CO insertion the η3-phosphinoketene complexes Cp(CO)Mn(η3-C(O)C(Ph)PR′R′′) (4c: R′ = R′′ = Ph, 4d: i-Pr2N; RR/SS-4e: R′ = Me, R′′ = Mes) in 80−95% yield. The η1-α-phosphinocarbene complexes 3a,b, bearing a mesityl substituent, were isolated in 57−65% yield. The availability of the phosphorus lone pair in such compounds was assessed by their reaction with borane, giving the corresponding adducts Cp(CO)2Mn═C(R)P(BH3)HMes (5a, R = Ph; 5b, R = Me). Complexes 3a,b slowly rearrange in a nonpolar solvent (benzene) to afford a mixture of the η3-phosphinoketene complexes Cp(CO)Mn(η3-C(O)C(R)PHMes) (4a, R = Ph; 4b, R = Me) and the η1-phosphaalkene complexes Cp(CO)2Mn(η1-P(Mes)═C(H)R (6a, R = Ph; 6b, R = Me), and ultimately pure 6a,b as a mixture of E/Z stereoisomers (6a, E/Z 60:40; 6b, E/Z 55:45). By contrast, a stereoselective rearrangement of 3a,b into E-6a,b was observed in a polar solvent (THF), the process being considerably accelerated in the presence of catalytic amounts of base. Deprotonation of 3a by n-BuLi at −80 °C affords the dicarbonyl phosphidocarbene anion [Cp(CO)2Mn═C(Ph)PMes]Li ([7a]Li), which undergoes an instantaneous CO insertion process upon addition of traces of weak acids (H2O, t-BuOH) to give the cyclic monocarbonylacyl anion [Cp(CO)2tiebar below startMn(η2-C(O)C(Ph)═tiebar below endPMes]Li ([8a]Li, R = Ph). Besides, deprotonation of 3a,b by t-BuOLi affords directly the cyclic monocarbonylacyl anions [8a,b]Li. The protonation of [8a]Li with various proton sources (HBF4·Et2O, NH4Claq, [Et3NH]Cl) leads to a stereoselective formation of E-6a, illustrating the key role of this anion in the stereoselective 3a → E-6a rearrangement. Alkylation of [8a]Li by MeI resulted in formation of η3-phosphinoketene complex RR/SS-4e as a single pair of diastereomers, whereas its treatment with iodine resulted in thermally unstable η3-phosphinoketene complex Cp(CO)Mn(η3-C(O)C(Ph)PIMes) (11), evolving at room temperature into the η1-phosphaalkene product Cp(CO)2Mn(η1-P(Mes)═C(I)Ph) (12) of Z conformation. The solid-state structures of 3b, 4c, 4d, RR/SS-4e, and Z-12 are reported.