AbstractThe reaction of the phosphinidene complex [Cp*P{W(CO)5}2] (1 a) (Cp*=C5Me5) with the anionic cyclo‐Pn ligand complex [(η3‐P3)Nb(ODipp)3]− (2, Dipp=2,6‐diisopropylphenyl) resulted in the formation of [{W(CO)5}2{μ3,η3:1:1‐P4Cp*}Nb(ODipp)3]− (3), which represents an unprecedented example of a ring expansion of a polyphosphorus‐ligand complex initiated by a phosphinidene complex. Furthermore, the reaction of the pnictinidene complexes [Cp*E{W(CO)5}2] (E=P: 1 a, As: 1 b) with the neutral complex [Cp′′′Co(η4‐P4)] (Cp′′′=1,2,4‐tBu3C5H2) led to a cyclo‐P4E ring (E=P, As) through the insertion of the pentel atom into the cyclo‐P4 ligand. Starting from 1 a, the two isomers [Cp′′′Co(μ3,η4:1:1‐P5Cp*){W(CO)5}2] (5 a,b), and from 1 b, the three isomers [Cp′′′Co(μ3,η4:1:1‐AsP4Cp*){W(CO)5}2] (6 a–c) with unprecedented cyclo‐P4E ligands (E=P, As) were isolated. The complexes 6 a–c represent unique examples of ring expansions which lead to new mixed five‐membered cyclo‐P4As ligands. The possible reaction pathways for the formation of 5 a,b and 6 a–c were investigated by a combination of temperature‐dependent 31P{1H} NMR studies and DFT calculations.