The new complexes [MoCl(CO)(η4-1,3-P2C2Bu(t)2)(η5-L)] (L = C5H5 1, C5Me5 2) are reported; facile oxidation of 1 with water or methanol affords a rare example of a mixed valence phosphaphosphonietinyl in [MoCl(CO){η3,λ3,λ5-PC2Bu(t)2-PH(OR)}(η5-C5H5)] (R = H, Me), which has also been structurally characterised for R = H.