Addition of one equivalent of the Group 15 trichlorides ECl₃ (E = P, As, or Sb) to the phosphido-bridged dimetallic anions [M₂Cp₂(CO)₄(μ-PH₂)]⁻ (M = Mo or W) afforded moderate yields of the complexes [M₂Cp₂(CO)₄(μ-η²-PE)], which feature a pseudo-tetrahedral M₂PE core. Treatment of the new complexes with [W(CO)₅(THF)] led to preferential co-ordination of this Lewis acid to the phosphorus atom. The additional co-ordination of a W(CO)₅ fragment to the As atom in [M₂Cp₂(CO)₄(μ-η²-PAs)] to give a 2∶1 adduct is reversible at room temperature on dissolution in solvents such as CH₂Cl₂ and THF; similar co-ordination to the Sb atom in [M₂Cp₂(CO)₄(μ-η²-PSb)] is not observed. The crystal structures of the 1∶1 adducts and of one 2∶1 adduct have been determined. The P–As distances in [W₂Cp₂(CO)₄{μ-η²-AsPW(CO)₅}] (2.217(5) Å) and that in [Mo₂Cp₂(CO)₄(μ-η²-AsP{W(CO)₅}₂)], in which the As and P atoms are disordered (2.223(3) Å), are intermediate between the P–P and As–As distances in homodiatomic E₂ complexes. Similarly, the P–Sb distances in the two crystallographically independent molecules of [Mo₂Cp₂(CO)₄{μ-η²-SbPW(CO)₅}] (2.401(2), 2.410(2) Å) are intermediate in length between the P–P and Sb-Sb distances in related homodiatomic E₂ complexes.