The influence of the steric properties of the ligands PR2Ph and L on the formation and properties of the complexes Mo(η6-PhPR2)(L)(PPh2CH2CH2PPh2), R = Et,L = PPhEt2 and R = Ph,L = PPh3, PR′3, CO,CNR, N2, H2

The reduction of MoCl₄(DPPE) (DPPE = PPh₂CH₂CH₂PPh₂) with Mg or Na/Hg in the presence of 2 PPhR₂ under Ar results in the formation of the new complexes Mo(η⁶-PhPR₂)(PPhR₂)(DPPE) when R is Ph (Ia) or Et(II). No η⁶-PhPR₂ complex is obtained when R is Me because this small ligand forms strong MoP σ-bonds; nor is one obtained for R = Cy because of too much steric crowding. The limits for η⁶-complexation can be quantified in terms of cone angle sums.Complex Ia is very similar to Mo(η⁶-PhPMePh)(PMePh₂)₃ (IIIa) in that both react at similar rates with a variety of small ligands L = PMePh₂, PMe₂Ph, PMe₃, P(OMe)₃, N₂, CO, CNBu^t and H₂ via dissociation of a labile σ-bonded ligand. Several other less crowded η⁶-arylphosphinemolybdenum complexes including II do not have labile ligands at 25°C. The new complexes Mo(η⁶-PhPPh₂)(L)(DPPE) have been characterized by ³¹P and ¹H NMR, IR and gas uptake measurements, Ia has a higher affinity for H₂ than IIIa possibly because Mo(η⁶-PhPPh₂)(H)₂(DPPE) adopts a non-fluxional trans-configuration. The ³¹P chemical shift of the σ-bonded ligand in 8 derivatives of Ia and 12 of IIIa correlate with the sum of the cone angles of the three σ-bonded ligands in each complex.