Backbone modified small bite-angle diphosphines: Synthesis and molecular structures of [M(CO)4{X2PC(R1R2)PX2}] (M = Mo, W; X = Ph, Cy; R1 = H, Me, Et, Pr, allyl, R2 = Me, allyl)

A range of new small bite-angle diphosphine complexes, [M(CO)4{X2PC(R1R2)PX2}] (M = Mo, W; X = Ph, Cy; R1 = H, Me, Et, Pr, allyl, R2 = Me, allyl), have been prepared via elaboration of the methylene backbones in [M(CO)4(X2PCH2PX2)] as a result of successive deprotonation and alkyl halide addition. When X = Ph it proved possible to replace both methylene protons but for X = Cy only one substitution proved possible. This is likely due to the electron-releasing nature of the cyclohexyl groups but may also be due to steric constraints. Attempts to prepare the bis(allyl) substituted complex [Mo(CO)4{Ph2PC(allyl)2PPh2}] were only moderately successful. The crystal structures of nine of these complexes are presented.