The reaction of BeCl₂ with carbodiphosphorane C(PPh ₃)₂; Experimental and theoretical studies

The use of 2-Br-fluorobenzene as the solvent permitted the isolation of the addition compound [Cl₂Be(C{PPh₃}₂)] (5) from the reaction of the carbodiphosphorane 1 (carbon) with BeCl₂ featuring a three coordinate beryllium atom. In other solvents such as THF, DCM, etc. deprotonation occurs with formation of the cations (HC{PPh ₃}₂)⁺ or (H₂C{PPh₃} ₂)²⁺. Compound 5 was characterized by an X-ray diffraction analysis. The analysis of the bonding situation in Cl₂Be←1 reveals that the orbital donation comes mainly from the σ lone-pair orbital of 1, whereas the π donation is rather weak. However, the Cl ₂Be←1 π donation appears strong enough to make the donation of a second donor species 1 unfavorable. The calculation of 1→Cl ₂Be←1 shows that the latter complex is higher in energy than 1→Cl₂Be and free 1. The carbodiphosphorane may bind, albeit weakly, a second BeCl₂ species in the complex Cl₂Be← 1→BeCl₂. The bond dissociation energies for the first and second BeCl₂ fragments are D_e = 31.8 kcal·mol ^-1 and 7.0 kcal·mol^-1. The dimerization of Cl ₂Be←1 is energetically slightly endoenergetic.