The formation of Lewis acid/base stabilised phosphanyltrielanes - A theoretical and experimental study

Theoretical investigations on the thermochemistry and the reaction mechanism of the formation of Lewis acid/base stabilised phosphanyltrielanes D · H₂EPH₂ · A (D = Lewis base, A = Lewis acid) were conducted. The reactions of EH₃ · D with A · PH₃ to form D · H₂EPH₂ · A and H₂ (E = B, Al, Ga; D = NH₃; A = BH₃, Cr(CO)₅) are all exothermic, regardless of whether donors and acceptors are present or absent. The lithium chloride elimination reactions between EH₂Cl · D and A · PH₂Li to give D · H₂EPH₂ · A and LiCl are endothermic for donor/acceptor stabilised compounds, if formation of gaseous LiCl is considered. If solid lithium chloride is considered all reactions are strongly exothermic. Studies of the transition state for H₂-elimination reactions between EH₃ · D and A · PH₃ to yield D · H₂EPH₂ · A and H₂ were only successful for E = Al, Ga. In these cases the reaction proceeds via a transition state featuring a five or six-coordinate group 13 element. Different donor molecules do not influence the activation energy of such H₂-elimination reactions, but nevertheless they have an effect on the reaction energy. The synthesis of the Cr(CO)₅ substituted phosphanyltrielanes [(CO)₅Cr(H₂PBH₂ · NMe₃)] (3a) and [(CO)₅Cr(H₂PAlH₂ · NMe₃)] (3b), as well as of the dinuclear complex [(CO)₈Cr₂(μ-HPBH₂ · NMe₃)₂] (4) are described, the latter as a subsequent reaction product of the photolysis of 3a. All compounds were characterised spectroscopically and by X-ray structure analysis.