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 H-2 (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 H-2-elimination reactions between EH3 (.) D and A (.) PH3 to yield D - H2EPH2 - A and H-2 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 H-2-elimination reactions, but nevertheless they have an effect on the reaction energy. The synthesis of the Cr(CO)(5) substituted phosphanyltrielanes [(CO)(5)Cr(H-2-PBH2 (.) NMe3))] (3a) and [(CO)(5)Cr(H2PAlH2 (.) NMe(3)A (3b)], as well as of the dinuclear complex [(CO)(8)Cr-2(mu-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. (c) 2006 Elsevier B.V.. All rights reserved.