The structures of the parent compounds of phosphanyl- and arsanylboranes, H₂BPH₂ and H₂BAsH₂, were calculated by DFT-B3LYP methods. Such compounds have not previously been obtained preparatively. By applying the concept of Lewis acid/base stabilisation, [(CO)₅W(H₂EBH₂·NMe₃)] (E=P (3), As (4)) derivatives have been synthesised by the metathesis reactions between Li[(CO)₅WEH₂] and ClH₂BNMe₃ (E=P, As). Comprehensive thermodynamic studies on these systems verify the high stability of the Lewis acid/base stabilised complexes. Unexpected based on the thermodynamic calculations, UV radiation of the phosphanylborane 3 leads to the dinuclear phosphanido-bridged complex [(CO)₈W₂(μ-PHBH₂· NMe₃)₂] (5) by H₂ and CO elimination.