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(H2EBH2NMe3)] (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(-PHBH2NMe3)2] (5) by H2 and CO elimination.