The halogenation of Lewis-acid/base-stabilised phosphanylboranes (1a) and arsanylboranes (1b) with CX4 (X = Cl, Br) leads selectively to the substitution of both protons at the pentel atom and the new compounds [(CO)(5)W(X2PBH2 center dot NMe3)] (2a: X = Cl, 2b: X = Br) and [(CO) W-5(X2AsBH2 center dot NMe3)] (3a: X = Cl, 3b: X = Br), respectively, are obtained. The new products were comprehensively characterised by spectroscopic methods and by X-ray crystallography. While compounds 2a and 2b show an antiperiplanar arrangement of the Lewis acid (W(CO)(5)) and the Lewis base (NMe3) in the solid state, a synclinal arrangement in 3a and 3b, respectively, was observed. Computational calculations of the optimised antiperiplanar and synclinal geometries of the compounds 2 and 3 in the gas phase slightly favour the antiperiplanar arrangement of the Lewis acid and the Lewis base for both compounds.