The addition of AlCl3 to four-coordinate boranes of the general formula (C-N-chelate)BCl2 results in halide abstraction and formation of three-coordinate borenium cations of the general formula [(C-N-chelate)BCl]+. The latter react with both arylstannanes and arylsilanes by boro-destannylation and -desilylation, respectively, to form arylated boranes. Catalytic quantities of AlCl3 were sufficient to effect high-yielding arylation of (C-N-chelate)BCl2. Boro-destannylation is more rapid than boro-desilylation and leads to double arylation at the boron center, whereas in reactions with arylsilanes either single or double arylation occurs dependent on the nucleophilicity of the arylsilane and on the electrophilicity of the borenium cation. The electrophilicity of the borenium cation derived from 2-phenylpyridine was greater than that of the benzothiadiazole analogues, enabling the boro-desilyation of less nucleophilic silanes and the direct electrophilic borylation of 2-methylthiophene.