The mechanism of the boron metathesis reaction of the transition-metal-aminoborylene complex $Cp(CO)_2FeBN- (CH_3)_2 + (8)$ with EX, where $EX=H_3PO (9ap)$, $H_3AsO (9bp), H_3PS (9aq), H_3AsS (9bq)$, $CH_3CHCH_2 (9cr), (NH_2)_2- CCH_2 (9dr), H_2CO (9ep)$, and $(NH_2)_2CO (9dp)$ is investigated at the B3LYP/LANL2DZ level. The analysis of bonding and charge distribution shows that the Fe-borylene complex (8) is a Fischer-type carbene analogue. The attack of the olefin takes place at the metal end of the M=C bond of the metal-carbene complex in olefin metathesis and proceeds via [2 + 2] cycloaddition, while in boron metathesis, the initial attack of the substrates takes place at the positively charged B atom of the Fe-borylene complex and forms the preferred acyclic intermediate. The energetics of boron metathesis is comparable to that of the olefin metathesis. Substrates that are polar and a have low-lying $σ^*$ molecular orbital (weak $σ$ bond) prefer the boron metathesis reaction. The relative stability of the metathesis products is controlled by the strength of the Fe-E and B-X bonds of the products 13 and 14, respectively. We have also investigated the possibility of a $β$-hydride-transfer reaction in the Fe-borylene complex.