Density functional theory calculations have been performed for the terminal cationic borylene complexes [(η⁵-C₅H₅)(CO)₂M{B(η⁵-C₅Me₅)}]⁺, [(η⁵-C₅H₅)(CO)₂M(BMes)]⁺, and [(η⁵-C₅H₅)(CO)₂M(BNMe₂)]⁺ (M = Fe, Ru, Os) using the exchange correlation functional BP86. The optimized bond lengths and angles of the complexes are in excellent agreement with experiment. The M-B bond distances in the complexes [(η⁵-C₅H₅)(CO)₂M{B(η⁵-C₅Me₅)}]⁺ (I, M = Fe; II, M = Ru; III, M = Os) are similar to those expected for single bonds on the basis of covalent radii predictions. In contrast, the optimized M-B bond distances in the complexes [( η⁵-C₅H₅)(CO)₂M(BMes)]⁺ and [(η⁵-C₅H₅)(CO)₂M(BNMe₂)]⁺ correspond to a Pauling bond order of 1.73-1.42. The contribution of the electrostatic interaction ΔE_elstat is significantly larger in all borylene complexes than the covalent bonding ΔE_orb, the [M]-BR bonding in the cationic borylene complexes having a greater degree of ionic (60.6-66.8%) than covalent character. The orbital interactions between metal and boron in [(η⁵-C₅H₅)(CO)₂M{B(η⁵-C₅Me₅)}]⁺, [(η⁵-C₅H₅)(CO)₂M(BMes)]⁺, and [(η⁵-C₅H₅)(CO)₂Fe(BNMe₂)]⁺ arise mainly from M←BR σ donation. The π-bonding contribution is, in all complexes, much smaller (9.0-17.3% of total orbital contributions).