Quasidegeneracy of (H2C = B = CH2)– borata-alkene synthon and (H3B–C≡CH)– boryl-substituted acetylene anion induced by an excess electron attachment

•Among all isomers of (BC2H4)–, (H2C = B = CH2)– and (H3B–C≡CH)– are the most stable.•(H2C = B = CH2)– and (H3B–C≡CH)– isomeric anions are nearly isoenergetic.•Excess electron binding energies of (BC2H4)– anions span the 1.71–4.32 eV range.•H3B–C≡CH isomer is the least stable among neutral isomers of BC2H4.•An excess electron attachment to neutral H3B–C≡CH renders it the most stable anion.