Ab initio calculations were carried out to investigate the structures, binding energies, bonding, and NMR spin–spin coupling constants of complexes HLB=BLH, for L=CO, NH3, OH2, PH3, SH2, and ClH. Both B–B and B–H bonds lengthen on complex formation relative to singlet HBBH, and except for L=CO, the B–B bonds are double bonds. The order of stability of the trans isomers correlates with the ordering of ligands in the spectrochemical series of ligand field theory. The trans isomer is always more stable than the corresponding cis. Inverse correlations are found between 1 J(B–B) and 1 J(B–H) and the corresponding B–B and B–H distances. For the trans isomers, 1 J(B–B) appears to be related to the ordering of ligands in the spectrochemical series, while 1 J(B–H) is related to the protonation energy of the ligand L.