Structures of neutral borane–ammonia complex H 3 BNH 3 1 and its protonated form H4BNH3+ 2 were calculated using ab initio method at the MP2/cc-pVTZ and MP2/cc-pVQZ levels. At the MP2/cc-pVQZ level the BN bond distance of 1 was found to be 1.646Å. This is close to the experimentally measured gas phase BN distance (r 0 ) of 1.672Å (microwave spectroscopy). At the same MP2/cc-pVQZ level, the BN bond distance of 2 was found to be only 1. 577Å. This is very close to the X-ray determined bond distance of 1.564Å. Protonation of 1 was found to be highly exothermic by 192.9kcal/mol. The 11 B NMR chemical shifts of 1 and 2 were also calculated by the correlated GIAO-CCSD(T) method. Both δ 11 B values of 1 and 2 were calculated to be close to the experimental value of H 3 BNH 3 1. For comparison, structures and 11 B NMR chemical shifts of the neutral complexes H 3 BX (X=PH 3 3, OH 2 5, SH 2 7 and CO 9) and their corresponding BH protonated structures were also computed.