Gas-phase clustering reactions N2H+(H-2)(n-1) + H-2 = N2H+(H-2)(n) with n = 1-13 were studied using a pulsed-electron beam mass spectrometer. Anomalous changes of binding energies in the cluster ion N2H+(H-2)(n) were observed, i.e., the bond energies of N2H+(H-2)(n) increase with n = 2 --> 5. The molecular orbital (MO) calculation revealed that the charge transfer takes place in the cluster ion N2H+(H-2)(n) from the H-2 ligands to the N2H+ moiety, with increase of n leading to the self-tightening of the bonds in the cluster Ions. The experimental bond energies of N2H+(H-2)(n) (n = 2-7) were reproduced by the ab initio MO calculations. The larger cluster ions N2H+(H-2)(n) (n = 10-13) were found to coexist with smaller ones with n = 6-9 in the temperature region of 65-40 K. This suggests the presence of the structural isomers.