Molecular geometry, electronic structure, and possible reaction mechanism of a series of mono-transition-metal-substituted Keggin-type polyoxometalate (POM)-dinitrogen complexes [PW11O39M(N-2)](n-) (M = Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Tc, Ru, Rh, Pd, Ag, Cd, W, Re, Os, Ir, Pt, Au, and Hg) have been investigated by using density functional theory (DFT) calculations with M06L functional. The calculated adsorption energy of N-2 molecule, N-N bond length, N-N stretching frequency, and the NBO charge on the coordinated N-2 moiety indicate that Mo-II-, Tc-II-, W-II-, Re-II-, and Os-II-POM complexes are significant for binding and activation of the inert N-2 molecule. The degree of the N-2 activation can be classified into the "moderately activated" category according to Tuczek's sense [J. Comput. Chem. 2006, 27, 1278]. Electronic structure and NBO analysis indicate that the terminal N atom of the coordinated N-2 molecule in these POM-dinitrogen complexes possesses more negative charge relative to the bridge N atom because Jahn-Teller distorted effects lead to an effective orbital mixture between sigma(2s)* orbital of N-2 and d(z)(2) orbital of transition metal center. And the mono-lacunary Keggin-type POM ligand with five oxygen donor atoms serves as a strong electron donor to the bivalent metal center. Meanwhile, a catalytic cycle for direct conversion of N-2 into NH3 has been systematically investigated based on a Re-POM complex along distal, alternating, and enzymatic pathways. The calculated free energy profile of the three catalytic cycles indicates that the distal mechanism is the favorable pathway in the presence of proton and electron donors.