We investigate the axial ligand effects on the diradical character (y) dependences of the second hyperpolarizabilities (gamma) of transition-metal dinuclear complexes, Mo(I)(2)(CO)(2), with different bond lengths (R) using the spin-unrestricted coupled-cluster method. Mo(I)(2)(CO)(2) exhibits intrinsic y-gamma correlation and dominant d sigma-electron contribution to the maximum gamma (gamma(max)), which are also observed in the bare dinuclear analogs. The axial-ligand coordination to Mo(I)(2) is found to cause the increase of the diradical character of the d sigma orbital, the emergence of large negative gamma for small R, and an enhancement of |gamma(max)| by a factor of similar to 30 as compared to the bare analogs. (c) 2014 Elsevier B.V. All rights reserved.