TDDFT calculations including scalar, spin-orbit and solvent effects were carried out on [Mo6Cl8Cl6](2), and [Mo6Cl8(py)(6)](4+) clusters. The calculations indicate that the contribution of the axial pyridine ligands in the active lowest unoccupied molecular orbitals which are involved in the higher intensity transitions in the [Mo6Cl8(py)(6)](4+) cluster are significant (similar to 31%). This situation differs from all the luminescent [M6X8L6] clusters, where most of the closely spaced excited states are located in the [M6X8](q+) core. Thus, the pyridine axial ligands switch off the cluster luminescence. (C) 2009 Elsevier B.V. All rights reserved.