Measurements of the third-order nonlinear optical responses of solutions of the metal-metal multiply bonded complexes Mo-2(OPri)(6), W-2(OBut)(6), M-2(NMe2)(6), M-2(O2CBut)(4), and M2Cl4(PMe3)(4) (M = Mo, W), using picosecond degenerate four-wave mixing at 1064 nm, are reported. These complexes display only very small instantaneous electronic polarizations when excited with cross-polarized beams, When the excitation beams are similarly polarized, a significant third-order optical response is detected, which is attributable to the formation of bulk thermal excitation gratings. Time-dependent measurements support this view.