A method for synthesizing a complex having the formula: [L3MO3]'' wherein L3 represents a tridentate ligand, M represents a metal selected from Tc an Re, and n is a charge from -2 to +1, the method comprising: a) reaction of pertechnetate or perrhenate with a reducing agent and L3, or b) reaction of pertechnetate or perrhenate with a Lewis acid and L3, wherein the synthesis (a) or (b) is carried out in an aqueous medium.