Studies were performed to study the complexation chemistry of 99mTc(CO)+3 with a new tridentate aminodihydroxymethyl phosphine (NP2) ligand with the 99mTc(CO)3(OH2)+3 synthon at tracer levels. A single, well-defined 99mTc(CO)3 NP2 complex is formed at pH 7.5 within 10 min at 60°C that exhibits high in vitro and in vivo stability.