Investigations on the preparation of four- and five-coordinate aluminum and gallium bis(amidophosphine) derivatives are reported. The reaction of the macrocyclic ligand precursor anti-Li-2(THF)(2)[P2N2] ([P2N2] = [PhP(CH2SiMe2NSiMe2CH2)(2)PPh]) with AlCl3 or GaCl3 in toluene at 25 degrees C leads to the formation of the four-coordinate species anti-MCl[P2N2] (M = Al (1), Ga (2)). An X-ray diffraction study of anti-GaCl[P2N2] shows it to be monomeric with a distorted tetrahedral geometry at Ga; only one of the phosphine donors of the [P2N2] ligand binds to the gallium, resulting in the retention of the anti-configuration. The solution NMR spectra are consistent with C-s symmetry. The addition of AlCl3 or GaCl3 to the macrocyclic ligand precursor syn-Li-2(dioxane)[P2N2] in toluene at 25 degrees C yields the five-coordinate complexes syn-MCl[P2N2] (M = Al (3), Ga (4)). The X-ray crystal structure of syn-GaCl[P2N2] reveals a trigonal bipyramidal geometry about the metal atom, necessitating the coordination of both phosphorus atoms. The solution NMR spectra are consistent with a C-2v symmetric complex. Heating the anti complexes results in the clean conversion to the syn complexes, with pyramidal inversion observed at phosphorus. The kinetics of this inversion were studied by H-1 NMR spectroscopy and found to be first-order. Barriers to pyramidal inversion (Delta G double dagger) were calculated to be 29.1 and 30.1 kcal mol(-1) for the aluminum and gallium complexes, respectively; these barriers are approximately 2-3 kcal mol(-1) lower than that determined for the metal-free, protonated compounds anti- and syn-H-2[P2N2]. It is suggested that the role that the metals play in this inversion, based on the values of Delta G double dagger, involves the large negative entropies of activation and thus help organize the transition state.