Ab initio calculations have been carried out on the parent phosphinoamide/iminophosphide anion H2PNH- and all of its fluoro derivatives. The geometries, optimized at the SCF level, reveal two real minima related by a saddle point. The two minima correspond to cis and trans conformations, and the saddle point, in most cases, corresponds to the rotational barrier about the PN bond. The energies of the cis and trans forms of H2PNH-, HFPNH-, and HFPNF- are degenerate within 1 kcal/mol at the MP4 level. The trans forms of H2PNF-, F2PNH-, and F2PNF- are unfavored relative to the cis forms by 4-6 kcal/mol. The transition states relating the two forms range from 8 kcal/mol for H2PNH- to 22 kcal/mol for HFPNF-. The P-N bond in the parent anion H2PNH- is significantly shortened with respect to a regular single-bond length. This effect mainly originates from some delocalization of the nitrogen in-plane n(sigma) lone pair into the symmetrical combination of the empty PH sigma* orbitals. The hyperconjugation is not sufficient, however, to describe H2PNH- as an iminophosphide H2P-=NH; the system is best described as a phosphinoamide anion H2P-NH- with the negative charge located mainly on nitrogen. Substitution of hydrogen for fluorine at phosphorus strengthens the P-N bond while substitution of hydrogen for fluorine at nitrogen weakens the P-N bond. The order of decreasing P-N bond strength is F2PNH-, HFPNH-, F2PNF-, HFPNF-, H2PNH-, H2PNF-, where F2PNH- is formally closest to an iminophosphide form. A model complex with lithium [Li(H2PNH)]2 has been investigated, and it does not exhibit the short P-Li contact that has been observed experimentally in [Li(Ph2PNPh)(OEt2)]2.