An unusual family of three-coordinate, d(8) and d(9) nickel phosphido and phosphinidene complexes containing the chelating 1,2-bis(di-tert-butylphosphino)ethane (dtbpe) ligand and a terminal PR(2)(-) or PR(2-) ligand have been prepared. The complexes (dtbpe)Ni[P(t-Bu)(2)] (2), [(dtbpe)Ni[=P(t-Bu)(2)](+)][PF(6)(-)] (3), [(dtbpe)Ni[=P(H)(dmp)](+)][PF(6)(-)] (5), and (dtbpe)Ni[=P(dmp)] (6) have been structurally characterized by single-crystal X-ray diffraction methods. The three-coordinate d(8) complexes exhibit Ni-P bond lengths and ligand geometries that indicate they participate in symmetry-allowed ligand-to-metal pi bonding involving phosphorus p-electrons and a metal-orbital of pi symmetry that lies in the Ni coordination plane. Compound 6 is a rare example of a late-transition-metal terminal phosphinidene complex.