A series of divalent, monovalent, and zerovalent nickel complexes supported by the electron-releasing, monoanionic tris(phosphino)borate ligands [PhBP_3] and [PhBP^(iPr)_3] ([PhBP_3] = [PhB(CH_2PPh_2)_3]-, [PhBP^(iPr)_3] = [PhB(CH_2PiPr_2)_3]-) have been synthesized to explore fundamental aspects of their coordination chemistry. The pseudotetrahedral, divalent halide complexes [PhBP_3]NiCl (1), [PhBP_3]NiI (2), and [PhBP^(iPr)_3]NiCl (3) were prepared by the metalation of [PhBP_3]Tl or [PhBP^(iPr)_3]Tl with (Ph_3P)_2NiCl_2, NiI_2, and (DME)NiCl_2 (DME = 1,2-dimethoxyethane), respectively. Complex 1 is a versatile precursor to a series of complexes accessible via substitution reactions including [PhBP_3]Ni(N_3) (4), [PhBP_3]Ni(OSiPh_3) (5), [PhBP_3]Ni(O-p-tBu-Ph) (6), and [PhBP_3]Ni(S-p-tBu-Ph) (7). Complexes 2−5 and 7 have been characterized by X-ray diffraction (XRD) and are pseudotetrahedral monomers in the solid state. Complex 1 reacts readily with oxygen to form the four-electron-oxidation product, {[PhB(CH_2P(O)Ph_2)_2(CH_2PPh_2)]NiCl} (8A or 8B), which features a solid-state structure that is dependent on its method of crystallization. Chemical reduction of 1 using Na/Hg or other potential 1-electron reductants generates a product that arises from partial ligand degradation, [PhBP_3]Ni(η^2-CH_2PPh_2) (9). The more sterically hindered chloride 3 reacts with Li(dbabh) (Hdbabh = 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene) to provide the three-coordinate complex [κ^2-PhBP^(iPr)_3]Ni(dbabh) (11), also characterized by XRD. Chemical reduction of complex 1 in the presence of L-type donors produces the tetrahedral Ni(I) complexes [PhBP_3]Ni(PPh_3) (12) and [PhBP3]Ni(CNtBu) (13). Reduction of 3 following the addition of PMe_3 or tert-butyl isocyanide affords the Ni(I) complexes [PhBP^(iPr)_3]Ni(PMe_3) (14) and [PhBP^(iPr)_3]Ni(CN^tBu) (15), respectively. The reactivity of these [PhBP_3]Ni^IL and [PhBP^(iPr)_3]NiI^L complexes with respect to oxidative group transfer reactions from organic azides and diazoalkanes is discussed. The zerovalent nitrosyl complex [PhBP_3]Ni(NO) (16) is prepared by the reaction of 1 with excess NO or by treating 12 with stoichiometric NO. The anionic Ni(0) complexes [[κ^2-PhBP_3]Ni(CO)_2][^nBu_4N] (17) and [[κ^2-PhBP^(iPr)_3]Ni(CO)_2][ASN] (18) (ASN = 5-azoniaspiro[4.4]nonane) have been prepared by reacting [PhBP_3]Tl or [PhBP^(iPr)_3]Tl with (Ph_3P)_2Ni(CO)_2 in the presence of R_4NBr. The photolysis of 17 appears to generate a new species consistent with a zerovalent monocarbonyl complex which we tentatively assign as {[PhBP_3]Ni(CO)}{^nBu_4N}, although complete characterization of this complex has been difficult. Finally, theoretical DFT calculations are presented for the hypothetical low spin complexes [PhBP_3]Ni(N^tBu), [PhBP^(iPr)_3]Ni(N^tBu), [PhBP^(iPr)_3]Ni(NMe), and [PhBP^(iPr)_3]Ni(N) to consider what role electronic structure factors might play with respect to the relative stability of these species
