Zwitterionic nickel(II) compounds with a borate-containing anionic bidentate phosphine ligand (PBP-) have been synthesized and some of them crystallographically characterized. The methyl complex, (PBP-)Ni+CH3(NCCH3), reacts with H2 to form an 12-membered macrocycle containing three Ni nuclei and reacts with carbon monoxide (CO) to afford the acetyl complex (PBP-)Ni+COCH3(CO). The square planar acetyl complex is stable in solution under one atmosphere of CO but undergoes irreversible decomposition under high CO pressure at room temperature. X-Ray crystallographic characterization of a decomposition product revealed fragmentation of the anionic borate under high pressure. (PBP-)Ni+COCH3(CO) is an active catalyst for ethylene-CO copolymerization, but its productivity is low likely due to its instability under high CO pressure.