Yeast inorganic pyrophosphatase catalyzes the hydrolysis of P1,P2-bidentate Co(NH3)4 pyrophosphate [Co(NH3)4PP] to the cis, bis(phosphate) complex Co(NH3)4(Pi)2, which is not stable at neutral pH and over a period of 24 h converts to HPO4(2-) and a mixture of bidentate Co(NH3)4(PO4) and monodentate Co(NH3)4(H2O)(HPO4). Concurrent with this process is the reduction and subsequent release of Co(H2O)6(2+) from the cobalt tetraammine bis(phosphate) complex and/or the cobalt tetraammine monophosphate complex. Bidentate tetraammine phosphatocobalt (III), hexaaquocobalt(II), orthophosphate, and two free water molecules cocrystallize [Co(NH3)4PO4 . Co(H2O)6(2+) . HPO4(2-) . 2H2O] from the reaction mixture in the triclinic space group Pi (Z = 2) with cell dimensions a = 6.849 (1) A, b = 11.693 (2) A, c = 12.630 (2) A, alpha = 65.60 (1) degree, beta = 88.98 (1) degree, and gamma = 73.04 (1) degree. The structure was solved by the heavy atom technique and refined to an R index of 0.040 by using 3077 intensities measured up to a 2 theta limit of 155 degrees. 31P NMR studies of the equilibrium mixture reveal that the equilibrium constant is a sensitive function of solution pH and temperature. Unlike the Co(NH3)4PP complex, there is evidence indicating that the Mg(H2O)4PP complex is degraded to monodentate Mg(H2-O)5PO4 in the enzyme active site.