A kinetic approach to assess the strengths of ligands bound to soluble Mn(III)

The soluble porphyrin ligand [α, β, γ, δ-tetrakis(4-carboxyphenyl)porphine (T(4-CP)P)] was used as a competitive complexing ligand to study Mn speciation in the sediment porewaters of four stations in the Laurentian Trough. Both Mn(II) and Mn(III) concentrations were determined simultaneously. The rate of recovery of Mn(II) complexes by the porphyrin shows similar kinetics, but Mn(III)-ligand complexes give different values for the kinetics of recovery, which can be used along with the steady-state approximation to determine the dissociation rate constant, kd, of the Mn(III)L complex(es). Dissociation rate constants for the recovery of inorganic Mn′ as the Mn(III)-porphyrin complex ranged from 2.28 × 10− 3 to 7.20 × 10− 3 s− 1. The kd values reflect the dissociation of the ligand from natural Mn(III)L complexes, which are slower to dissociate than Mn(III) bound to pyrophosphate but faster than desferrioxmaine-B bound to Mn(III). A comparison of the kd values of natural Mn(III)L and Fe(III)L complexes from previous studies indicates that Mn(III)L complexes dissociate faster than Fe(III)L complexes. Because Kcond = kf / kd, we estimate Kcond at 1.39 to 4.35 × 1011 M− 1 for these Mn′L complexes [where Mn′ is defined as all inorganic forms of Mn(III)] from the diffusion-controlled rate of water exchange (kex) for Mn(H2O)63 + and its hydroxo species, which is the upper limit for the value of kf for Mn′ reacting with ligands. These Kcond values for Mn′L complexes are similar to those found for Fe′L in open ocean waters.