To do so, we begin with a rheological characterization of the dynamics of reversible polymer networks with histamine-M 2+ complexes of different coordination numbers. We adopt a variation of the approach of Grindy et al., in which histidine-functionalized 4-arm poly(ethylene glycol) (4-PEG-histidine) with Ni 2+ was used to study the viscoelasticity of polymer networks under different histidine-Ni 2+ stoichiometries. In our study, we investigate the metal-ligand stoichiometrydependent rheological properties of 4-PEG-histamine and M 2+ networks buffered with MOPS, which is a Good's buffer at physiological pH that shows minimal interaction with transition metals such as Cu 2+ , Ni 2+ , and Zn 2+ and is thus used as standard buffers in binding assays. This is in contrast to the previous study in which the gels buffered by phosphate-buffered saline (PBS), where PO4 -groups in PBS can interact significantly with the M 2+ . The use of PBS can therefore pose complications in studying the stability of the resulting coordination complexes, especially since interaction between M 2+ and anionic species can cause a change in the coordination number of the metal centers. The storage modulus 𝐺′(𝜔) and loss modulus 𝐺′′(𝜔) of 4-PEG-histamine-M 2+ networks under different stoichiometries are shown in Fig. (𝑇 = 5 , 𝐶 ). The rheological data exhibits Maxwell-like viscoelastic behavior, as expected based on prior results and as expected of transient networks. In the ML3-coordinated networks with Ni 2+ , Cu 2+ , and Zn 2+ , we observe an ordering of the dissociation times of the 4-PEG network, indicated by the cross-over time 𝜏 -= 1/𝜔 -, to be Ni 2+ > Cu 2+ > Zn 2+ in agreement with the refs. However, this trend is completely different in a network formed with a ML2 metal-ligand coordination. We find that the Ni 2+ network completely loses its ability to form a viscoelastic network as indicated by the purely viscous response of the network. The Cu 2+ network becomes slightly weaker, indicated by the slight lowering of the viscoelastic moduli of the network. The Zn 2+ network actually becomes slightly more viscous, as indicated by an increase in 𝐺′′(𝜔). Comparing just the complex shear viscosity 𝜂 * (𝜔) = 𝐺 * (𝜔)/𝜔 of these networks (which is a measure of the zero-shear viscosity (𝜂 + ) of the network via the Cox-Merz rule), we find that for ML2 stoichiometric conditions, the ordering of dissociation times is Cu 2+ > Zn 2+ > Ni 2+ (Fig. ).