Ligand flexibility and counterion-driven diversity in morpholine-based Cd(II), Cu(II), Zn(II), and Ni(II) complexes:...

Counterion identity and ligand flexibility play crucial yet often overlooked roles in governing structural properties and influencing functional behavior of metal complexes. We report a systematic investigation of a series of morpholine-based Cd(II), Cu(II), Zn(II), and Ni(II) complexes that demonstrate how subtle variations in counterion identity and the donor environment can profoundly influence solid-state architectures, supramolecular motifs, and anticancer activity. Single-crystal X-ray diffraction reveals an unusual Cd(II) mixed-crystal system, previously reported to contain both a neutral [CdL(NO₃)₂] species with a staggered cis-(NO₃)₂ motif and an ionic [CdL(H₂O)(NO₃)]NO₃ species cocrystallized in a single lattice. DFT calculations of the experimental structure reveal spatial separation of the HOMO (localized on the neutral 2a site) and LUMO (on the ionic 2b site), providing an electronic rationale for the solid-state stabilization of 2b despite its thermodynamic instability in solution. In contrast, the perchlorate analogues of Cd(II) and Cu(II) form discrete chair-like supramolecular dimers mediated by N-H⋯Cl hydrogen bonding. Expanding the scope, a series of Ni(II) complexes featuring stepwise substitution of monodentate ligands (H₂O, CH₃CN, ^tBuNC) reveal geometric isomerism in the H₂O complex and the formation of Ni-C bonds in the ^tBuNC derivative, with all three structures exhibiting consistent equatorial coordination by the monodentate donors. This O → N → C donor progression is supported by crystallographic, spectroscopic, and conductivity data. Cytotoxicity assays reveal differences in activity among the complexes, with trends consistent with an influence of counterion identity; however, these observations remain preliminary due to the absence of complementary control experiments and stability under physiological conditions.