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Formation of Complex Ions03:45

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A type of Lewis acid-base chemistry involves the formation of a complex ion (or a coordination complex) comprising a central atom, typically a transition metal cation, surrounded by ions or molecules called ligands. These ligands can be neutral molecules like H2O or NH3, or ions such as CN− or OH−. Often, the ligands act as Lewis bases, donating a pair of electrons to the central atom. These types of Lewis acid-base reactions are examples of a broad subdiscipline called coordination...
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In complexation reactions, metal cations are the electron pair acceptors, and the ligands are the electron pair donors. The stability of the metal complexes depends primarily on the complexing ability of the central metal ion and the nature of the ligands. Generally, the complexing ability of the metal ion depends on the size and charge of the ion. As the metal ion size increases, the stability of the metal complexes decreases, provided that the valency of the metal ion and the ligands remain...
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Complexation reactions take place when dative or coordinate covalent bonds form between metal ions and ligands. The compounds formed in these reactions are called coordination compounds. The number of bonds formed between the metal ion and the ligands is called its coordination number. Generally, most metal ions in an aqueous solution are solvated by water molecules and thus exist as aqua complexes.
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EDTA titrations are usually carried out in highly basic conditions, where the fully deprotonated form of EDTA, Y4−, actively complexes with the free metal ions in the solution. Several metal ions precipitate as hydrous oxide (hydroxides, oxides, or oxyhydroxides) under these conditions, lowering the concentration of free metal ions in the solution. For this reason, auxiliary complexing agents or ligands such as ammonia, tartrate, citrate, or triethanolamine are used in EDTA titrations to...
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In complexation reactions, metal atoms or cations interact with ligands to form donor-acceptor adducts called metal complexes. Ligands that bind through one donor site are monodentate, ligands with two donor sites are bidentate, and those with more than two donor sites are polydentate ligands. For example, ethylene diamine is a bidentate ligand that binds through two nitrogen donor atoms, forming a five-membered ring. EDTA is a polydentate ligand that binds through four oxygen and two nitrogen...
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The mitochondrial cristae membrane is the primary site for the oxidative phosphorylation (OXPHOS) process of energy conversion mediated through respiratory complexes I to V. These complexes have been widely studied for decades, and it has been proven that they form supramolecular structures called respiratory supercomplexes (SC). These higher-order complexes may be crucial in maintaining the biochemical structure and improving the physiological activity of the individual complexes while...
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Multinuclear complex formation between Ca(II) and gluconate ions in hyperalkaline solutions.

Attila Pallagi1, Éva G Bajnóczi, Sophie E Canton

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Polyhydroxy carboxylates like gluconate significantly increase portlandite solubility in alkaline solutions. Stable multinuclear calcium-gluconate complexes form, crucial for modeling radioactive waste repositories and actinide interactions.

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Area of Science:

  • Geochemistry
  • Materials Science
  • Radiochemistry

Background:

  • Cementitious materials are used in radioactive waste repositories.
  • Polyhydroxy carboxylates, like gluconate, are present in alkaline repository environments.
  • Understanding calcium-gluconate complexation is vital for repository performance assessment.

Purpose of the Study:

  • To investigate the structure and equilibria of calcium-gluconate complexes in alkaline solutions.
  • To determine the stability and speciation of these complexes.
  • To identify the binding sites of gluconate to calcium.

Main Methods:

  • Potentiometric measurements using H2/Pt electrodes.
  • X-ray Absorption Spectroscopy (XAS).
  • (1)H Nuclear Magnetic Resonance (NMR) spectroscopy.
  • Electrospray Ionization Mass Spectrometry (ESI-MS).
  • Conductometry and freezing-point depression experiments.

Main Results:

  • Gluconate significantly enhances portlandite solubility.
  • Highly stable mononuclear and multinuclear calcium-gluconate complexes are formed.
  • Predominant species are neutral multinuclear complexes: [Ca2Gluc(OH)3](0) and [Ca3Gluc2(OH)4](0).
  • Calcium binding involves the carboxylate group and oxygen atoms on carbons 2 and 3 of gluconate.

Conclusions:

  • Multinuclear calcium-gluconate complexes dominate in alkaline solutions.
  • These findings are critical for thermodynamic modeling of radioactive waste repositories.
  • The identified calcium-gluconate complexes are precursors to actinide complexation.