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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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The hemoglobin in the blood, the chlorophyll in green plants, vitamin B-12, and the catalyst used in the manufacture of polyethylene all contain coordination compounds. Ions of the metals, especially the transition metals, are likely to form complexes.
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Metal ions can be separated from one another by complexation with organic ligands–the chelating agent– to form uncharged chelates. Here, the chelating agent must contain hydrophobic groups and behave as a weak acid, losing a proton to bind with the metal. Since most organic ligands used in this process are insoluble or undergo oxidation in the aqueous phase, the chelating agent is initially added to the organic phase and extracted into the aqueous phase. The metal-ligand complex is...
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Complexation Equilibria: Factors Influencing Stability of Complexes01:09

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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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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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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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Lead(II) complex formation with L-cysteine in aqueous solution.

Farideh Jalilehvand1, Natalie S Sisombath, Adam C Schell

  • 1Department of Chemistry, University of Calgary , 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada.

Inorganic Chemistry
|February 20, 2015
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Summary

This study reveals how lead(II) and L-cysteine form complexes in alkaline solutions, identifying dithiolate and trithiolate structures using advanced spectroscopy. These findings offer insights into lead coordination in biological systems.

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Area of Science:

  • Coordination Chemistry
  • Bioinorganic Chemistry
  • Spectroscopy

Background:

  • L-cysteine (H2Cys) is a multidentate chelator capable of coordinating with metal ions.
  • Understanding lead(II) complexation with cysteine is crucial for elucidating lead interactions in biological systems, such as proteins and enzymes.
  • Previous studies have explored lead-cysteine interactions, but detailed structural characterization in alkaline aqueous solutions remains an area of interest.

Purpose of the Study:

  • To investigate the structural characteristics and coordination modes of lead(II) complexes formed with L-cysteine in alkaline aqueous solutions.
  • To determine the influence of L-cysteine concentration and pH on the speciation and structure of lead(II)-cysteine complexes.
  • To establish spectroscopic fingerprints for different lead(II) coordination environments with cysteine for potential application in biological systems.

Main Methods:

  • Solution studies utilizing a combination of spectroscopic techniques: 207Pb, 13C, and 1H NMR, Pb LIII-edge X-ray absorption (EXAFS), and UV-vis spectroscopy.
  • Electrospray ion mass spectrometry (ESI-MS) was employed for complementary analysis.
  • Systematic variation of H2Cys/Pb(II) mole ratios and lead(II) concentrations (0.01-0.1 M) at controlled pH (9.1-10.4).

Main Results:

  • At low H2Cys/Pb(II) ratios, dithiolate complexes ([Pb(S,N-Cys)2](2-) and [Pb(S,N,O-Cys)(S-HCys)](-)) dominated, with specific Pb-ligand bond distances determined.
  • At high free cysteinate concentrations, conversion to trithiolate ([Pb(S,N-Cys)(S-HCys)2](2-)) and minor PbS3-coordinated ([Pb(S-HCys)3](-)) complexes was observed.
  • Spectroscopic data (207Pb NMR chemical shifts and UV-vis absorption bands) provided distinct signatures for dithiolate and trithiolate species, correlating with coordination environments.

Conclusions:

  • The study successfully characterized lead(II) complexation with L-cysteine in alkaline solutions, revealing pH and ligand concentration-dependent speciation.
  • The identified coordination modes (dithiolate, trithiolate, and PbS3) and their spectroscopic signatures provide valuable models for understanding lead interactions in biological macromolecules.
  • This research offers essential spectroscopic fingerprints for structural elucidation of lead(II) coordination in proteins and enzymes.