ligand

Examples of ligand in the following topics:

• Coordination Number, Ligands, and Geometries

  • Many ligands are capable of binding metal ions through multiple sites, usually because the ligands have lone pairs on more than one atom.
  • Thus, the halides and pseudohalides are important anionic ligands.
  • For example, trans-spanning ligands are bidentate ligands that can span coordination positions on opposite sides of a coordination complex.
  • A bridging ligand links two or more metal centers.
  • These shapes are defined by orbital overlap between ligand and metal orbitals and ligand-ligand repulsions, which tend to lead to certain regular geometries.

• Reactions of Coordination Compounds

  • The atom within a ligand that is bonded to the central atom or ion is called the donor atom.
  • The ions or molecules surrounding the central atom are called ligands.
  • The central atom or ion, together with all ligands, comprise the coordination sphere.
  • One important indicator of reactivity is the rate of degenerate exchange of ligands.
  • Complexes where the ligands are released and rebound rapidly are classified as labile.

• Octahedral Complexes

  • Octahedral complexes have six ligands symmetrically arranged around a central atom, defining the vertices of an octahedron.
  • When two or more types of ligands are coordinated to an octahedral metal center, the complex can exist as isomers.
  • The number of possible isomers can reach 30 for an octahedral complex with six different ligands (in contrast, only two stereoisomers are possible for a tetrahedral complex with four different ligands).
  • The dz2 and dx2−y2 (the so-called eg set), which are aimed directly at the ligands, are destabilized.
  • Rearrangements where the relative stereochemistry of the ligands change within the coordination sphere

• Chelating Agents

  • Chelating agents are ligands for metals that bind via multiple atoms, thus taking up several coordination sites on the metal.
  • Chelating agents, unlike the other ligands in coordination compounds, bind via multiple atoms in the ligand molecule, not just one.
  • The chelate effect describes the enhanced affinity of chelating ligands for a metal ion compared to the affinity of a collection of similar nonchelating (monodentate) ligands for the same metal.
  • In (1), the bidentate ligand ethylenediamine forms a chelate complex with the copper ion.
  • Thus, proteins, polysaccharides, and polynucleic acids are excellent polydentate ligands for many metal ions.

• Crystal Field Theory

  • Crystal field theory states that d or f orbital degeneracy can be broken by the electric field produced by ligands, stabilizing the complex.
  • The Crystal Field Theory (CFT) is a model for the bonding interaction between transition metals and ligands.
  • It describes the effect of the attraction between the positive charge of the metal cation and negative charge on the non-bonding electrons of the ligand.
  • The electrons in the d orbitals of the central metal ion and those in the ligand repel each other due to repulsion between like charges.
  • It arises due to the fact that when the d orbitals are split in a ligand field, some of them become lower in energy than before.

• Coloring Agents

  • This approach is described by the ligand field theory (LFT) and the molecular orbital theory (MO).
  • A charge transfer band entails promotion of an electron from a metal-based orbital into an empty ligand-based orbital (Metal-to-Ligand Charge Transfer or MLCT).
  • The converse will also occur: excitation of an electron in a ligand-based orbital into an empty metal-based orbital (Ligand to Metal Charge Transfer or LMCT).
  • Since the nature of the ligands and the metal can be tuned extensively, a variety of colors can be obtained.
  • Changing the metal or the ligand can change the color of the coordination complex.

• Metal Cations that Act as Lewis Acids

  • Ligands create a complex when forming coordinate bonds with transition metals ions; the transition metal ion acts as a Lewis acid, and the ligand acts as a Lewis base.
  • Common ligands include H2O and NH3 ; examples of complexes include the tetrachlorocobaltate(II) ion, [CoCl4]2- and the hexaqua-iron(III) ion, [Fe(H2O)6]3+.
  • Nearly all compounds formed by the transition metals can be viewed as collections of the Lewis bases—or ligands—bound to the metal, which functions as the Lewis acid.
  • Examples of several metals (V, Mn, Re, Fe, Ir) in coordination complexes with various ligands.
  • All these metals act as Lewis acids, accepting electron pairs from their ligands.

• Isomers in Coordination Compounds

  • In octahedral complexes—with four of one ligand and two of another—and square planar complexes—with two of one ligand and two of another—there are two different arrangements of the same atoms with the same bonds.
  • In cis molecules, the two ligands are on the same side of the complex.
  • In a fac isomer, any two identical ligands are adjacent or cis to each other.
  • This type of isomerism occurs when the center ion of the complex is also a potential ligand.
  • Linkage isomerism occurs with ambidentate ligands that can bind in more than one place.

• Naming Coordination Compounds

  • When naming a complex ion, the ligands are named before the metal ion.
  • Write the names of the ligands in the following order: neutral, negative, positive.
  • Polydentate ligands (e.g., ethylenediamine, oxalate) receive bis-, tris-, tetrakis-, etc.
  • The total charge on the ligands is -4.
  • The coordination number of ligands attached to more than one metal (bridging ligands) is indicated by a subscript to the Greek symbol μ placed before the ligand name.

• Tetrahedral and Square Planar Complexes

  • Tetrahedral complexes have ligands in all of the places that an octahedral complex does not.
  • In contrast, the dxy,dyz, and dxz axes lie directly on top of where the ligands go.
  • The removal of a pair of ligands from the z-axis of an octahedron leaves four ligands in the x-y plane.
  • The removal of the two ligands stabilizes the dz2 level, leaving the dx2-y2 level as the most destabilized.
  • These compounds typically have sixteen valence electrons (eight from ligands, eight from the metal).