Transition Metals & Gemstone Colors

• The presence of transition metal ions like chromium(III), iron(II), and iron(III) in colorless minerals results in the various colors of gemstones.

Properties of Transition Metals

• Transition metals usually possess incomplete d subshells or produce ions with incomplete d subshells.
• Zinc, cadmium, and mercury do not possess this property.
• Characteristics include unique coloring, creation of paramagnetic substances, catalytic capabilities, and a propensity to generate complex ions.
• Atomic radii decrease and electronegativities and ionization energies rise steadily across any period from left to right.
• 3d electrons shield 4s electrons from increasing nuclear charge which make atomic radii decrease less rapidly.
• Electronegativities and ionization energies increase only slightly from Scandium to Copper.
• Most transition metals, other than copper, should react with strong acids to produce hydrogen gas, however most transition metals are inert due to a protective oxide layer.

Electron Configurations of Transition Metal Ions

• When transition metals form cations, electrons are removed from the 4s orbitals before the 3d orbitals.
• The outer electron configuration of Iron(II) is 3d6, not 4s23d4.
• Transition metals display variable oxidation states in compounds.
• Common oxidation states for each element include +2, +3 or both with +3 being more stable at the beginning of the series, whereas +2 states are more stable at the end.
• The highest oxidation state is +7, which is seen in manganese.
• Transition metals usually display their highest oxidation states in compounds with very electronegative elements such as oxygen and fluorine.

Coordination Compounds

• It usually consists of a complex ion and counter ion, and may not always contain complex ions.
• Coordination compounds stems from the work of Alfred Werner, who proposed Werner's coordination theory.
• Most elements exhibit primary and secondary valence.
• Werner determined that primary valence corresponds to the oxidation number and secondary valence to the coordination number.
• Molecules or ions surrounding the metal in a complex ion are ligands.
• Ligands participate in Lewis acid-base reactions.
• Lewis bases donate one or more electron pairs.

Ligands

• The atom in a ligand that is directly bound to the metal atom is the donor atom.
• Coordination number is the number of donor atoms surrounding the central metal atom in a complex ion.
• Depending on the number of donor atoms present, ligands are classified as monodentate, bidentate, or polydentate.
• Bidentate and polydentate ligands are called chelating agents because they can hold the metal atom like a claw.

Oxidation Number of Metals

• In a complex ion, the net charge is the sum of charges on the central metal atom and ligands.
• In the [PtCl6]2− ion, the oxidation number of platinum is +4.
• If ligands do not bear net charges, the oxidation number of the metal equals the charge of the complex ion.

Naming Coordination Compounds

• The rules for naming coordination compounds are as follows:
  • The cation is named before the anion.
  • Within a complex ion, the ligands are named first and the metal ion is named last.
  • Anionic ligands' names end with the letter 'o', and neutral ligands are called by the molecule's name.
  • Use Greek prefixes (di-, tri-, tetra-, penta-, hexa-) to name multiple ligands of a particular type.
  • The oxidation number of the metal is written in Roman numerals following the name of the metal.
  • If the complex is an anion, its name ends in '-ate'.
  • For example, in K4[Fe(CN)6] the anion [Fe(CN)6]4− is called hexacyanoferrate(II) ion.

Geometry of Coordination Compounds

• Structure and the coordination number of the metal atom relate to each other.
• Coordination compounds may exhibit geometric isomers and optical isomers (enantiomers).
• Many coordination compounds do not have stereoisomers.
  • Coordination number 2 has a linear structure
  • Coordination number 4 has a tetrahedral or square planar structure
  • Coordination number 6 has an octahedral structure
• Square planar complex ions with two different monodentate ligands can exhibit geometric isomerism.
• Geometric isomers have different properties, such as melting point, boiling point, color, solubility in water, and dipole moment.
• Certain octahedral complex ions can give rise to enantiomers.

Crystal Field Theory

• Crystal field theory addresses color and magnetic properties.
• It explains the bonding in complex ions in terms of electrostatic forces.
• A central metal atom is surrounded by lone pairs of electrons and all five d orbitals experience electrostatic repulsion. As a result, the five d orbitals in an octahedral complex split between two sets of energy levels:
• Higher level with two orbitals (dx2-y2 and dz2)
• Lower level with three equal-energy orbitals (dxy, dyz, and dxz)
• Crystal field splitting (∆) is the energy difference between two sets of d orbitals in a metal atom when ligands are present.
• D-to-d transition must occur for a transition metal complex to show color.
• Ions with d0 or d10 electron configurations are usually colorless.
• The magnitude of crystal field splitting is measured using spectroscopy to determine the wavelength at which light is absorbed.
• Spectrochemical series is a list of ligands arranged in increasing order of splitting the d orbital energy levels.
• Magnetism is impacted by D.
• The arrangement of electrons is determined by the amount of stability gained by having maximum parallel spins versus the investment in energy required to higher d orbitals.
• A distinction between low- and high-spin complexes can be made only if the metal ion contains more than three and fewer than eight d electrons.
• The splitting pattern for a tetrahedral ion is the reverse of that for octahedral complexes and most tetrahedral complexes are high-spin complexes.

Reactions of Coordination Compounds

• Complex ions undergo ligand exchange or substitution reactions in solution.
• Kinetic lability is a complex ion's tendency to react.
• Labile complexes undergo rapid ligand exchange reactions.
• The smaller the activation energy, the larger the rate constant, and hence the greater the rate.
• Inert complex ions go under slow exchange reactions.
• Most complex ions containing Co3+, Cr3+ are kinetically inert.

Coordination Compounds in Living Systems

• They are essential in the storage and transport of oxygen, as electron transfer agents, as catalysts, and in photosynthesis.