Properties of Transition Metals

• The presence of trace amounts of transition metal ions like Cr³⁺, Fe²⁺, and Fe³⁺ in colorless minerals leads to the various colors of gemstones.

Properties of Transition Metals

• Transition metals commonly have partially filled d subshells or form ions with incomplete d subshells.
• Zinc, cadmium, and mercury do not fit the electron configuration characteristic, and do not belong in the transition metal category.
• Notable properties include distinct coloring, forming paramagnetic compounds, catalytic activity, and forming complex ions.
• When reading across a period, there's an increase in atomic numbers, electrons in the outer shell, and nuclear charge via protons
• Atomic radii decrease rapidly from sodium to argon, while electronegativities and ionization energies consistently rise.
• The nuclear charge increases from Sc to Cu, but electrons get added to the inner 3d subshell to shield the 4s electrons.
• Electronegativities and ionization energies increase slightly from scandium to copper, relative to sodium to argon.
• All transition metals except copper should react with strong acids to form hydrogen gas, however, most are inert or slow to react due to a protective oxide layer

Electron Configurations

• Calcium has the electron configuration [Ar]4s².
• Scandium across to copper, electrons are added to the 3d orbitals.
• Scandium has the outer electron configuration 4s²3d¹, titanium is 4s²3d², etc.
• Chromium and copper are exceptions with outer electron configurations of 4s¹3d⁵ and 4s¹3d¹⁰.
• When first-row transition metals make cations, electrons are first taken from the 4s orbitals and then the 3d orbitals.

Oxidation States

• Transition metals display variable oxidation states in compounds
• Common oxidation states include +2 and +3
• The +3 oxidation states are more stable at the start of the series
• The +2 oxidation states are more stable towards the end.
• Manganese reaches the highest oxidation state at +7 (4s²3d⁵).
• Transition metals usually display their highest oxidation states in compounds containing highly electronegative elements like oxygen and fluorine.

Coordination Compounds

• Coordination compounds typically contain a complex ion and a counter ion.
• Coordination compounds can be understood from the work of Alfred Werner, who created Werner's coordination theory.
• Werner postulated elements exhibit primary valence, the oxidation number, and secondary valence, the coordination number.
• Use [Co(NH3)6]Cl3 to indicate that ammonia molecules/cobalt atoms form a complex ion.
• Molecules or ions surrounding the metal in a complex ion are ligands and act as Lewis bases by donating electrons to metals, Lewis acids.
• Coordination number in coordination compounds is the number of donor atoms surrounding the central metal atom in a complex ion.

Geometry of Coordination Compounds

• Structure and coordination number of the metal atom relate to each other

• Coordination Number 2: Linear

• Coordination Number 4: Tetrahedral or square planar

• Coordination Number 6: Octahedral

• Stereoisomers are compounds with the same types/numbers of atoms bonded together but different spatial arrangements.

• Geometric and optical isomers (enantiomers) are two types of stereoisomers.

• Square planar complex ions with two different monodentate ligands can exhibit geometric isomerism.

• Complexes with a coordination number of 6 have octahedral geometry.

• Geometric isomers are possible in octahedral complexes when two or more different ligands are present.

Crystal Field Theory

• Crystal field theory explains bonding in complex ions using only electrostatic forces
• Two types of electrostatic interaction:
  • Attraction between the positive metal ion and the negatively charged ligand or the negatively charged end of a polar ligand.
  • Electrostatic repulsion between the lone pairs on the ligands and the electrons in the d orbitals of the metals.
• Five d orbitals experience electrostatic repulsion, and the magnitude relies on the d orbital involved.
• The name "crystal field" is associated with the theory used to explain solid, crystalline material properties, also used to study coordination compounds.

Crystal Field Splitting

• The five d orbitals in an octahedral complex are split between two sets of energy levels

• Crystal field splitting (Δ) is the energy difference between two sets of d orbitals when ligands are present.

• The magnitude of Δ depends on the metal and the ligand nature: a direct effect on color /magnetic properties.

• White light is a combination of all colors, and a substance will appear black if it absorbs all visible light, and white if none

• If an object absorbs all light but reflects green, it appears green.

• Absorption happens when the energy of a photon equals the energy difference between the ground/excited state.

• Measure crystal field splitting by using spectroscopy to determine the wavelength in which light is absorbed.

• I⁻ < Br⁻ < Cl⁻ < OH⁻ < F⁻ < H2O < NH3 < en < CN⁻ < CO is a spectrochemical series; it is a list of ligands arranged in increasing order of their abilities to split the d orbital energy levels.

• CO and CN⁻ are strong-field ligands, and halide/hydroxide ions are weak-field ligands.

Magnetic Properties

• Crystal field splitting determines magnetic properties of a complex ion
• In a complex ion, the actual number of unpaired electrons/spins can be found by magnetic measurements
• A distinction between low- and high-spin complexes can be made only if the metal ion has more than three and fewer than eight d electrons

Reactions of Coordination Compounds

• Complex ions undergo ligand exchange (or substitution) reactions in solution.
• Stability in a complex ion and its tendency to react is called kinetic lability
• Kinetic lability is useful when studying ligand exchange reactions,
• Stability context measured by the species' formation constant
• Complexes that undergo rapid ligand exchange reactions are called labile complexes.
• Inert complex exchanges ligands order of hours or even days.
• Most complex ions containing Co³⁺, Cr³⁺, and Pt²⁺ are kinetically inert.

Coordination Compounds in Living Systems

• Coordination compounds are essential in the storage and transport of oxygen, electron transfer agents, catalysts, and photosynthesis.
• Hemoglobin, containing four folded long chains called subunits, functions as an oxygen carrier for metabolic processes.