Z Metalle und Komplexe Metallic Properties and Coordination Chemistry Fundamentals

Classification and Periodic Trends of Metals

• Position in the Periodic System: Approximately $80 \%$ of all known elements are metals. In the periodic table, they are located to the left of the elements Boron ($B$), Silicon ($Si$), Germanium ($Ge$), Antimony ($Sb$), and Astatine ($At$).

• Elemental Classification:

  • Metals (White in Figure 1)

  • Semimetals/Metalloids (Grey in Figure 1)

  • Non-metals (Blue in Figure 1)

• Periodic Trends:

  • Within a Period: Metallic character decreases from left to right. This direction marks the transition from metals to semimetals and then to non-metals.

  • Within a Group: Metallic character increases from top to bottom.

  • Consequence of Trends: The transition point between metals and non-metals shifts toward higher group numbers as the period increases.

  • In the $2^{nd}$ Period: The transition occurs between groups $2$ and $13$.

  • In the $6^{th}$ Period: The transition occurs between groups $16$ and $17$.

Physical Properties of Metals

• Metallic Luster: Metals exhibit a characteristic surface shine known as metallic luster. This is defined by opacity and high reflectivity, a property utilized in applications such as mirrors.

• Ductility: Metals are capable of being stretched and plastically deformed without breaking. This behavior is called ductility.

  • Example: Gold Leaf: Very thin layers can be produced. Gold leaf is approximately $0.1\,\mu m$ thick and consists of roughly $230$ layers of atoms.

• Electrical and Thermal Conductivity:

  • Metals are excellent conductors of heat and electricity.

  • Coinage Metals: The best conductivity values are found in Group $11$ elements.

  • Temperature Dependence: Electrical conductivity in metals is temperature-dependent and is proportional to the reciprocal of the temperature. As temperature increases, the electrical conductivity decreases due to increased lattice vibrations ("Gitterschwingungen").

• Melting and Boiling Points: Metals are characterized by high melting and boiling points, which are associated with high enthalpies of vaporization.

Models of Metallic Bonding

• Electron Gas Model (Elektronengasmodell):

  • This is a simple model for understanding metallic bonds.

  • Structure: Positive atomic cores ("Ionenrümpfe") form a fixed lattice. This lattice is surrounded by valence electrons that move freely through the structure as an "electron gas."

  • Binding Force: The lattice is held together by the electrostatic attraction between the positive atomic cores and the negatively charged electron gas.

  • Explanation of Properties:

  • Ductility: When lattice planes slide past each other, the binding forces remain intact due to the fluid nature of the electron gas.

  • Electrical Conductivity: This arises from the free mobility of the electrons within the gas.

• Band Model (Bändermodell):

  • This provides a more precise explanation by considering the orbitals of individual atoms in a crystal.

  • Interaction: Two atoms of the same metal have identical (degenerate) energy levels. As they move closer, their energy levels interact.

  • Molecular Orbitals (MOs): The combination of $n$ atomic orbitals leads to the formation of $n$ molecular orbitals. This lifts the degeneracy, making MOs energetically distinguishable.

  • Formation of Bands: In a metal crystal, the extremely dense sequence of energy states forms an "energy band."

Detailed Band Model Analysis of Lithium

• Electron Configuration of Lithium: $1s^2 2s^1 2p^0$

• Band Separation: In Lithium, the band formed from the $1s$ atomic orbitals is distinct and separated from the band formed by the $2s$ orbitals.

• Forbidden Zone (Band Gap): The energy region where no energy levels exist between bands is called the "forbidden zone" or "band gap."

• Overlapping Bands: In Lithium, the energy bands formed by the $2s$ and $2p$ orbitals are so strongly split that they overlap.

• Conductivity Requirements:

  • Because the distance between energy levels within a band is extremely small, energy changes occur almost continuously.

  • Electrons are delocalized, making Lithium electrically conductive.

  • Condition: Electrical conductivity cannot occur if the bands are either completely empty or completely filled.

Introduction to Complex Compounds

• Definition: A complex compound (or coordination compound) consists of a coordination center surrounded by a ligand shell.

• Coordination Center: This is usually a central atom or ion, often transition metals because they possess vacant $d$-orbitals.

• Ligands: These are ions or molecules that surround the center.

• Coordination Number (CN): The total number of ligands chemically bonded to the central particle is referred to as the coordination number.

• Nature of the Bond: Unlike standard covalent bonds, the ligands contribute all the bonding electrons. This is termed a coordinate bond ("koordinative Bindung").

Coordination Bonding as a Lewis Acid-Base Reaction

• Lewis Acid-Base Interaction:

  • Ligand: Acts as the Lewis Base or electron pair donor.

  • Central Particle: Acts as the Lewis Acid or electron pair acceptor.

• Terminology Distinction: The term "electron pair" is emphasized to distinguish these roles from electron donors/acceptors in redox reactions.

• The bond formation: A Lewis base uses an electron pair from a filled orbital to bind into an empty orbital (frequently a $d$-orbital in transition metals) of the Lewis acid.

• The $18$-Electron Rule:

  • This is the transition metal equivalent to the Octet Rule of the $2^{nd}$ period.

  • Complexes with noble gas configurations are generally especially stable.

  • To reach noble gas configuration in the $4^{th}$, $5^{th}$, or $6^{th}$ periods, an atom requires $18$ electrons ($2s + 6p + 10d$).

Nomenclature of Complex Compounds

• Order of Names: The name of the ligand is stated first, followed by the name of the central atom.

• Ligand Suffix: Ligands are typically identified by the suffix "-o" attached to the ion or molecule name.

  • Examples: sulfato ($SO_4^{2-}$), chlorido ($Cl^-$).

  • IUPAC Update: Previously, anions ending in "-id" (like Fluoride or Oxide) dropped the "-id" (fluoro, oxo). IUPAC has abolished this special rule to achieve uniformity; they are now named as fluorido and oxido.

• Special Ligand Names:

  • Water: aqua

  • Ammonia: ammine (spelled with "mm" to avoid confusion with the organic class of amines).

• Quantifying Ligands: The number of identical ligands is indicated by Greek prefixes: mono, di, tri, tetra, penta, hexa, etc.

• Oxidation State: The oxidation number of the central atom must be written in Roman numerals inside parentheses at the end of the name to avoid ambiguity (e.g., Potassium hexacyanidoferrate).

Naming Categorization by Complex Charge

• Formula Ordering: Anionic ligands are written before neutral ligands.

• Naming Ordering: Ligands are listed in alphabetical order within the name.

  • Example: $[CrCl_2(H_2O)_4]^+$ is named Tetraaquadichloridochromium(III).

• Kationic Complexes: Examples provided like $[Ag(NH_3)_2]Cl$ (Diamminesilver(I) chloride).

• Neutral Complexes: Examples provided like $[Ni(CO)_4]$ (Tetracarbonylnickel(0)).

• Anionic Complexes:

  • The name of the central atom ends with the suffix -ate.

  • Certain elements use their Latin roots when named as an anion:

  • Silver: Argentate

  • Gold: Aurate

  • Iron: Ferrate

  • Copper: Cuprate

  • Lead: Plumbate

  • Tin: Stannate

  • Example: $Na[Ag(CN)_2]$ (Sodium dicyanidoargentate(I)).

Cation Acids (Kationensäuren)

• Mechanics of Cation Acids: Using the complex cation $[Fe(OH_2)_6]^{3+}$ (Hexaaquairon(III) cation) as an example, the O-H bond in the water ligand is weakened because the Oxygen atom is bonded to the iron central atom.

• Polarization: The high charge density of the central ion causes strong polarization of the water molecule, making it easier to release a proton compared to pure water.

• Protolyse Equilibrium: A new equilibrium is established, leading to a decrease in pH value:

  • $[Fe(OH_2)_6]^{3+} + H_2O \rightleftharpoons [Fe(OH_2)_5(OH)]^{2+} + H_3O^+$

• Classification:

  • These complexes react acidicly and are called Cation Acids.

  • Generally, complexes with central ions of high charge density act as cation acids.

  • Salts of metal ions with high charge density that can form aqua complexes are also considered cation acids.