Coordination compounds are fascinating molecules with a central metal atom bonded to surrounding ligands. These compounds play crucial roles in biology and industry, from hemoglobin carrying oxygen to catalysts driving chemical reactions.

The structure and properties of coordination compounds depend on the metal, ligands, and bonding. Understanding their nomenclature, isomerism, and electronic structure helps explain their diverse applications in medicine, manufacturing, and beyond.

Coordination Compounds

Characteristics of coordination compounds

• Molecules consisting of a central metal atom or ion bonded to surrounding molecules or ions called ligands
  • Central metal atom or ion usually a transition metal (iron, copper, cobalt) or inner transition metal (lanthanides, actinides)
  • Ligands can be neutral molecules ($\text{H}_2\text{O}$, $\text{NH}_3$) or ions ($\text{Cl}^-$, $\text{CN}^-$)
• Bonds between metal and ligands are coordinate covalent bonds where both electrons in the bond come from the ligand
• Entire coordination compound enclosed in square brackets in chemical formulas with metal listed first followed by ligands ($[\text{Co}(\text{NH}_3)_6]^{3+}$)
• Can be cationic, anionic, or neutral depending on charges of metal and ligands
  • Cationic example: $[\text{Co}(\text{NH}_3)_6]^{3+}$
  • Anionic example: $[\text{Fe}(\text{CN})_6]^{4-}$
  • Neutral example: $[\text{Pt}(\text{NH}_3)_2\text{Cl}_2]$
• The central atom and its surrounding ligands form the coordination sphere

Monodentate vs polydentate ligands

• Monodentate ligands bond to central metal atom through only one atom forming a single coordinate covalent bond
  • Examples: $\text{H}_2\text{O}$, $\text{NH}_3$, $\text{Cl}^-$, $\text{CN}^-$
• Polydentate ligands bond to central metal atom through two or more atoms forming multiple coordinate covalent bonds
  • Bidentate ligands bond through two atoms (ethylenediamine, $\text{H}_2\text{NCH}_2\text{CH}_2\text{NH}_2$)
  • Hexadentate ligands bond through six atoms (EDTA, ethylenediaminetetraacetic acid)
• Polydentate ligands form more stable complexes due to chelate effect where entropy of system increases when multiple monodentate ligands replaced by a single polydentate ligand

Nomenclature for coordination compounds

• Name consists of ligands followed by central metal atom or ion
• Ligands named first in alphabetical order followed by metal
  • Anionic ligands end in "-o" (chloro for $\text{Cl}^-$, cyano for $\text{CN}^-$)
  • Neutral ligands named as molecule (aqua for $\text{H}_2\text{O}$, ammine for $\text{NH}_3$)
• Greek prefixes (di-, tri-, tetra-) used to indicate number of each type of ligand ($[\text{Co}(\text{NH}_3)_6]^{3+}$ named hexaamminecobalt(III) ion)
• Oxidation state of metal indicated by Roman numeral in parentheses after metal name
  • Oxidation state calculated by considering charges of ligands and overall charge of complex
• The coordination number, which is the number of ligand donor atoms bonded to the central atom, is often included in the name

Isomerism in coordination complexes

• Geometric isomers have same chemical formula but different spatial arrangements of ligands around central metal atom
  • Square planar complexes with formula $[\text{MA}_2\text{B}_2]$ can have cis and trans isomers where A and B ligands are adjacent or opposite each other
• Optical isomers are non-superimposable mirror images of each other arising from presence of chiral centers in complex
  • Octahedral complexes with formula $[\text{MA}_3\text{B}_3]$ where A and B are different ligands can have optical isomers
  • Isomers designated as Δ (delta) and Λ (lambda) based on direction of twist of ligands around metal center
• Coordination compounds with optical isomers can exhibit different biological activities and chemical properties due to different spatial arrangements

Electronic Structure and Properties

• The spectrochemical series ranks ligands based on their ability to cause d-orbital splitting in the central metal atom
• D-orbital splitting occurs when ligands interact with the central metal, causing energy differences between d orbitals
• Complexes can be classified as high-spin or low-spin depending on how electrons fill the split d orbitals
  • High-spin complexes have electrons occupying all d orbitals before pairing
  • Low-spin complexes have electrons pairing in lower energy d orbitals before occupying higher energy ones

Applications of coordination compounds

• Hemoglobin, oxygen-carrying protein in red blood cells, contains iron(II) ion coordinated to porphyrin ligand
  • Oxygen binds to iron center allowing hemoglobin to transport oxygen throughout body
• Chlorophyll, green pigment in plants, contains magnesium ion coordinated to porphyrin ligand
  • Complex plays crucial role in photosynthesis by absorbing light energy
• Vitamin B12 is coordination compound with cobalt ion center essential for red blood cell formation and proper nervous system function
• Cisplatin, square planar platinum(II) complex, used as anticancer drug
  • Compound binds to DNA interfering with cell division and causing apoptosis in cancer cells
• Used as catalysts in various industrial processes such as Monsanto process for production of acetic acid
  • Catalyst, rhodium complex cis-$[\text{Rh}(\text{CO})_2\text{I}_2]^-$, facilitates reaction between methanol and carbon monoxide to produce acetic acid