Coordination compounds are the backbone of inorganic chemistry. They're made up of a central metal atom surrounded by ligands. Naming these compounds follows specific rules that help chemists understand their structure and properties.

Knowing how to name and write formulas for coordination compounds is crucial. It helps you communicate their composition, predict their behavior, and understand their role in various chemical reactions and applications.

Nomenclature of Coordination Compounds

Components of Coordination Compounds

• Coordination compounds consist of a central metal atom or ion bonded to a group of surrounding molecules or ions called ligands
• The identity of the ligands and the oxidation state of the central metal atom determine the name of a coordination compound
• Ligands are named before the central metal atom, and their names are separated by a space (cobalt(III) hexaammine chloride)
• Anionic ligands end in the suffix -o (chloro, cyano, hydroxo), while neutral ligands typically use their molecular name (aqua, ammine, carbonyl)

Prefixes and Suffixes in Nomenclature

• If there are multiple ligands of the same type, prefixes such as di-, tri-, tetra-, penta-, and hexa- are used to indicate the number of each ligand (dichlorobis(ethylenediamine)cobalt(III))
• If the complex is an anion, the suffix -ate is added to the metal name, and the entire complex is named as a salt with the cation (potassium hexacyanocobaltate(III))
• The oxidation state of the central metal atom is written as a Roman numeral in parentheses after the metal name (cobalt(III))

Oxidation States in Coordination Compounds

Determining Oxidation States

• The oxidation state of the central metal atom is determined by the overall charge of the complex and the charges of the ligands
• The sum of the oxidation state of the metal and the charges of the ligands must equal the overall charge of the complex
• Neutral ligands do not contribute to the overall charge of the complex, while anionic ligands contribute negative charges

Calculating Oxidation States

• The oxidation state of the metal can be calculated by subtracting the sum of the ligand charges from the overall charge of the complex
• For example, in $[Co(NH_3)_6]Cl_3$, the overall charge is +3 (from the three chloride anions), and each ammine ligand is neutral, so the oxidation state of cobalt is +3
• In $[Fe(CN)_6]^{4-}$, the overall charge is -4, and each cyanide ligand contributes a -1 charge, so the oxidation state of iron is calculated as -4 - (-6) = +2

Ligands and Prefixes in Nomenclature

Common Ligands

• Common anionic ligands include halides (chloro, bromo, iodo, fluoro), cyanide (cyano), hydroxide (hydroxo), and oxalate (oxalato)
• Common neutral ligands include water (aqua), ammonia (ammine), and carbon monoxide (carbonyl)
• Polydentate ligands, which bond to the metal through multiple atoms, have special names such as ethylenediamine (en) and bipyridine (bipy)

Prefixes for Multiple Ligands

• Greek prefixes (di-, tri-, tetra-, penta-, hexa-) are used to indicate the number of each type of ligand in the complex (hexaamminecobalt(III) chloride)
• If the ligand itself contains a Greek prefix (e.g., ethylenediamine), alternate prefixes such as bis-, tris-, and tetrakis- are used to avoid confusion (tris(ethylenediamine)cobalt(III) chloride)

Formula Writing for Coordination Compounds

Formula Components and Order

• The formula of a coordination compound is written with the central metal atom first, followed by the ligands in alphabetical order
• The ligands are enclosed in parentheses, with the number of each ligand indicated by a subscript ($[Co(NH_3)_6]Cl_3$)
• The overall charge of the complex is written as a superscript outside the parentheses ($[Fe(CN)_6]^{4-}$)

Anionic Complexes and Oxidation States

• If the complex is an anion, it is written in square brackets with the cation outside the brackets (potassium hexacyanocobaltate(III), $K_3[Co(CN)_6]$)
• The oxidation state of the metal is not explicitly written in the formula but can be determined from the charges of the ligands and the overall charge of the complex