IUPAC Recommendations (2004)

1. The naming of Negatively Charged Ligands (Anionic Ligands) – 2004 IUPAC Recommendations

As per the 2004 IUPAC draft recommendations, the names of negatively charged ligands (anionic ligands) must end in "o". This rule standardizes the naming of anions by replacing their traditional endings (ide, ite, ate) as follows:

1. If the anion name ends in "-ide", replace it with "-ido"
   
   

   • Example: Chloride (Cl⁻) → Chlorido

   • Example: Cyanide (CN⁻) → Cyanido

2. If the anion name ends in "-ite", replace it with "-ito"
   
   

   • Example: Nitrite (NO₂⁻) → Nitrito

   • Example: Sulfite (SO₃²⁻) → Sulfito

3. If the anion name ends in "-ate", replace it with "-ato"
   
   

   • Example: Nitrate (NO₃⁻) → Nitrato

   • Example: Carbonate (CO₃²⁻) → Carbonato

2. Donor Atom Specification (Kappa Notation, κ)

When a ligand can coordinate through different donor atoms, the κ (kappa) notation is used to specify the coordination site.

• Example:

  • [NiBr₂(Me₂PCH₂CH₂PMe₂)] → Dibromido[ethane-1,2-diylbis(dimethylphosphane)-κ²P]nickel(II)

[ Note: 1. Since the phosphorus atoms directly bind to the metal, the κ²P notation is used to specify that two phosphorus atoms are involved in coordination. 

2. "ethane-1,2-diyl" indicates the -CH₂CH₂- backbone.

3."bis(dimethylphosphane)" represents the two PMe₂ groups attached at positions 1 and 2.]

• • [Co(NO₂)(NH₃)₅]²⁺ → Pentaammine-nitrito-κN-cobalt(III) (when bound via nitrogen)

  • [Co(ONO)(NH₃)₅]²⁺ → Pentaammine-nitrito-κO-cobalt(III) (when bound via oxygen)

3.  Naming Bridging Ligands in Polynuclear Complexes

• The Greek letter "μ" (mu) is used before the ligand name to indicate a bridging ligand.

• The number of bridges is specified using di-, tri-, tetra-, etc. when multiple ligands are involved.

• If a ligand bridges more than two metal centers, the number of metal centers involved is indicated as a right subscript, e.g., μ₃, μ₄.

Examples :

 [{(NH₃)₅Co(μ-OH)Co(NH₃)₅]}⁴⁺

1. Identify the central metal atoms

   • Two Cobalt (Co) atoms are present.

2. Identify the ligands

   • One hydroxido (OH⁻) ligand acts as a bridge.

   • Each cobalt atom is coordinated by five ammine (NH₃) ligands.

3. Apply Bridging Notation

   • Since one hydroxido ligand is bridging, we use μ-hydroxido.

   • Both cobalt atoms are coordinated by five ammine ligands, so we use bis(pentaamminecobalt).

4. Apply Charge Notation

   • The overall charge is 4+, so we include (4+) at the end.

5. Final Name:
   μ-hydroxido-bis(pentaamminecobalt)(4+)

Other Examples with μ-notation

1. [Cr₂(μ-Cl)₂Cl₄]²⁻ → Di-μ-chlorido-tetrachloridodichromate(II)
   (Two chromium atoms connected by two bridging chlorido ligands and four terminal chlorido ligands)

2. [{RuCl₅(μ-Cl)2RuCl₅]}²⁻ → Di-μ-chlorido-bis(pentachloridoruthenate)(2−)
   (Two ruthenium centers bridged by two chloride ligands, each also having five terminal chloride ligands)

4. Metal-Metal Bonding in Polynuclear Complexes

• Metal-metal bonds are explicitly indicated using a long dash (—) between metal symbols.

• This notation clearly differentiates metal-metal bonded species from simply coordinated metal centers.

Examples 

 [Mn₂(CO)₁₀]

1. Identify the central metal atoms

   • Two Manganese (Mn) atoms are present.

2. Identify the ligands

   • Ten carbonyl (CO) ligands are present.

3. Recognizing the Metal-Metal Bond

   • There is a direct metal-metal (Mn—Mn) bond in this structure.

4. Apply Naming Rules

   • Decacarbonyl → Ten CO ligands.

   • Dimanganese → Two Mn atoms.

   • (Mn—Mn) → Explicit metal-metal bonding notation.

5. Final Name:
   Decacarbonyl(dimanganese)(0)(Mn—Mn)

[Re₂Cl₈]²⁻

1. Identify the central metal atoms

   • Two Rhenium (Re) atoms are present.

2. Identify the ligands

   • Eight chlorido (Cl⁻) ligands are present.

3. Recognizing the Metal-Metal Bond

   • The two rhenium atoms share a direct metal-metal bond (Re—Re).

4. Apply Naming Rules

   • Octachlorido → Eight Cl⁻ ligands.

   • Dirhenate(2−) → Two Re atoms with an overall charge of 2−.

   • (Re—Re) → Explicit metal-metal bonding.

5. Final Name:
   Octachloridodirhenate(2−) (Re—Re)

Other Examples with Metal-Metal Bonds

1. [Rh₂(O₂CMe)₄] → Tetrakis(acetato)dirhodium(II) (Rh—Rh)
   (Two rhodium centers bonded with four acetate ligands)

2. [Mo₂Cl₄(μ-Cl)₂]²⁻ → Di-μ-chlorido-tetrachloridodimolybdenum(2−) (Mo—Mo)
   (Two molybdenum atoms bridged by two chlorido ligands, each also having two terminal chlorido ligands)

5. Oxidation State and Charge Notation

1. TheThe oxidation state of the central atom is given in Roman numerals in parentheses immediately after the metal’s name.

[Cr(NH₃)₆]³⁺ → Hexaamminechromium(III) ion

2. Alternatively, The net charge of a coordination entity can also be indicated in Arabic numerals in parentheses after the complex’s name.

[CoCl(NH₃)₅]²⁺ → Pentaamminechloridocobalt(2+) ion

6. Rules for Naming Isomers in Coordination Chemistry 

As per IUPAC recommendations provide systematic rules for naming stereoisomers and structural isomers of coordination compounds. These guidelines cover the spatial configuration and the relative connectivity of ligands in complex compounds.

The IUPAC rules define two broad categories of isomers:

1. Structural (constitutional) isomers – differ in connectivity of atoms.

2. Stereoisomers – have the same connectivity but differ in spatial arrangement.

1.Structural Isomers

Structural isomers can be further classified into:

• Linkage Isomers – differ in which donor atom of a ligand is bonded to the metal.

• Coordination Isomers – involve the exchange of ligands between cations and anions.

• Ionization Isomers – differ in which ligand is displaced by a counterion in solution.

• Hydrate (Solvate) Isomers – differ in the presence of water molecules inside or outside the coordination sphere.

Example of Linkage Isomerism:

• [Co(NO₂)(NH₃)₅]²⁺ → Pentaammine-nitrito-κN-cobalt(III) (N-bonded)

• [Co(ONO)(NH₃)₅]²⁺ → Pentaammine-nitrito-κO-cobalt(III) (O-bonded)

Stereoisomers

Stereoisomers can be divided into:

1. Geometrical (diastereoisomers)

   • cis-trans isomerism (square planar and octahedral complexes)

   • fac-mer isomerism (octahedral complexes)

2. Optical Isomers (Enantiomers)

   • Non-superimposable mirror images (∆ and Λ notation for octahedral complexes)

2. Geometrical Isomerism

cis-trans Isomerism (Square Planar and Octahedral Complexes)

• cis-isomer: Two identical ligands are adjacent to each other.

• trans-isomer: Two identical ligands are opposite to each other.

Example (Square Planar Complexes)

• cis-[PtCl₂(NH₃)₂] → cis-diamminedichloridoplatinum(II)

• trans-[PtCl₂(NH₃)₂] → trans-diamminedichloridoplatinum(II)

Example (Octahedral Complexes)

• cis-[CoCl₂(NH₃)₄]⁺ → cis-tetraammine-dichloridocobalt(III)

• trans-[CoCl₂(NH₃)₄]⁺ → trans-tetraammine-dichloridocobalt(III)

fac-mer Isomerism (Octahedral Complexes)

• facial (fac-) isomer: Three identical ligands are arranged at the same face of the octahedron.

• meridional (mer-) isomer: Three identical ligands form a meridian, spanning across different faces.

Example:

• fac-[CoCl₃(NH₃)₃] → fac-triamminetrichloridocobalt(III)

• mer-[CoCl₃(NH₃)₃] → mer-triamminetrichloridocobalt(III)

3. Optical Isomerism (Enantiomers)

• Enantiomers are non-superimposable mirror images.

• They are labeled using ∆ (Delta) and Λ (Lambda) notation.

• The ∆ (Delta) form has a right-handed twist.

• The Λ (Lambda) form has a left-handed twist.

Example (Octahedral Complexes with Bidentate Ligands)

• ∆-[Co(en)₃]³⁺ → ∆-Tris(ethane-1,2-diamine)cobalt(III)

• Λ-[Co(en)₃]³⁺ → Λ-Tris(ethane-1,2-diamine)cobalt(III)

4. Absolute Configuration Naming (R/S and C/A Notation)

R/S Convention for Tetrahedral Centres

General Rule:

• The R/S system, originally developed for chiral carbon centers, is directly applicable to tetrahedral metal centers without modifications.

• The ligands attached to the metal center are assigned priority numbers based on atomic number.

• Determination of R or S:

  • The lowest-priority ligand (priority 4) is placed at the back.

  • The remaining three ligands are ranked from highest (1) to lowest (3) based on atomic number.

  • If the sequence 1 → 2 → 3 moves clockwise, the configuration is R.

  • If the sequence moves counterclockwise, the configuration is S​.

Example 1: R/S Assignment in a Tetrahedral Metal Complex

For a tetrahedral metal complex of iron: [FeI(CO)(PPh3​)(C5H5)]

• Ligands: Iodide (I), Carbonyl (CO), Triphenylphosphine (PPh₃), and the Cyclopentadienyl (C5H5)

• Priority order (based on atomic number):

  1. Iodide (I) (highest priority)

  2. Phosphine (P)

  3. Carbonyl (C)

  4. Cyclopentadienyl (C5H5) (lowest priority)

• Assigning the sequence:

  1. If I → P → C moves clockwise, the complex is R.

  2. If I → P → C moves counterclockwise, the complex is S

R/S Convention for Trigonal Pyramidal Centres

General Rule:

• Trigonal pyramidal centers are handled by placing a 'phantom atom' of lowest priority in an imaginary fourth position.

• The same priority rules are applied as in tetrahedral systems.

• The viewer looks down the vector from the central metal to the lowest-priority ligand.

• A clockwise sequence of 1 → 2 → 3 is assigned R, while counterclockwise is S.

Example 2: R/S Naming in a Trigonal Pyramidal Complex

For a sulfoxide-like metal complex:

[M(NH3​)(CH3​)(H2​O)]

• Priority order:

  1. H₂O (Oxygen, atomic number 8)

  2. NH₃ (Nitrogen, atomic number 7)

  3. CH₃ (Carbon, atomic number 6)

  4. Phantom lone pair (lowest priority)

• If the rotation is clockwise, the isomer is R.

• If counterclockwise, the isomer is S​

​C/A Convention for Absolute Configuration in Octahedral Complexes

General Rule:

• The C/A notation replaces the R/S notation for octahedral centers.

• Ligands are assigned priority numbers based on atomic number (CIP rules).

• A reference axis is chosen, defined by:

  • The highest-priority ligand (Priority 1) and

  • The lowest-priority trans ligand (highest numerical value).

• The configuration is determined by:

  • C (Clockwise): If the three remaining ligands in the perpendicular plane are ordered in a clockwise manner.

  • A (Anticlockwise): If the sequence is counterclockwise​

Skew-Line Convention (Δ/Λ) for Chelated Complexes

General Rule:

• Used specifically for tris(bidentate) and bis(bidentate) complexes.

• Based on the helical arrangement of ligand strands.

• Δ (Delta) → Right-handed helical twist.

• Λ (Lambda) → Left-handed helical twist