Coordination Compounds

Classification and Characteristics of Ligands in Coordination Compounds

Definition of Ligands and Their Classification: Ligands are molecules or ions that bond to a central metal atom in a coordination complex. They are categorized based on their electrical charge and donor atoms.
Negative Ligands:
    * Cyanide ion ( $CN^-$ ): Carries a charge of $-1$ . The donor atom is Carbon ( $C$ ). In a complex, it is named "cyano" or "cyanide".
    * Halide ions ( $X^-$ such as $F^-$ , $Cl^-$ , $Br^-$ , $I^-$ ): Carry a charge of $-1$ . The donor atom is the halogen ( $X$ ). In a complex, these are named fluoride, chloride, bromide, or iodide (note: later rules specify the "-o" ending such as "chlorido").
    * Sulphato ( $SO_4^{2-}$ ): A negative ligand which ends in "-ato".
    * Oxalato ( $C_2O_4^{2-}$ ): A negative ligand ending in "-ato".
Positive Ligands: These are relatively rare and typically end in the suffix "-ium".
    * Hydrazinium ion ( $NH_3^+$ or $NH_2-NH_3^+$ ): Carries a charge of $+1$ . The donor atom is Nitrogen ( $N$ ). Its name in a complex is "hydrazinium".
    * Nitrosonium ion ( $NO^+$ ): Carries a charge of $+1$ . The donor atom is Nitrogen ( $N$ ). Its name in a complex is "nitrosonium".
    * Nitronium ion ( $NO_2^+$ ): Carries a charge of $+1$ . The donor atom is Nitrogen ( $N$ ). Its name in a complex is "nitronium".
Neutral Ligands: These molecules carry zero charge.
    * Methylamine ( $CH_3NH_2$ ): The donor atom is Nitrogen ( $N$ ). Its name in a complex is "methylamine".
    * Ammonia ( $NH_3$ ): The donor atom is Nitrogen ( $N$ ). Its name in a complex is "ammine".
    * Water ( $H_2O$ ): The donor atom is Oxygen ( $O$ ). Its name in a complex is "aqua" or "aquo".
    * Carbonyl ( $CO$ ): A neutral ligand.
    * Triphenylphosphine ( $P(C_6H_5)_3$ ): A neutral organic molecule.
    * Pyridine ( $C_5H_5N$ or $py$ ): A neutral organic molecule.
    * Thiourea ( $H_2NCSNH_2$ or $tu$ ): A neutral organic molecule.
    * Ethane-1,2-diamine ( $NH_2CH_2CH_2NH_2$ ): A neutral organic molecule.

Types of Coordination Complexes

Coordination complexes are classified into three types based on the net charge of the complex ion:

Cationic Complex: The complex ion carries a net positive charge.
* Examples: $[Co(NH_3)_6]^{3+}$ , $[Ni(NH_3)_6]^{2+}$ .
Anionic Complex: The complex ion carries a net negative charge.
* Examples: $[Ag(CN)_2]^-$ , $[Fe(CN)_6]^{4-}$ .
Neutral Complex: The complex carries no net electrical charge.
* Examples: $[Ni(CO)_4]$ , $[Co(NH_3)_3Cl_3]$ .

Rules for Writing Formulas of Mononuclear Coordination Compounds

Ordering of Ions: The formula of the cation (whether simple or complex) is always written first, followed by the formula of the anion.
Coordination Sphere: The entire coordination entity—whether it is charged or neutral—must be enclosed within square brackets.
Central Metal Atom: Within the coordination sphere (the square brackets), the symbol of the central metal atom is written first.
Ligand Arrangement: The symbols or formulas of the ligands are listed after the metal atom. They must be arranged alphabetically based on their names, regardless of their electrical charge. For example: $[Co(NH_3)_4(H_2O)Cl]Cl_2$ .

Rules for Nomenclature of Mononuclear Coordination Compounds

Order of Naming Ions:
    * The positive ion (cation) is always named first, followed by the negative ion (anion).
    * The complex part of the name starts with a small letter and is written as a single word.
    * Example: $[Co(NH_3)_6]Cl_3$ is named "hexaamminecobalt (III) chloride".
    * Non-ionic or molecular complexes are also given a one-word name. Example: $[Co(NH_3)_3(NO_2)_3]$ is named "triamminetrinitrocobalt".
Naming Specific Ligand Types:
    * Negative Ligands: Their names end in "-o". If the original name of the anion ends in "-ide", "-ite", or "-ate", the final "e" is replaced by "o" (resulting in "-ido", "-ito", or "-ato"). Examples: $CN^-$ (cyano), $Cl^-$ (chloro), $Br^-$ (bromo), $SO_4^{2-}$ (sulphato), $C_2O_4^{2-}$ (oxalato).
    * Neutral Ligands: These generally use the name of the molecule without special endings. Examples: $NH_3$ (ammine), $H_2O$ (aqua), $CO$ (carbonyl).
    * Positive Ligands: These end in the suffix "-ium". Examples: $NH_2-NH_3^+$ (hydrazinium), $NO^+$ (nitrosonium), $NO_2^+$ (nitronium).
    * Organic Ligands: Organic free radicals retain their original names, such as $CH_3$ (methyl), $C_2H_5$ (ethyl), $C_6H_5$ (phenyl), or $C_5H_5$ (cyclopentadienyl). Neutral organic molecules also use their standard names, such as methylamine or pyridine.
    * Ambidentate Ligands: These are unidentate ligands with more than one potential coordinating atom. To indicate the specific point of attachment, the symbol of the donor atom is placed after the name of the group, separated by a hyphen.
Numerical Prefixes for Ligand Quantity:
* To indicate multiple ligands of the same type, prefixes such as "di-", "tri-", "tetra-", "penta-", and "hexa-" are used.
* Example: Four $NH_3$ molecules are named "tetra ammine".
Alphabetical Ordering in Naming:
    * All ligands are named in alphabetical order, irrespective of charge, before naming the metal atom.
    * Note: Numerical prefixes (di, tri, etc.) are not considered when determining alphabetical order.
    * Example: $[Pt(NH_3)_4Cl(NO_2)]SO_4$ is named "tetra ammine chloridonitroplatinum (IV) sulphate".
Ending of the Central Metal Atom Name:
    * The ligands are named first, followed by the metal atom.
    * The ending of the metal's name depends on whether the complex ion is a cation, anion, or neutral. If the complex is an anion, the metal name typically ends in "-ate".
    * Example: $[Cu(NH_3)_4]SO_4$ is named "tetraamminecopper (II) sulphate".

Examples of Complex Compounds and IUPAC Names

Standard Examples:
    * $[Co(NH_3)_6]Cl_3$ : hexaamminecobalt (III) chloride
    * $[Cr(H_2O)_4Cl_2]NO_2$ : tetraaquadichloridochromium (III) nitrate
    * $[Co(NH_3)_4Cl(NO_2)]NO_3$ : tetraamminechloridonitrocobalt (III) nitrate
    * $K_3[Fe(C_2O_4)_3]$ : potassium trioxalatoferrate (III)
Comprehensive List of IUPAC Names:
    1. $[Ag(NH_3)_2]Cl$ : Diamminesilver(I) chloride
    2. $[Co(en)_2Cl_2]Cl$ : Dichloridobis(ethane-1,2-diamine) cobalt(III) chloride
    3. $[Cu(NH_3)_4]SO_4$ : Tetraamminecopper(II) sulphate
    4. $[Co(CO_3)(NH_3)_4]Cl$ : Tetraamminecarbonatocobalt(III) chloride
    5. $[Cr(NH_3)_3(H_2O)_3]Cl_3$ : Triamminetriaquachromium(III) chloride
    6. $K_3[Fe(CN)_5NO]$ : Potassiumpentacyanidonitrosylferrate(II)
    7. $Na_2[Ni(EDTA)]$ : Sodium 2,2',2",2'''-(ethane-1,2-diyldinitrilo) tetraacetatonickelate(II)
    8. $[PdI_2(ONO)_2(H_2O)_2]$ : Diaquadiiododinitrito-κO palladium(IV)
    9. $[Cr(PPh_3)(CO)_5]$ : Pentacarbonyltriphenylphosphanechromium(0)
    10. $[Co(NO_2)_3(NH_3)_3]$ : Triamminetrinirito-κNcobalt(III)
    11. $[Co(NH_3)_5CN][Co(NH_3)(CN)_5]$ : Pentaamminecyanido-kCcobalt(III) amminepentacyanido-kCcobaltate(III)
    12. $[Pt(py)_4][PtCl_4]$ : Tetrapyridineplatinum(II) tetrachloridoplatinate(II)
    13. $[Co(NH_3)_4Cl_2][Cr(CN)_6]$ : Tetraamminedichloridocobalt(III) hexacyanido-KCchromate(III)
    14. $[Ag(NH_3)_2]^+$ : diamminesilver(I) ion
    15. $[Co(NH_3)_5Cl]$ : pentaamminechlorocobalt(III) ion (Note: complex ion charge omitted in transcript, name remains pentaamminechlorocobalt(III) ion)
    16. $[FeF_6]^{3-}$ : Hexafluoridoferrate(III)ion (Transcript lists as $[FeF_6]^{4-}$ but name is given as Hexafluoridoferrate(II)ion; transcript page 7 item xvi is $[FeF_6]^{4-}$ : Hexafluoridoferrate(II)ion).