Coordination Compounds (Nomenclature And Isomerism)

Nomenclature Of Coordination Compounds

The naming of coordination compounds follows a systematic set of rules established by IUPAC to ensure unambiguous identification.

Formulas Of Mononuclear Coordination Entities

Mononuclear Coordination Entity: An entity containing a single central metal atom.

Rules for Writing Formulas:

Central Metal First: The chemical symbol of the central metal atom/ion is written first.
Ligands Follow: Ligands are written next, in alphabetical order, regardless of their charge.
Coordination Sphere in Brackets: The central metal atom and its directly attached ligands are enclosed in square brackets `[]` to indicate the coordination sphere.
Counter Ions Outside: Counter ions (ions outside the coordination sphere that balance the charge) are written outside the square brackets. Cations are written first, followed by anions.
Subscripts for Stoichiometry: Subscripts are used to indicate the number of ligands and counter ions.
Number of Ligands: If a ligand appears more than once, it is enclosed in parentheses, and the number of such ligands is indicated by a subscript outside the parentheses (e.g., $(NH_3)_4$).
Charge of the Complex Ion: The charge of the complex ion, if it exists, is indicated by a superscript outside the square brackets (e.g., $[Co(NH_3)_6]^{3+}$).

Example:

Potassium hexacyanoferrate(II): $K_4[Fe(CN)_6]$
Tetraamminecopper(II) sulfate: $[Cu(NH_3)_4]SO_4$

Naming Of Mononuclear Coordination Compounds

Rules for Naming:

Cation First, Then Anion: The naming follows the pattern of simple ionic compounds – the cation is named before the anion.
Naming the Coordination Sphere:

Ligands Named First: Ligands are named before the central metal atom.
Alphabetical Order: If there are multiple ligands, they are named in alphabetical order (ignoring prefixes like 'di', 'tri', 'tetra').
Prefixes for Number of Ligands: Prefixes like di-, tri-, tetra-, penta-, hexa- are used to indicate the number of monodentate ligands. For polyatomic or complex ligands, prefixes like bis-, tris-, tetrakis- are used, and the ligand name is enclosed in parentheses.
Naming Anionic Ligands: The ending '-o' is added to the name of anionic ligands (e.g., $F^-$ is fluoro, $Cl^-$ is chloro, $CN^-$ is cyano, $SO_4^{2-}$ is sulfato, $O^{2-}$ is oxo, $OH^-$ is hydroxo).
Neutral Ligands: Neutral ligands are named as such, with some exceptions (e.g., $H_2O$ is aqua, $NH_3$ is ammine, $CO$ is carbonyl, $NO$ is nitrosyl).
Naming the Central Metal:

If the complex ion is cationic or neutral, the name of the metal is used as is (e.g., iron, cobalt, nickel).
If the complex ion is anionic, the ending '-ate' is added to the name of the metal (e.g., iron becomes ferrate, cobalt becomes cobaltate, chromium becomes chromate, gold becomes aurate, lead becomes plumbate, tin becomes stannate).

Oxidation State of Central Metal: The oxidation state of the central metal atom is indicated by a Roman numeral in parentheses immediately after the name of the metal (e.g., iron(III), cobalt(II)).

Linking Cation and Anion Names: The name of the cation (which could be a simple ion or the complex ion) and the anion are written together, separated by a space.

Examples:

$[Co(NH_3)_6]Cl_3$: Hexamminecobalt(III) chloride
$[Ni(en)_2Cl_2]$: Dichloridobis(ethylenediamine)nickel(II)
$K_4[Fe(CN)_6]$: Potassium hexacyanoferrate(II)
$[Cr(H_2O)_4Cl_2]Cl$: Tetraamminedichlorochromium(III) chloride
$[Pt(NH_3)_2Cl_2]$: Diamminedichloroplatinum(II)

Isomerism In Coordination Compounds

Isomerism is the phenomenon where compounds have the same chemical formula but differ in the arrangement of their atoms or ions. In coordination compounds, isomerism can arise from differences in the connectivity of atoms (structural isomerism) or the spatial arrangement of ligands (stereoisomerism).

Geometric Isomerism

Definition: Geometric isomerism occurs in coordination compounds when ligands occupy different positions around the central metal atom, leading to different spatial arrangements of ligands. This is common in complexes where ligands are not identical or when they occupy different positions in the coordination sphere.

Occurrence: Common in square planar ($MA_2B_2$, $MABCL$) and octahedral ($MA_2B_4$, $MA_3B_3$, $MABCDEFG$) complexes.

Types of Geometric Isomerism:

In Square Planar Complexes ($[Ma_2b_2]$ type):

Cis isomer: Similar ligands are adjacent to each other (at 90°).
Trans isomer: Similar ligands are opposite to each other (at 180°).

Example: $[Pt(NH_3)_2Cl_2]$ exists as cis-diamminedichloroplatinum(II) (used as anti-cancer drug) and trans-diamminedichloroplatinum(II).

In Octahedral Complexes ($[Ma_4b_2]$ type):

Cis isomer: The two similar ligands (b) are at adjacent positions.
Trans isomer: The two similar ligands (b) are at opposite positions.

Example: $[Co(NH_3)_4Cl_2]Cl$.

In Octahedral Complexes ($[Ma_3b_3]$ type):

Facial (fac) isomer: The three identical ligands (a or b) occupy one face of the octahedron.
Meridional (mer) isomer: The three identical ligands lie in a plane passing through the central metal atom, dividing the octahedron into two halves.

Example: $[Co(NH_3)_3Cl_3]$.

Optical Isomerism

Definition: Optical isomerism occurs when a coordination compound and its mirror image are non-superimposable, i.e., they are chiral. These isomers rotate the plane of polarized light in opposite directions.

Occurrence: Common in complexes that lack elements of symmetry, particularly in chiral octahedral complexes.

Requirement: Typically requires the absence of a plane of symmetry and a center of symmetry within the coordination sphere.

Examples:

$[Ma_2b_2]$ Square Planar: If $a$ and $b$ are different, geometric isomers exist, but $MA_2B_2$ type square planar complexes generally do not exhibit optical isomerism unless the ligands themselves are chiral.
$[Ma_2b_2c_2]$ Octahedral: Can exhibit both geometric (cis/trans) and optical isomerism. The cis isomers are often optically active.
$[M(en)_3]^{n+}$ type Octahedral: Complexes with three bidentate ligands are chiral and exhibit optical isomerism.

Example: $[Co(en)_3]^{3+}$ exists as two enantiomers, $\Delta$ and $\Lambda$. The cis isomer of $[Co(en)_2Cl_2]^+$ also exhibits optical isomerism.

Linkage Isomerism

Definition: Linkage isomerism occurs when a ligand can coordinate to the central metal atom through different donor atoms.

Ambidentate Ligands: Ligands that possess more than one donor atom, but only one coordinates at a time, are called ambidentate ligands.

Examples:

Thiocyanato ($SCN^-$) vs. Isothiocyanato ($NCS^-$): Can coordinate through sulfur or nitrogen.

$[Co(NH_3)_5(SCN)]^{2+}$ (thiocyanato complex) vs. $[Co(NH_3)_5(NCS)]^{2+}$ (isothiocyanato complex)

Nitro ($NO_2^-$) vs. Nitrito ($ONO^-$): Can coordinate through nitrogen or oxygen.

$[Cr(NH_3)_5(NO_2)]^{2+}$ (nitro complex) vs. $[Cr(NH_3)_5(ONO)]^{2+}$ (nitrito complex)

Coordination Isomerism

Definition: This type of isomerism occurs when there is an interchange of ligands between the two

Definition: This type of isomerism occurs when there is an interchange of ligands between the two complex ions constituting a coordination compound.

Requirement: Requires at least two coordination entities, one being cationic and the other anionic.

Example: Consider the pair $[Co(NH_3)_6][Cr(CN)_6]$ and $[Cr(NH_3)_6][Co(CN)_6]$. Both compounds contain the same set of ions ($[Co(NH_3)_6]^{3+}$, $[Cr(CN)_6]^{3-}$, $Co^{3+}$, $[Cr(NH_3)_6]^{3+}$, $[Co(CN)_6]^{3-}$), but the central metal atoms and ligands are exchanged between the cation and anion.

Ionisation Isomerism

Definition: Ionisation isomerism occurs when isomers differ in the nature of the counter ion and the ligand. An ionizable group in the primary valency sphere of one isomer acts as a ligand in the other isomer, and vice versa.

Example: $[Co(NH_3)_5Br]SO_4$ and $[Co(NH_3)_5SO_4]Br$.

In the first complex, sulfate ($SO_4^{2-}$) is the counter ion, and bromide ($Br^-$) is the ligand.
In the second complex, bromide ($Br^-$) is the counter ion, and sulfate ($SO_4^{2-}$) is the ligand.

These isomers precipitate different ions when treated with suitable reagents (e.g., $BaCl_2$ will precipitate $BaSO_4$ from the first complex but not from the second).

Solvate Isomerism

Definition: Solvate isomerism is a special case of ionisation isomerism where solvent molecules are involved as either ligands or counter ions.

Example: $[Cr(H_2O)_6]Cl_3$ (hexaaquachromium(III) chloride) and $[Cr(H_2O)_5Cl]Cl_2 \cdot H_2O$ (pentaquachloridochromium(III) chloride monohydrate) and $[Cr(H_2O)_4Cl_2]Cl \cdot 2H_2O$ (tetraaquadichloridochromium(III) chloride dihydrate).

In the first isomer, all water molecules are ligands.
In the second isomer, one water molecule is outside the coordination sphere as water of crystallisation.
In the third isomer, two water molecules are outside the coordination sphere.