What is the primary difference between a double salt and a coordination compound?

The fundamental difference lies in their behavior in solution. A double salt, such as Mohr's salt ($\text{FeSO}_4 \cdot (\text{NH}_4)_2\text{SO}_4 \cdot 6\text{H}_2\text{O}$), dissociates completely into its constituent ions when dissolved in water, losing its identity. You would detect $\text{Fe}^{2+}$, $\text{NH}_4^+$, and $\text{SO}_4^{2-}$ ions. In contrast, a coordination compound, like $\text{K}_4[\text{Fe(CN)}_6]$, retains its complex ion identity in solution. It dissociates into counter ions and the complex ion (e.g., $\text{K}^+$ and $[\text{Fe(CN)}_6]^{4-}$), but the complex ion itself remains intact and does not break down into $\text{Fe}^{2+}$ and $\text{CN}^-$ ions.

How do I determine the oxidation state of the central metal in a coordination compound?

To determine the oxidation state, you need to know the charge of the overall complex ion and the charges of all the ligands. Assign a variable (e.g., 'x') to the metal's oxidation state. Sum the charges of the metal and all ligands, and equate this sum to the net charge of the complex ion. For example, in $[\text{Co(NH}_3)_5\text{Cl}]^{2+}$, $\text{NH}_3$ is neutral (0) and $\text{Cl}^-$ is -1. So, $x + 5(0) + (-1) = +2$, which gives $x = +3$. Thus, cobalt is in the +3 oxidation state.

What is the significance of the spectrochemical series?

The spectrochemical series is an experimentally determined list of ligands arranged in increasing order of their ability to cause d-orbital splitting ($\Delta_o$ or $\Delta_t$). This series is crucial in Crystal Field Theory (CFT) because it helps predict whether a complex will be high spin or low spin, and consequently, its magnetic properties and color. Strong field ligands (at the high end of the series, like $\text{CN}^-$, $\text{CO}$) cause large splitting, favoring electron pairing (low spin). Weak field ligands (at the low end, like $\text{I}^-$, $\text{Br}^-$) cause small splitting, leading to high spin complexes.

Why are transition metals commonly found in coordination compounds?

Transition metals are ideal for forming coordination compounds due to several key characteristics. Firstly, they possess vacant d-orbitals of appropriate energy, which can readily accept electron pairs from ligands to form coordinate covalent bonds. Secondly, they exhibit variable oxidation states, allowing them to form a wide range of complexes. Thirdly, their relatively small size and high effective nuclear charge enable them to attract and hold ligands effectively. Finally, the presence of partially filled d-orbitals allows for d-d electronic transitions, which are responsible for the vibrant colors observed in many transition metal complexes.

Explain the difference between cis-trans and optical isomerism.

Both are types of stereoisomerism, meaning they have the same connectivity but different spatial arrangements. **Cis-trans isomerism (geometrical)** arises from the different positions of identical ligands relative to each other (adjacent or opposite) around the central metal. It's common in square planar and octahedral complexes. For example, in $[\text{Pt(NH}_3)_2\text{Cl}_2]$, cis-platin has $\text{NH}_3$ groups adjacent, while trans-platin has them opposite. **Optical isomerism** occurs when a complex is non-superimposable on its mirror image, making it chiral. These isomers (enantiomers) rotate plane-polarized light in opposite directions. This is common in octahedral complexes with bidentate ligands, such as $[\text{Co(en)}_3]^{3+}$, which lacks a plane of symmetry.

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Nomenclature of Coordination Compounds

Coordination Compounds

Chemistry

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Version 1Updated 22 Mar 2026

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Coordination compounds, often referred to as complexes, are a distinct class of chemical substances characterized by a central metal atom or ion, typically a transition metal, bonded to a surrounding array of molecules or ions called ligands. These bonds are primarily coordinate covalent bonds, where the ligands donate electron pairs to the central metal. The unique properties of these compounds, …

Quick Summary

Coordination compounds are fascinating chemical entities where a central metal atom or ion, typically a transition metal, is bonded to a specific number of surrounding molecules or ions called ligands.

These bonds are coordinate covalent, with ligands donating electron pairs. The number of such bonds defines the coordination number, and the entire metal-ligand assembly is termed the coordination sphere.

Ligands can be monodentate, bidentate, or polydentate, with polydentate ligands forming stable chelate rings. Understanding their nomenclature, based on IUPAC rules, is crucial for systematic naming. Isomerism, both structural (ionization, hydrate, linkage, coordination) and stereoisomerism (geometrical, optical), explains the diverse forms these compounds can take.

Bonding theories like Valence Bond Theory (VBT) and Crystal Field Theory (CFT) elucidate their geometry, magnetic properties, and color. VBT uses hybridization to predict structure and magnetism, while CFT explains d-orbital splitting due to ligand fields, accounting for color and providing a more nuanced view of magnetic behavior based on the spectrochemical series.

These compounds are vital in biology, medicine, and industry.

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Key Concepts

Calculating Oxidation State of Central Metal

The oxidation state of the central metal ion is a fundamental property that dictates its electronic…

Identifying Isomerism Type

Identifying the type of isomerism requires careful comparison of the structural formulas and ligand…

Predicting Magnetic Behavior using VBT

Valence Bond Theory (VBT) predicts the magnetic behavior (paramagnetic or diamagnetic) of a complex based on…

Central Metal — Lewis acid, usually transition metal.

Ligand — Lewis base, electron pair donor (monodentate, bidentate, polydentate).

Coordination Number — Number of metal-ligand bonds (e.g., 4, 6).

IUPAC Naming — Cation first, ligands (alphabetical) then metal, oxidation state (Roman numeral), '-ate' for anionic complex.

Isomerism — Structural (Ionization, Hydrate, Linkage, Coordination), Stereoisomerism (Geometrical, Optical).

VBT — Hybridization (

\text{sp}^3

\text{dsp}^2

\text{d}^2\text{sp}^3

\text{sp}^3\text{d}^2

), Geometry (Tetrahedral, Square Planar, Octahedral), Magnetic properties (unpaired electrons).

CFT — d-orbital splitting (

\Delta_o

\Delta_t

), Spectrochemical Series (ligand field strength), High spin/Low spin, CFSE, Color (d-d transitions), Magnetic properties.

Magnetic Moment —

\mu = \sqrt{n(n+2)}\,\text{BM}

(n = unpaired electrons).

Spectrochemical Series (partial) —

\text{I}^- < \text{Br}^- < \text{Cl}^- < \text{F}^- < \text{H}_2\text{O} < \text{NH}_3 < \text{en} < \text{CN}^- < \text{CO}

To remember the spectrochemical series for common ligands (weak to strong field):

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Iodide (

\text{I}^-

)

Bromide (

\text{Br}^-

)

Sulfide (

\text{S}^{2-}

)

Chloride (

\text{Cl}^-

)

Sulfate (

\text{SO}_4^{2-}

)

Fluoride (

\text{F}^-

)

Oxalate (

\text{C}_2\text{O}_4^{2-}

)

Oxide (

\text{O}^{2-}

)

Hydroxide (

\text{OH}^-

)

Nitrate (

\text{NO}_3^-

)

Ethylenediamine ('en')

Nitrite (

\text{NO}_2^-

)

Cyanide (

\text{CN}^-

)

Carbonyl (CO)

(Note: This is a slightly expanded version, focus on the most common ones for NEET like $\text{I}^-, \text{Br}^-, \text{Cl}^-, \text{F}^-, \text{H}_2\text{O}, \text{NH}_3, \text{en}, \text{CN}^-, \text{CO}$ )

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Nomenclature of Coordination Compounds