Coordination Chemistry – Complex Ions & Ligands

Definitions

Coordination Compound
- Contains one or more complex ions (or neutral complexes) as constituents.
- Overall chemical/physical behaviour is governed by the complexes it contains.
Complex (Coordination Complex/Ion)
- Any species in which ligands are coordinated (bonded) to a central metal atom/ion.
- Previously introduced when discussing Lewis acids/bases and formation constants.
- Can be cationic, anionic, or neutral.
Ligand
- Electron-pair donor (Lewis base).
- Forms a coordinate (dative) covalent bond: both electrons of the shared pair come from the ligand.
- Possible charge states:
- Anionic (mono- or poly-atomic)
- Neutral molecules
- Because of the ligand’s electron donation, the metal centre behaves as a Lewis acid.
Metal Centre
- Often a cation in examples but may also be a neutral atom.
- Accepts electron pairs to create metal–ligand bonds.

Illustrative Formation Series: $\text{Co}^{3+}$ with $\text{Cl}^-$ and $\text{NH}_3$

All examples contain six total ligands (coordination number 6).
The charge on the complex equals the sum of the metal charge and all ligand charges.
- $[\text{Co}(\text{NH}3)6]^{3+}$
- Six neutral $\text{NH}_3$ ligands → overall $+3$ .
- $[\text{CoCl}(\text{NH}3)5]^{2+}$
- One $\text{Cl}^-$ lowers charge by one → overall $+2$ .
- $[\text{CoCl}2(\text{NH}3)_4]^{+}$
- Two $\text{Cl}^-$ lower charge by two → overall $+1$ .
- $[\text{CoCl}3(\text{NH}3)_3]$
- Three $\text{Cl}^-$ cancel metal charge → neutral complex.
- $[\text{CoCl}4(\text{NH}3)_2]^{-}$
- Four $\text{Cl}^-$ exceed metal charge by one → complex anion.

Coordination Number (CN)

Definition: Number of points of attachment (metal–ligand bonds) around the central metal.
Typical range: $2 \leq \text{CN} \leq 12$ .
- $6$ most common
- $4$ next
- $3$ and $5$ are rare.

Worked Examples

$[\text{CoCl}(\text{NO}2)(\text{NH}3)_4]^+$
- Count ligands → CN $=6$ .
- Oxidation state $x$ of Co:
 $x +(-1)+(-1)+4(0)=+1 \Rightarrow x=+3$ .
$[\text{Ni}(\text{CN})_4\text{I}]^{3-}$
- CN $=5$ (four CN⁻ + one I⁻).
- Oxidation state $y$ of Ni:
  $y +4(-1)+(-1) = -3 \Rightarrow y = +2$ .

(Tracking oxidation state is essential for predicting magnetic behaviour, redox chemistry, and crystal-field splitting, topics introduced later.)

Common Geometries & Characteristic Angles

Geometry	Typical CN	Key Angles	Visual/Conceptual Notes
Linear	$2$	$180^{\circ}$	e.g.

Square Planar	$4$	$90^{\circ},\ 180^{\circ}$	All five atoms in one plane; wedges/dashes show the 2-D plane is perpendicular to page. Common for $d^8$ metals like Pt(II), Pd(II).
Tetrahedral	$4$	$109.5^{\circ}$	Familiar VSEPR shape; bonds alternate in/out of the screen. Typical for $d^{10}$ metals such as Zn(II).
Octahedral	$6$	$90^{\circ}$	Looks like square planar with extra axial ligands above & below. Most ubiquitous coordination environment.

(Other geometries—trigonal bipyramidal, pentagonal bipyramidal, cuboctahedral, etc.—exist but are beyond this course’s scope.)

Ligand Denticity (Number of "Bites")

Monodentate (Unidentate)
- One donor atom → one metal–ligand bond per ligand.
- Examples (Lewis structures in lecture):
- $\text{Cl}^-$
- $\text{OH}^-$
- $\text{NH}_3$
Polydentate
- Donates $\ge 2$ lone-pair sites from different atoms to different coordination sites on the metal.
- Terminology:
- Bidentate (two sites)
- Tridentate, tetradentate, …
- Hexadentate (six sites)

Representative Polydentate Ligands & Significance

Ligand	Abbreviation	Denticity	Structural/Functional Highlights
Ethylenediamine	en	2 (bidentate)	Two N donor atoms connected by flexible –CH2–CH2– link; free rotation about $\sigma$ bonds lets ligand twist so N–M–N angle $\approx 90^{\circ}$ .
Ethylenediamine-tetraacetate	EDTA$^{4-}$	6 (hexadentate)	Wraps completely around metal; forms extremely stable complexes; widely used in analytical titrations, medicine (heavy-metal chelation therapy), and water softening.

Chelate & Chelation
- When a polydentate ligand forms a ring with the metal, the complex is called a chelate (from Greek chele = crab’s claw).
- Rings usually contain 5 or 6 atoms for optimal stability.
- Polydentate ligand = chelating agent; the process = chelation.
- Chelate effect: polydentate ligands often give significantly larger formation constants than an equivalent number of comparable monodentate ligands—important in biochemical metal sequestration and industrial extraction.

Example Chelate Complex

$\text{Pt}^{2+}$ coordinated by two ethylenediamine ligands: $[\text{Pt}(en)_2]^{2+}$
- Only 2 ligands, yet **4 donor atoms → CN $=4$ .
- Each en forms a 5-membered ring with Pt, enhancing stability.

Broader Context & Implications

Lewis Acid–Base Perspective: Metals (acids) + Ligands (bases) generalises main-group acid–base theories to transition-metal chemistry.
Formation (Stability) Constants previously introduced quantify the equilibrium governing chelate/complex formation; understanding CN, denticity, and geometry is prerequisite for predicting $K_f$ trends.
Biological Relevance:
- Natural chelators (e.g., hemoglobin’s porphyrin, siderophores) secure essential metals while discriminating against toxic ones.
- EDTA used clinically to bind Pb$^{2+}$, Hg$^{2+}$.
Industrial Applications:
- Water treatment, photographic fixing, catalysis, mineral extraction all exploit selective complex formation.

Quick Reference: Numerical/Statistical Facts

Coordination numbers observed: $2\text{–}12$ (common $6,\ 4$ ).
Standard bond angles:
- Linear $180^{\circ}$
- Square planar $90^{\circ},\ 180^{\circ}$
- Tetrahedral $109.5^{\circ}$
- Octahedral $90^{\circ}$ .
Charge-balance & oxidation-state determination rely on the equation:
$\sum (\text{oxidation state of metal}) + \sum (\text{ligand charges}) = \text{overall complex charge}.$

These notes encompass every key definition, example, structural feature, and conceptual link from the transcript, providing a complete study guide for coordination chemistry fundamentals.