Coordination Chemistry – Comprehensive Bullet-Point Notes

Coordination Compounds / Complexes

• Coordination (or complex) compounds are Lewis‐acid/Lewis‐base adducts.

  • Central metal atom/ion = Lewis acid (electron-pair acceptor).

  • Ligand = Lewis base (has at least one lone pair to donate).

• Coordinate (dative) bond: both shared electrons originate from donor atom of ligand and enter an empty metal orbital.

• Atom of ligand directly attached to metal = donor atom.

• The whole metal-ligand entity inside brackets = coordination sphere.

  • Primary (inner) sphere = ligands directly bound.

  • Secondary (outer) sphere = counter-ions / solvated species.

• Example evolution of historical vs. modern formulations (cobalt–ammonia complexes)

  • $\text{“CoCl}<em>3\cdot6\,NH</em>3” \;\longrightarrow\; [Co(NH<em>3)</em>6]Cl_3$ (orange-yellow; 3 free Cl⁻)

  • $\text{“CoCl}<em>3\cdot5\,NH</em>3”\;\longrightarrow\;[Co(NH<em>3)</em>5Cl]Cl_2$ (purple; 2 free Cl⁻)

  • $\text{“CoCl}<em>3\cdot4\,NH</em>3”$ splits into trans- and cis-$[Co(NH<em>3)</em>4Cl_2]Cl$ (green vs. violet; 1 free Cl⁻).

Types of Complex Ions

• Cationic: positive charge inside brackets.

  • Examples: $[Cr(NH<em>3)</em>4Cl<em>2]^+$, $[Co(H</em>2O)_6]^{2+}$.

• Anionic: negative charge inside brackets.

  • Examples: $[Fe(CN)<em>6]^{4-}$, $[CuCl</em>4]^{2-}$, $K<em>2[PtCl</em>6]$.

• Neutral: overall zero charge.

  • Examples: $Pt(NH<em>3)</em>2Cl<em>2$, $[Ir(NH</em>3)<em>3Cl</em>3]$.

Ligands

• Definition: atom/ion/molecule bonded directly to a metal centre via donation of a lone pair.

• Denticitiy – number of donor sites

  • Monodentate (one donor): $H<em>2O$ (aqua), $NH</em>3$ (ammine), $Cl^-$ (chloro), $CN^-$ (cyano), $NO_2^-$ (nitro / nitrito).

  • Bidentate (two donors): ethylenediamine (en), 2,2’-bipyridine (bipy), oxalate (ox), acetylacetonate (acac), glycinate (gly).

  • Polydentate (>2): diethylenetriamine (dien, tridentate); EDTA$^{4-}$ (hexadentate).

• Chelating ligand: a polydentate ligand that forms one or more rings with the metal.

  • The resulting ringed complex = chelate.

  • Common chelators: en, ox, EDTA.

• Charge categories

  • Anionic ligands (end in “-o” in names): chloro $Cl^-$, fluoro $F^-$, hydroxo $OH^-$, oxo $O^{2-}$, cyano $CN^-$, nitro/nitrito $NO_2^-$, thiocyanato / isothiocyanato $SCN^-$/$NCS^-$, etc.

  • Neutral ligands: aqua $H<em>2O$, ammine $NH</em>3$, carbonyl $CO$, nitrosyl $NO$, dinitrogen $N<em>2$, dioxygen $O</em>2$, pyridine (py), bipyridine (bipy), phenanthroline (phen), PPh$_3$.

  • Cationic ligands: hydrazinium $NH<em>2NH</em>3^+$, nitrosonium $NO^+$, nitronium $NO<em>2^+$, anilinium $C</em>6H<em>5NH</em>3^+$.

Oxidation State of Central Metal

• Calculated by formal charge balance:
  $\text{Metal charge} + \sum (\text{ligand charges}) = \text{overall charge of complex}$

• Cationic complex example: $[Mn(H<em>2O)</em>6]^{2+}$
  $x + 6(0) = +2 \;\Rightarrow\; x = +2$ → Mn(II).

• Anionic complex example: $[PtCl_4]^{2-}$
  $x + 4(-1) = -2 \;\Rightarrow\; x = +2$ → platinate(II).

• Neutral complex example: $[Cr(H<em>2O)</em>4Cl_2]$
  $x + 0 + 2(-1) = 0 \Rightarrow x = +2$ → chromium(II).

Coordination Number (CN) & Common Geometries

• Definition: number of donor atoms directly bonded to the metal (not number of ligands!).

• Typical range $2 \le CN \le 16$; depends on metal size, d-electron count, ligand bulk.

• CN = 1: extremely rare (e.g. Tl(I), In(I) aryl complexes).

• CN = 2 (linear): $[Ag(NH<em>3)</em>2]^+$, $[Au(CN)<em>2]^-$, $Hg(CN)</em>2$ (d$^{10}$).

• CN = 3: trigonal planar/pyramidal/T-shaped; sterically crowded; example $[Au(PPh<em>3)</em>3]^+$.

• CN = 4

  • Tetrahedral: d$^0$ or d$^{10}$ (e.g. $[Ni(CO)<em>4]$, $MnO</em>4^-$, $[Cu(py)_4]^+$).

  • Square planar: d$^8$ (Ni(II), Pd(II), Pt(II)): $PtCl<em>2(NH</em>3)<em>2$, $[PdCl</em>4]^{2-}$.

• CN = 5: trigonal bipyramidal or square pyramidal.

  • Examples: $[CuCl<em>5]^{3-}$ (TBP), $[Ni(CN)</em>5]^{3-}$ (square pyramidal).

• CN = 6: most common; octahedral or trigonal prismatic.

  • Octahedral examples: $[Co(NH<em>3)</em>6]^{3+}$, $[Cu(H<em>2O)</em>6]^{2+}$.

  • Trigonal prism less common (e.g. $[Mo(S<em>2C</em>2R<em>2)</em>3]^{2-}$).

• CN = 7: pentagonal bipyramid, capped octahedron, capped trigonal prism; e.g. $[ZrF_7]^{3-}$.

• CN = 8: square antiprism, dodecahedron; common for larger lanthanides/actinides $[Mo(CN)_8]^{4-}$.

• CN = 9–16: tricapped trigonal prism (9), etc.; common in f-block chemistry (e.g. $[La(H<em>2O)</em>9]^{3+}$).

Nomenclature Rules

1. Write formula

   • Metal symbol first; then ligands (neutral before anionic); entire coordination sphere in brackets; cation before anion.

   • Example build-up:

   1. Metal $Co$

   2. Add ligands $NH<em>3,\;Cl^-$ → $Co(NH</em>3)<em>4Cl</em>2$

   3. Put in brackets $[Co(NH<em>3)</em>4Cl_2]$

   4. Add counter-anion $Cl^-$ → $[Co(NH<em>3)</em>4Cl_2]Cl$ (neutral overall).

2. Name

   • Cationic part first.

   • Inside coordination sphere: list ligands alphabetically ignoring numerical prefixes.

   • Anionic ligand names end in “-o” (chloro, cyano, nitrato, oxalato…).

   • Neutral ligand special names: ammine, aqua, carbonyl, nitrosyl; others keep molecule name (pyridine).

   • Use prefixes to denote quantity: di-, tri-, tetra-, penta-, hexa-.

     • If ligand name already contains a prefix or is polydentate, use bis-, tris-, tetrakis- etc. and put ligand name in parentheses.

   • Metal: unchanged in cationic/neutral complexes; ends in “-ate” in anionic complexes (special Latin roots: ferrate, cuprate, argentate, aurate, plumbate, stannate, cobaltate, nickelate, borate).

   • Oxidation state of metal in Roman numerals immediately after metal name, in parentheses.

3. Examples

   • $[Co(NH<em>3)</em>4Cl_2]Cl$ → tetraamminedichlorocobalt(III) chloride.

   • $[Co(NH<em>3)</em>5Br]SO_4$ → pentaamminebromocobalt(III) sulfate.

   • $Na<em>3[FeCl(CN)</em>5]$ → sodium chloropentacyanoferrate(III).

   • $[Fe(bipy)_3]^{2+}$ → tris(bipyridine)iron(II) ion.

   • $[Co(en)<em>3]Cl</em>3$ → trichlorotris(ethylenediamine)cobalt(III) chloride.

   • $[Co(py)<em>4(AsPh</em>3)<em>2]Cl</em>3$ → tetrakis(pyridine)bis(triphenylarsine)cobalt(III) chloride.

   • $(NH<em>4)</em>2[CuCl_4]$ → ammonium tetrachlorocuprate(II).

   • $Na[B(NO<em>3)</em>4]$ → sodium tetranitratoborate(III).

   • $[Ti(H<em>2O)</em>6][CoCl_6]$ → hexaaquatitanium(III) hexachlorocobaltate(III).

Isomerism in Coordination Chemistry

Complexes with same empirical formula can differ by arrangement of atoms/bonds.

Structural Isomers

• Ionization isomers: exchange of ligands between coordination sphere and counter-ion.

  • $[Co(NH<em>3)</em>5Br]SO<em>4$ (violet, Co–Br bond) vs. $[Co(NH</em>3)<em>5SO</em>4]Br$ (red, Co–OSO$_3$ bond).

• Hydration isomers: swap of coordinated $H_2O$ and water of crystallization / counter-ions.

  • Chromium(III) chloride examples:

  • $[Cr(H<em>2O)</em>6]Cl_3$ (violet) – all 3 Cl⁻ outside; gives full AgCl ppt.

  • $[CrCl(H<em>2O)</em>5]Cl<em>2\cdot H</em>2O$ (grey-green).

  • $[Cr(H<em>2O)</em>4Cl<em>2]Cl\cdot2H</em>2O$ (bright-green).

• Linkage isomers (ambidentate): ligand binds through alternative donor atoms.

  • $SCN^-$ can bind via S (thiocyanato-) or N (isothiocyanato-).

  • $[Co(NH<em>3)</em>5(SCN)]^{2+}$ = pentaamminethiocyanatocobalt(III).

  • $[Co(NH<em>3)</em>5(NCS)]^{2+}$ = pentaammineisothiocyanatocobalt(III).

  • $NO<em>2^-$: nitro- (N-bonded) vs. nitrito- (O-bonded). Orange $[Co(NH</em>3)<em>5(NO</em>2)]Cl<em>2$ vs. red $[Co(NH</em>3)<em>5(ONO)]Cl</em>2$.

• Coordination isomers: cationic and anionic complex ions exchange ligands while maintaining overall ratios.

Stereoisomers

Geometrical (cis/trans, fac/mer)

• Square-planar [MX$<em>2$Y$</em>2$]: cis- vs. trans-$[Pt(NH<em>3)</em>2Cl_2]$.

• Octahedral [MX$<em>4$Y$</em>2$]: cis- and trans-$[Co(NH<em>3)</em>4Cl_2]^+$.

• Octahedral with bidentate ligands [M(L$<em>2$)X$</em>2$]^+$</strong>: cis / trans isomers for$[Cr(en)2Cl2]^+$.</p></li><li><p><strong>Octahedral [MX$3$Y$3$]</strong>:</p><ul><li><p><strong>fac</strong> (three identical ligands on one triangular face) vs. <strong>mer</strong> (three in equatorial plane).</p></li><li><p>Example fac- and mer-$[CoCl3(NH3)_3]$.</p></li></ul></li></ul><h5 id="d95e3d9b-3f3d-4068-8f63-806fe58405f5" data-toc-id="d95e3d9b-3f3d-4068-8f63-806fe58405f5" collapsed="false" seolevelmigrated="true">Optical (Enantiomers)</h5><ul><li><p><strong>Chiral complex</strong>: non-superimposable mirror image; rotates plane-polarized light.</p><ul><li><p>Two forms: <strong>dextrorotatory (d or +)</strong> and <strong>levorotatory (l or –)</strong>.</p></li></ul></li><li><p><strong>Achiral</strong>: possesses internal mirror plane → optically inactive.</p></li><li><p>Octahedral complexes with 3 bidentate ligands (e.g.$[Co(en)_3]^{3+}$) are chiral (Δ / Λ configurations).</p></li><li><p><strong>Square-planar</strong> trans complexes generally achiral.</p></li><li><p>For$[Co(NH3)4Cl_2]^+$:</p><ul><li><p>Trans-isomer has mirror plane → not optically active.</p></li><li><p>Cis-isomer can be chiral and exhibit a pair of enantiomers (d-cis / l-cis).</p></li></ul></li></ul><h3 id="07099366-be65-4712-b450-d92a84873bf2" data-toc-id="07099366-be65-4712-b450-d92a84873bf2" collapsed="false" seolevelmigrated="true">Practical / Conceptual Connections</h3><ul><li><p>Coordination chemistry underpins color, magnetism, biological metal sites (e.g. hemoglobin, B$_{12}).

• Chelation therapy uses EDTA to sequester heavy metals.

• Isomerism explains differing reactivity & biological activity for compounds with same formula (cis-platin vs. transplatin).

• Geometrical & optical control crucial in catalysis, drug design, material science.

Numerical & Miscellaneous Data

• Common prefixes (1–10): mono (rarely used), di, tri, tetra, penta, hexa, hepta, octa, nona, deca.

• Alternative prefixes for complex ligand names: bis, tris, tetrakis, pentakis, hexakis…

• Maximum observed coordination number so far ≈ 16$(e.g.$[CsXeF_7]$$ type, large cluster anions).

Ethical / Philosophical Notes

• Nomenclature clarity prevents medical or industrial errors (e.g. correct drug vs. toxic isomer).

• Understanding ligand behavior informs environmental remediation strategies (complexation of pollutants).

End of comprehensive bullet-point study notes on coordination chemistry.