Complex Ions: The Chemistry of Coordination
The Basic Components: Metal and Ligands
At the heart of every complex ion is a partnership. The central metal ion is usually a transition metal like iron, copper, or cobalt. These metals have a special ability: they have empty spaces in their electron orbitals where other particles can attach.
The particles that attach are called ligands. A ligand is a molecule or ion that has at least one atom with a lone pair of electrons it is willing to "donate" to the metal. The atom that does the donating is called the donor atom. Common ligands include water ($H_2O$), ammonia ($NH_3$), chloride ions ($Cl^-$), and cyanide ions ($CN^-$).
The bond that forms is a coordinate covalent bond (also called a dative bond). It's a special type of covalent bond where both electrons in the shared pair come from the same atom—the ligand.
Coordination Number and Geometry
The coordination number is simply the number of donor atoms that are bonded to the central metal ion. It tells us how many "connections" the metal has. Common coordination numbers are 2, 4, and 6.
The coordination number determines the geometry—the three-dimensional shape—of the complex ion. The ligands arrange themselves around the metal to be as far apart as possible, minimizing repulsion.
| Coordination Number | Geometry | Example | Diagram |
|---|---|---|---|
| 2 | Linear | $[Ag(NH_3)_2]^+$ | M—L—M |
| 4 | Tetrahedral or Square Planar | $[ZnCl_4]^{2-}$ (tetra.) $[Ni(CN)_4]^{2-}$ (square) | L L – M – L L |
| 6 | Octahedral | $[Co(NH_3)_6]^{3+}$ $[Fe(H_2O)_6]^{2+}$ | Four ligands in a square, one above and one below the metal. |
Naming Complex Ions: A Simple System
Naming complex ions follows logical rules. Let's break it down with the example of $[Cu(NH_3)_4]^{2+}$.
Ligands First: Name the ligands in alphabetical order. Use specific names:
- $NH_3$ = ammine (note: two m's).
- $H_2O$ = aqua.
- $Cl^-$ = chloro.
- $CN^-$ = cyano.
For multiple identical ligands, use prefixes: di (2), tri (3), tetra (4), penta (5), hexa (6). So, four $NH_3$ ligands = tetraammine.
- Metal Next: Name the central metal. If the complex is a cation (positive) or neutral, use the regular metal name (e.g., copper). If the complex is an anion (negative), change the metal name ending to "-ate" (e.g., ferrate for iron, cuprate for copper). Our example is a cation, so we use "copper".
- Oxidation State: Use Roman numerals in parentheses to show the metal's charge. For $[Cu(NH_3)_4]^{2+}$, each neutral $NH_3$ has charge 0, so the copper must have a +2 charge to give the total +2 charge. This is copper(II).
Putting it together: tetraamminecopper(II) ion.
Where the Magic Happens: Color and Magnetism
Complex ions are famous for their brilliant colors. Why? It's all about d-orbitals and light. In a transition metal ion, the five d-orbitals normally have the same energy. When ligands approach, their electron pairs repel the d-electrons of the metal unevenly, splitting the d-orbitals into groups with slightly different energy levels.
When white light hits the complex, an electron can absorb a specific color (a specific photon of energy) to jump from a lower d-orbital to a higher one. The color we see is the complementary color of the light that was absorbed. For example, the complex $[Ti(H_2O)_6]^{3+}$ absorbs green and yellow light, so it appears purple.
This d-orbital splitting also explains magnetic properties. Electrons fill the split d-orbitals according to specific rules. If all electrons are paired, the substance is diamagnetic (weakly repelled by a magnet). If there are unpaired electrons, it is paramagnetic (attracted into a magnetic field).
Complex Ions in Action: From Biology to Technology
These are not just lab curiosities; they are essential to life and modern industry.
1. Oxygen Transport: The red color of blood comes from hemoglobin. At its core is a complex ion called heme, which has an iron(II) ion ($Fe^{2+}$) coordinated to a large organic ring. This iron can bind to an oxygen molecule ($O_2$) as a sixth ligand, forming oxyhemoglobin. This is how oxygen is carried from your lungs to your cells.
2. Catalysis: Many industrial chemical reactions need catalysts to speed them up. A famous example is the Haber Process for making ammonia ($NH_3$) from nitrogen and hydrogen. The iron catalyst works because nitrogen gas can bind to the iron surface, weakening the strong triple bond between nitrogen atoms and making it easier to react with hydrogen.
3. Photography (Traditional): In black-and-white film, light exposes silver halide crystals ($AgBr$). During development, a "fixer" (usually sodium thiosulfate, $Na_2S_2O_3$) dissolves the unexposed $AgBr$ by forming a very stable, soluble complex ion: $[Ag(S_2O_3)_2]^{3-}$. This complexation reaction removes the unreacted silver, fixing the image on the film.
4. Pigments and Dyes: Many historic and modern colors come from complex ions. Prussian Blue, a deep blue pigment, is a mixed-valence iron complex. The vibrant colors in stained glass windows, like the red from gold nanoparticles or the blue from cobalt complexes, are also due to coordination chemistry.
Important Questions
A ligand forms a direct, directional covalent bond with the metal by donating a lone pair of electrons. An ionic bond is a non-directional electrostatic attraction between fully formed positive and negative ions. In a complex like $[CoCl_4]^{2-}$, the $Cl^-$ ions are ligands covalently bonded to cobalt. In salt $NaCl$, the $Cl^-$ is ionically attracted to all surrounding $Na^+$ ions, not bonded to just one.
Absolutely! The overall charge is the sum of the metal ion's charge and the charges of all ligands. If they cancel out, the complex is neutral. A famous example is the anticancer drug Cisplatin, $[Pt(NH_3)_2Cl_2]$. Platinum has a +2 charge, two $NH_3$ ligands are neutral (0), and two $Cl^-$ ligands give -2. (+2) + 0 + (-2) = 0. This neutral complex can cross cell membranes to fight cancer.
Transition metals have partially filled d-orbitals that are accessible in energy. These orbitals are spatially oriented in a way that allows them to accept electron pairs from ligands. Also, transition metal ions are small and have high positive charge densities, which strongly attract the lone pairs of electrons on ligands. Main group metals like sodium or calcium lack these suitable empty orbitals of the right energy.
Footnote
1 Ligand: A molecule or ion that donates a pair of electrons to a central metal atom/ion to form a coordinate bond.
2 Coordination Number (CN): The number of donor atoms bonded to the central metal ion in a complex.
3 Transition Metal: An element from groups 3-12 on the periodic table, characterized by having partially filled d-orbitals.
4 Diamagnetic: A substance with all electrons paired, weakly repelled by a magnetic field.
5 Paramagnetic: A substance with one or more unpaired electrons, attracted into a magnetic field.
6 Hemoglobin: The iron-containing oxygen-transport metalloprotein in the red blood cells of vertebrates.