Complex Compounds

Team Chemistry -
Created by: Team Chemistry -, Last Updated: April 27, 2024

Complex Compounds

Complex compounds, also known as coordination compounds, often fascinating to explore, are chemical structures formed when a central metal atom or ion binds with surrounding molecules or ions called ligands. Imagine the metal at the heart of these compounds as a sun, with ligands orbiting around it like planets. This bond creates a unique, stable entity known for its diverse properties and applications, from vibrant colors in art to crucial roles in biological systems and industrial processes.

What are Complex Compounds?

Complex compounds are special types of chemicals where a central metal atom or ion is surrounded by several other atoms, ions, or molecules, called ligands. Think of it as a social gathering where the metal atom is the host and the ligands are guests, attaching themselves to the host in a specific arrangement. This unique bonding creates a whole new entity with properties distinct from those of the individual attendees. These compounds play a crucial role across various sectors, including environmental science, health care, and industrial processes, making them a fascinating subject of study that connects the dots between chemistry and real-world applications.

Complex Compounds Structure

Imagine building a model where at the center is a metal piece, and around it, you attach different beads – this is essentially the structure of complex compounds. At the heart of these compounds is a metal atom or ion, kind of like the main character in a story. Surrounding this central character are various other atoms, ions, or molecules, known as ligands, which can be thought of as supporting characters. These ligands are attached to the central metal by bonds, acting like bridges that connect the ligands to the metal. This setup forms a complex, intricate structure, where the metal and its surrounding ligands work together, creating a compound with unique properties. This fascinating architecture not only makes complex compounds essential in many scientific fields but also a captivating topic for anyone keen on understanding the microscopic world of chemistry.

Complex Compounds Examples



  • This is the star player in your blood that carries oxygen from your lungs to the rest of your body. Structurally, it’s a complex compound with iron at its center, making it possible to bind with oxygen.
  • Equation: Fe²⁺ + O₂ ↔ FeO₂²⁺


  • Think of chlorophyll as the plant world’s solar panels. With a magnesium ion at its heart, chlorophyll captures sunlight to help plants make food through photosynthesis.
  • Equation: Mg + (C₅₅H₇₂O₅N₄) → C₅₅H₇₂MgO₅N₄

Vitamin B12

  • Essential for your brain and blood, Vitamin B12 features a cobalt ion nestled within a complex organic structure, helping in nerve function and red blood cell formation.
  • Equation: Co + (C₆₃H₈₈CoN₁₄O₁₄P) → C₆₃H₈₈N₁₄O₁₄PCo


  • These are like the power generators in cells, using iron to transfer electrons during cellular respiration. They play a crucial role in the energy production of every cell.
  • Equation: Fe³⁺ + e⁻ → Fe²⁺


  • Similar to hemoglobin but found in muscle cells, myoglobin stores oxygen, using its iron center to release oxygen when your muscles work hard.
  • Equation: Fe²⁺ + O₂ ↔ FeO₂²⁺


  • This complex compound acts as an iron storage locker in your body, releasing iron when needed. It’s vital for managing the body’s iron balance. E
  • Equation: Fe³⁺ → [FeO(OH)]₈[FeO(H₂PO₄)].


  • Another name for Vitamin B12, cobalamin is crucial for the brain and blood. Its complex structure with a cobalt ion supports vital bodily functions.
  • Equation: Co + C₆₃H₈₈CoN₁₄O₁₄P → C₆₃H₈₈N₁₄O₁₄PCo.


  • This enzyme breaks down harmful hydrogen peroxide into water and oxygen in our bodies, with iron playing a key role in the process.
  • Equation: 2H₂O₂ → 2H₂O + O₂.


  • Essential for plants, nitrogenase contains molybdenum and iron and helps convert atmospheric nitrogen into a form plants can use to grow.
  • Equation: N₂ + 8H⁺ + 8e⁻ + 16ATP → 2NH₃ + H₂ + 16ADP + 16Pᵢ.


  • Found in the blood of some invertebrates, hemocyanin uses copper, not iron, to transport oxygen, turning blue when oxygenated.
  • Equation: Cu+ O₂ ↔ CuO₂

How to Name Complex Compounds

Step 1: Identify the Ligands

Start by naming the ligands (the atoms, ions, or molecules attached to the central metal atom) in alphabetical order. For negatively charged ligands, use the ending ‘-o’, such as chloro for chlorine. Neutral ligands like water (aqua) and ammonia (ammine) have special names.

Step 2: Name the Central Metal

Next, name the central metal atom or ion. If the complex ion is an anion (negatively charged), the metal’s name ends in ‘-ate’, such as ferrate for iron.

Step 3: Specify Oxidation State

Indicate the oxidation state (charge) of the central metal in Roman numerals in parentheses right after the metal’s name. This shows how many electrons the metal has gained or lost.

Step 4: For Anionic Complexes:

If the complex part of the compound is an anion, add ‘ate’ to the end of the metal’s name. For example, copper in a negative complex becomes cuprate.

Step 4: Add Prefixes for Multiple Ligands:

If there are multiple ligands of the same type, use prefixes like di-, tri-, tetra- to show how many there are. For example, a complex with two chlorine ligands would have ‘dichloro’ before the metal’s name.

Step 5: Cation Before Anion:

If the complex is part of a salt, name the cationic part (positive charge) first followed by the anionic part (negative charge), just like naming regular ionic compounds.

Complex Compounds Properties

Physical Properties

ColorOften vibrant and varied
SolubilityVaries widely among compounds
Magnetic PropertiesPresent in some compounds
ConductivitySome are conductors, others are insulators
Melting/Boiling PointsGenerally high but can vary


Complex compounds can display a wide array of vibrant colors. This is because of the way their metal centers and ligands interact with light, absorbing certain wavelengths and reflecting others, which is what we see as color.


The solubility of complex compounds in water or other solvents can vary greatly. Some dissolve easily, making solutions, while others remain solid. This property is crucial for their use in various applications, from medicine to industrial processes.

Magnetic Properties

Certain complex compounds exhibit magnetic properties due to the presence of unpaired electrons in their metal centers. These properties make them useful in various technologies, including magnetic resonance imaging (MRI).


Complex compounds can behave as conductors or insulators of electricity, depending on their structure. This characteristic is essential in the field of electronics and materials science, where they are used in making semiconductors and other components.

Melting/Boiling Points

Generally, complex compounds have high melting and boiling points, indicating strong bonds within their structures. However, these points can vary based on the nature of the metal and ligands involved, affecting their stability and how they are used in different environments.

Characteristics of Complex Compounds

Varied Coordination Numbers

The coordination number of a complex compound refers to the number of ligand (molecule or ion) attachments to the central metal atom. This number can vary, influencing the shape and properties of the compound. For instance, some might have four ligands attached, while others might have six.

Distinct Geometrical Structures

These compounds can adopt different geometrical shapes based on the coordination number and the types of ligands attached. Common shapes include tetrahedral, square planar, and octahedral. Each shape influences the compound’s chemical reactivity and physical properties.

Colorful Appearances

One of the most striking features of complex compounds is their vibrant colors. The color results from the way these compounds absorb and reflect specific wavelengths of light, a property utilized in many industries, such as dyes and pigments.

Variable Oxidation States

The central metal atom in a complex compound can exhibit various oxidation states, affecting the compound’s reactivity and stability. This variability is crucial for catalysis in industrial and biological processes.

Ligand Exchange Reactions

Complex compounds can undergo ligand exchange reactions, where one or more ligands are replaced by other ligands. This characteristic is important for understanding reaction mechanisms and designing catalysts.

Diverse Magnetic Properties

The magnetic properties of complex compounds depend on the arrangement of electrons around the central metal atom. Some are diamagnetic (not attracted to magnets), while others are paramagnetic (attracted to magnets), playing a role in technologies such as MRI.


What Are Complex Compounds Examples?

  • Hemoglobin, found in red blood cells, binds oxygen for body-wide transport. Chlorophyll, the green pigment in plants, facilitates photosynthesis with a magnesium core. Vitamin B12, crucial for health, features a cobalt ion in a corrin ring. Cisplatin, a platinum-based chemotherapy drug, targets cancer with platinum bonded to chloride and ammonia.

What Makes Complex Compounds From Simple Substances?

  1. Metal ions acting as Lewis acids: Metal ions, with their positive charge and vacant orbitals, act as Lewis acids by accepting electron pairs.
  2. Ligands acting as Lewis bases: Ligands, with their lone electron pairs, act as Lewis bases by donating electrons to the metal ion.
  3. Formation of coordination bonds: The electron pair donation from ligand to metal results in the formation of coordination bonds, creating the complex structure.

What Are the Most Complex Compounds?

  • DNA (Deoxyribonucleic Acid):
  • Though not a complex compound by strict chemistry standards, DNA stands out as an exceptionally complex molecule, crucial for the genetics of all living beings and many viruses.
  • Proteins and Enzymes: Proteins, with their diverse and intricate structures, perform a vast array of functions within organisms. Enzymes, a special class of proteins, catalyze biochemical reactions, displaying remarkable specificity and efficiency.
  • Chlorophyll and Hemoglobin: As complex compounds, both chlorophyll and hemoglobin are incredibly sophisticated, given their roles in photosynthesis and oxygen transport, respectively. Their complex structures enable the critical life processes of energy conversion and respiration.

Can Complex Compounds Have Different Shapes?

Yes, complex compounds can have various shapes, including linear, square planar, tetrahedral, octahedral, and trigonal bipyramidal. The shape of a complex compound depends on the number of ligands attached to the central metal and the repulsions between electron pairs in the ligand’s coordination environment.

Can Complex Compounds Exhibit Isomerism?

Yes, complex compounds can exhibit several types of isomerism, including geometric and optical isomerism. This variability in structure can lead to compounds with different physical and chemical properties, even though they contain the same types and quantities of atoms.

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