Coordination Chemistry: Bonding in Coordinated Compounds

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  • 0:00 Intro to Coordinated Compounds
  • 2:54 Bonding in Coordinated…
  • 6:39 Shapes of Coordinated…
  • 7:37 Coordinated Compounds…
  • 9:20 Lesson Summary
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Lesson Transcript
Instructor: Elizabeth (Nikki) Wyman

Nikki has a master's degree in teaching chemistry and has taught high school chemistry, biology and astronomy.

Discover what a coordinated compound is. Understand how bonding occurs in coordinated compounds and some of the possible shapes coordinated compounds can be. Learn the uses in nature and industry for coordinated compounds.

Intro to Coordinated Compounds

If I asked what color human blood was, most people would say 'red, of course!' But human blood isn't always bright red. When it's in our veins, returning to the heart without oxygen, it's dark.

If that's not a shock, did you know that lobsters and other mollusks have clear blood and that when it interacts with oxygen it turns blue?

Believe it or not, these cool color changes result from the bonding of coordination compounds with oxygen. Coordination compounds are not your average compounds. They are highly organized compounds made of molecules, ions and atoms. Before we dive into the definition of a coordination compound, let's review some terms first.

An ion is a charged particle. A cation is a positively charged ion, and an anion is a negatively charged ion. Transition metals are metals in groups 1B-10B on the periodic table. They lose electrons to form cations; some transition metals can form more than one type of cation.

Now for a new term. A ligand is a neutral molecule or ion that has a lone pair of electrons and acts as a Lewis base. The negatively charged lone pairs on the ligand are attracted to positive particles. Common examples of ligands are water (H2 O), ammonia (NH3 ) and cyanide (CN-).

I think we're ready to tackle coordinated compounds now. Coordination compounds are made up of a transition metal with attached ligands.

If the coordination compound is an ion, it is called a complex ion. A complex ion is an ion made of transition metals with attached ligands. Complex ions often occur bonded to counter ions, or ions that have a charge opposite of the complex ion.

An example of a coordination compound is Cr(NH3)6 3+. In this compound, chromium is the transition metal with six ammonia ligands surrounding it. The overall charge of the compound is 3+, which is also the charge on the chromium ion. We can infer this because the ligands have no charge. Because this compound has a charge it is also considered a complex ion. It is likely this compound would occur bonded to three negatively charged counter ions, in which case it would be called a coordinated complex. Imagine this compound is bonded to three chloride (Cl -) ions. In this case, the formula for the compound would be Cr(NH3)6 Cl3.

Makes complete sense, right? If you said no, don't worry! Even the brightest chemistry student (or teacher) is apt to struggle with coordinated compounds. We'll spend the rest of this lesson learning more in depth about coordination compounds, including how they bond and why they are important.

Bonding in Coordinated Compounds

Transition metals are up to all sorts of unusual activities in the chemical world, like giving color to compounds or performing vital functions in living things. Many of their unique abilities have to do with their electron configurations. Because they are so special, we often find transition metals as the center of attention, literally. In coordinated compounds the transition metal is in the middle of the complex ion.

Transition metals involved in the complex ion have two sets of valence electrons participating in bonding. The first set of bonding electrons is called primary valence, and it is the oxidation number of the metal. The oxidation number can be determined by looking at the charge on the transition metal ion. Copper 2+ (Cu 2+), for example, has an oxidation number of 2. Sometimes this number must be inferred based on the overall charge of the complex ion. The primary valence electrons are involved in typical ionic bonds.

The second set of transition metal valence electrons are called secondary valence, usually referred to as the coordination number. The secondary valence electrons are involved with bonding to the ligands. The coordination number indicates the number of ligands that a metal ion is bonded to.

Like with many things in chemistry, sometimes there are no easy explanations for why certain phenomena occur. Coordination numbers are examples of these; there is really no easy way to predict how many ligands a transition metal is capable of bonding with, and some metals are capable of multiple coordination numbers. The table below shows some examples of typical coordination numbers for some common metal ions. The nickel 2+ ion can have coordination numbers of either four or six. The silver one ion can only have a coordination number of 2.

Metal ion Coordination Number
Ag+ 2
Cu+ 2, 4
Fe2+ 6
Ni2+ 4, 6
Cu2+ 4, 6
Cr3+ 6
Co3+ 6

Ligands bond to transition metals by sharing a lone pair of electrons. This type of interaction is a Lewis acid-base reaction, where the metal ion is the Lewis acid and the ligand is the Lewis base. The resulting bond in which one species donates both bonding electrons is called a coordinate covalent bond.

Here are some examples of coordination compounds in a table that identifies the components of each compound. In every case, notice how the transition metal ion is in the center of the ligands.

Compound Ligand (Lewis Base) Metal Ion (Lewis Acid) Coordination Number
[Cu(NH3)4] 2+ NH3 Cu 2+ 4
[Ag(Cl2)]- Cl - Ag+ 2
[Fe(H2 O)6] 3+ H2 O Fe 3+ 6

The first example, Cu(NH3)4 2+, has one copper ion (Cu) and four NH3 ligands. The number of ligands is equal to the coordination number for copper, 4. The charge on the complex ion is also the same as the charge on the copper ion, 2+ because the ligands have no charge.

The second example, Ag(Cl2)- , has one silver ion (Ag) and two chloride ion ligands. The number of chloride ligands is equal to the coordination number for silver, 2. The charge on the complex ion is negative one. To find the charge on the silver ion, we must subtract the number of negative ions contributing to the total charge. A -1 charge minus 2- is +1. The charge on silver must be +1.

The third example Fe(H2 O)6 3+, has one iron (Fe) ion and six H2 O ligands. The coordination number for iron is 6, and the charge on iron is 3+ since water is a neutrally charged molecule.

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