Shapes of Ion Complexes in Transition Metals

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Transition metals are able to form a variety of shapes when they bind with different ligands. We will learn about the shapes that can form and when each type will form.

Ion Complex Shapes

An exciting new form of chemotherapy uses cisplatin, which has been found to be very effective against many different forms of cancer. Cisplatin has a platinum transition metal in the center, connected to two chlorines and two ammonias. It forms a unique complex shape: a square planar complex; most transition metals with 4 ligands form a tetrahedral shape instead.

Transition metals can have any of four possible ion complex shapes. The ion complex shape is the orientation of ligands, or molecules that are attached to the transition metal. The shape that a metal takes depends on the coordination number (number of ligands) and how large the ligands are. The possible shapes are:

  • Linear
  • Square Planar
  • Tetrahedral
  • Octahedral

Linear

A linear complex ion forms when the ligands and the metal form a straight line. This can only occur with silver (I) complexes with 2 ligands attached.

The most commonly used linear complex occurs in Tollens' reagent. Tollens' reagent is used to differentiate between ketones and aldehydes. Tollens' reagent contains complexes of silver connected to two ammonia groups.

Tollens` reagent contains complexes of silver and two ammonia molecules in a linear shape
Linear tollens

Square Planar

As we discussed earlier, the square planar shape is not very common. The square planar shape forms when the ligands around a metal are all on the same plane; in other words, it is completely flat. This occurs when there are 4 ligands attached to a transition metal with a d8 electron configuration. When there are 4 bonds this means that there are 4 other unpaired electron pairs. There are two ligands pointing up and two ligands pointing down; none are pointing to the sides.

The most common square planar complexes occur with platinum (II) metals. But they can also occur with rhodium (I), iron (I), palladium (II) and gold (III).

Cisplatin has a square planar structure.
Cisplatin structure

Cisplatin has a platinum (II) as the transition metal. It is connected to two chlorine atoms and two ammonia groups. Since it is cis, we know that the two largest groups (chlorine) are pointing in opposite directions, so one is up and one is down.

All of the elements are in the same plane, so it can easily be drawn and pictured in a 2D environment.

Tetrahedral

The tetrahedral complex shape is the most common shape for a transition metal with 4 ligands. It has one ligand pointing up, one down, and the other two are pointing to the front and back. This occurs with any transition metal when the attached ligands are large. Large ligands include chlorine, bromine, iodine, and others. Most transition metals want to form the octahedral, but large ligands get in the way of forming this octahedral, so only a tetrahedral, with 4 ligands, can form.

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