Back To CourseAP Chemistry: Tutoring Solution
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Some of life's most critical processes can be attributed to coordinate covalent bonds and the coordination compounds that contain them. For example, oxygen is distributed throughout our body via the heme group, an iron containing coordination compound present in blood cells.
Before diving into the world of coordination compounds and coordinate covalent bonds, let's review ionic compounds. Ionic compounds are composed of one or more cations (positive ions) and one or more anions (negative ions) that are bonded together because they are oppositely charged. The positive and negative charges balance each other so that the overall charge on the compound is zero.
Coordination compounds are just like slightly more complicated ionic compounds. In a coordination compound, one of the ions has to be a complex ion. It's usually pretty easy to identify which ion is the complex ion because it looks complex! Here are two ions that bond to form a coordination compound. Which one is the complex ion?
If you noticed that the first ion is vastly more complicated than the simple chloride ion, nice work! Your observations have helped you correctly identify a complex ion. Try again to identify the complex ion:
This time, the second ion shown is more complex.
A complex ion is made of two things, a metal ion and compounds called ligands. Ligands are neutral molecules or ions containing lone electron pairs that can bond with the metal ion. Common ligands are ammonia (NH3), water (H2 O) and halide ions (Cl-, Br-).
Ligands are considered Lewis bases because they are sharing their electron pairs with the metal ion. As you may recall, metal ions are always positive, so they are quite attractive to lone pairs of electrons. The resulting bond between the metal ion and the ligands are known as coordinate covalent bonds.
In this image, a coordinate covalent bond is shown in red between the platinum metal ion and the lone pair on nitrogen NH3. Notice that in the previous examples, both complex ions contain metal ions and several ligands. Not all of the ligands have to be the same. In the first example, [Pt(NH3)3 Br]+, there are three ammonia ligands, NH3, and one bromide ion ligand. Each ligand has formed a bond with the platinum ion for a total of four coordinate covalent bonds. In the second example [CoF6)]3-, there are six fluoride ligands, each forming coordinate covalent bonds with the cobalt ion for a total of six bonds.
As you might have observed, complex ions can be positive or negative. Like simpler ionic compounds, complex ions will bond with ions of the opposite charge called counter ions, forming a neutral compound. A complete coordination compound is written with the complex ion in brackets and the counter ions outside the brackets. There should be no charge on the compound because it is neutral.
Here are the same examples written as complete compounds:
Coordination compounds exist because some metals can do some funky things with their electrons. Some elements, like the alkali metals in column one, are capable of bonding with just their outermost set of electrons, known as valence electrons, so they are limited in the number of bonds they can make. Other metals are not though. Some metals, like transition metals, are thought to have two sets of electrons that can participate in bonding, primary valence electrons, and secondary valence electrons.
Secondary valence electrons are involved in the formation of coordination covalent bonds, or bonds between the metal ion and the ligands. The number of bonds a metal is capable of making is known as the coordination number. Metals are capable of having multiple coordination numbers. Six and four are the most common coordination numbers.
|Metal ion||Coordination Numbers|
Unfortunately, there's no easy way to predict the coordination number of a transition metal. It can be easily deducted given the formula of a complex ion or coordination compound.
Primary valence electrons are involved in forming ionic bonds with the counter ions. The charge of the transition metal within the complex ion is known as the oxidation number. The oxidation number of the transition metal can be determined by first finding the charge of the complex ion and then considering the charges of any ions that are also ligands.
The total charge of the counter ions must be equal and opposite to the complex ion's charge.
Let's break down two coordination compounds and determine the complex ion, counter ions, ligands, transition metal ion, coordination number, number of coordinate covalent bonds and oxidation number.
The first example: [Co(NH3)5 Cl]Cl2
Work from the outside in and start with the counter ions. Counter ions are on the outside of the brackets. In this case there are two Cl- ions because of the subscript 2 after Cl, and that the charge on each ion is -1 because Cl is a halide and always forms -1 ions.
The complex ion looks like this: [Co(NH3)5 Cl] 2+. The charge on the complex ion is 2+ because there were two Cl- ions countering its charge.
There are six ligands present, five ammonia molecules and one Cl- ion. Therefore, the coordination number is six and there are six coordinate covalent bonds. The transition metal, which is always listed first, looks like this: Co3+ and, therefore, the oxidation number is 3+. This is because the net charge of the complex ion is 2+. Within the complex ion there is one chloride ion with a -1 charge. If one considers that the presence of the negative ion would decrease the net charge from 3+ to 2+, one can infer that the charge on the Cobalt ion is 3+.
The second example: [Cr(H2 O)5 Br]Br2
Coordination compounds are neutral compounds made of a complex ion and counter ions. Complex ions contain a positively charged transition metal ion and several ligands. Ligands are neutral molecules or ions with at least one set of lone pairs. A coordinate covalent bond forms between a metal ion and a ligand. In this bond, a ligand shares a lone pair of electrons with the positively charged metal ion. The charge on the transition metal ion is known as the oxidation number. The number of coordinate covalent bonds in a complex ion is known as the coordination number. And, finally, counter ions are usually monatomic ions with charges opposite the complex ion.
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Back To CourseAP Chemistry: Tutoring Solution
16 chapters | 181 lessons
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