Question Video: Identifying Dative Covalent Bonds

Which of the following molecules and ions does not contain a dative covalent bond? [A] CO [B] HNO₃ [C] BF4− [D] CS₂ [E] NH4+


Video Transcript

Which of the following molecules and ions does not contain a dative covalent bond? A) CO, B) HNO₃, C) BF4 minus, D) CS₂, or E) NH4 plus.

First, let’s remind ourselves what we mean by a dative covalent bond. In a normal covalent bond, two atoms are held together by sharing a pair of electrons. One electron in this pair comes from one atom, and the other belongs to the other atom. In a dative covalent bond, we still have a pair of electrons shared between two atoms, forming our bond. But in this case, both electrons originated from the same atom. So what we need to do to answer this question is to look at each of the molecules and ions on our list and draw some dot-and-cross diagrams to show the origin of each of the electrons in the bonds.

Remember though that the question is asking us which of these does not contain a dative covalent bond. This means that four out of the five molecules and ions we’re given should contain a dative bond. Sometimes you might hear these referred to as coordinate bonds.

Let’s start with CO, carbon monoxide. When drawing our dot-and-cross diagrams, we only really need to focus on the valence electrons. So here we have our molecule with a carbon and an oxygen atom. And now we just have to place the electrons. The carbon atom in our CO molecule contains four valence electrons, while our oxygen contains six valence electrons.

You may have seen carbon monoxide drawn like this, with a triple bond between the carbon and the oxygen. A triple bond suggests that there should be three pairs of electrons in the shared area down the middle. We will denote the electrons from carbon as dots and those from oxygen as crosses for this diagram. We can easily draw in two normal covalent bonds with two pairs of electrons and one electron in each pair coming from the oxygen and one electron in each pair coming from the carbon.

We now have two electrons left to place in carbon and four left to place in oxygen. If we were to create one more covalent bond with one electron from carbon and one for oxygen, both the carbon and oxygen would be left with an odd number of electrons, which is not ideal. So this is definitely not the case. What we actually end up with is one lone pair on the carbon, one lone pair on the oxygen, and then a shared lone pair donated by the oxygen. And this creates our dative covalent bond.

In this question, we’re looking for a molecule or ion that does not contain a dative covalent bond, which means that carbon monoxide is not the correct answer.

Let’s move on to HNO₃. You may have seen the structure of HNO₃ drawn in two different ways, one of which showing the delocalised nature of the bonding between the nitrogen and the two oxygen atoms. Let’s try to convert these into a dot-and-cross structure. It’s easiest to draw this in a dot-and-cross diagram as the nondelocalised structure.

Again, we’re only going to be interested in the valence electrons. Each oxygen atom is going to contribute six valence electrons. Each nitrogen will contribute five and hydrogen just the one. The OH bond is easy to depict. In this diagram, we’ll use dots for the electrons in hydrogen and nitrogen and crosses for the oxygen.

This first oxygen is fairly straightforward to fill in, with one covalent bond to the hydrogen, one to the nitrogen, and two lone pairs. Now let’s focus on the nitrogen. If we draw the nitrogen oxygen double bond in with our dots and crosses, we are left with two electrons in nitrogen unaccounted for. This bottom oxygen has four, which we can easily display as two lone pairs. So we’re left with six electrons on our last oxygen and two unaccounted for electrons on our nitrogen.

If we were to arrange these so that they were all in the same shell, we would have a full shell of eight electrons. We can do this by arranging the six oxygen electrons into lone pairs and using the two electrons in nitrogen to form a dative covalent bond. So just like carbon monoxide, this molecule does contain a dative covalent bond and therefore is not the correct answer to our question.

Remember of course that, in reality, this molecule exists with some delocalisation. So this dot-and-cross diagram isn’t entirely accurate.

Now let’s look at BF4 minus. Boron can contribute three valence electrons and fluorine seven electrons each. Electrons from boron will be denoted by a dot and those from fluorine with a cross. Don’t forget though that this is BF4 minus. So we’re going to have an extra electron somewhere to account for the charge.

Once we’ve drawn in three boron-to-fluorine covalent bonds, the boron has already run out of valence electrons. This means it has no electrons to contribute to the final BF bond. We can consider this last fluorine as fluorine minus. BF4 minus of course is formed by the reaction of BF₃ plus an F⁻. This is a Lewis acid–Lewis base reaction. So our F⁻ is going to have eight valence electrons. This means it is able to donate two electrons to the boron–fluorine bond. And this is our dative covalent bond.

We can add in the rest of the fluorine lone pairs for completeness. So this ion does contain a dative covalent bond so is not the correct answer to our question.

Now let’s look at CS₂. You can think of CS₂ as having a similar structure to CO₂ since both oxygen and sulphur are in the same group of the periodic table so have the same number of valence electrons. Each carbon is going to contribute four valence electrons and each sulphur six. Here electrons from carbon will be denoted by dots and those from sulphur with crosses.

Let’s first put in the two covalent bonds between carbon and sulphur each side. After drawing in these electrons, we can see that carbon’s four initial valence electrons have now been used up. And its outer shell contains eight electrons. So this satisfies the octet rule.

Sulphur, meanwhile, still has four electrons to place. And we can arrange these simply as two lone pairs on each sulphur atom. As you can see, in this molecule, there are no dative covalent bonds, just normal covalent bonds. So this is a correct answer to our question. But let’s check the last answer just to be safe.

In this diagram, nitrogen will have five valence electrons and hydrogen one electron each. However, again, we have an ion in this case. Here it’s a cation. So we’re going to be missing one electron to give it the overall charge. NH4 plus, the ammonium ion, is usually formed when ammonia reacts with H⁺ from an acid. In a curly arrow diagram, we would draw this with an arrow from the nitrogen lone pair going towards the H⁺. And this can be seen in our dot-and-cross diagram.

We have three normal covalent bonds between nitrogen and three of the hydrogens. Nitrogen still has two electrons unaccounted for. And these are the lone pair that we saw in our curly arrow diagram. These two final electrons in the nitrogen lone pair go on to form a dative covalent bond with our H⁺. So here we have the dot-and-cross structure for the ammonium ion. And we can see it does contain a dative covalent bond. So this is not a correct answer.

From these molecules and ions, the only one which does not contain a dative covalent bond is CS₂.

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