Question

Chemistry question? Bond lengths.

Predict the ordering, from shortest to longest, of the bond lengths in CO, `CO_2`, and `(CO_3)^(2-)`.

I know that the answer is CO < `CO_2` < `(CO_3)^(2-)`, but why does `(CO_3)^(2-)` have a longer bond length than `CO_2`?

Answer 1

The order is `"CO < CO"_2 < "CO"_3^"2-"`.

Explanation:

You can order the bond lengths by determining the bond order (BO) of the `"C-O"` bonds.

(a) `bb"CO"`

The Lewis structure of `"CO"` is `:"C≡O":`

There is a triple bond, so BO = 3.

(b) `bb"CO"_2`

The Lewis structure of `"CO"_2` is `":"stackrelcolor(blue)(". .")("O")"=C="stackrelcolor(blue)(". .")("O")":"`

Each `"C-O"` bond is a double bond, so BO = 2.

(c) `bb"CO"_3^"2-"`

We can draw three different Lewis structures for `"CO"_3^"2-"`:

The actual structure is a resonance hybrid in which the bond lengths are identical and an average of those in the three structures.

The ion has a double bond and two single bonds. Averaged over the three bonds the bond order is

`"BO" = (2 + 1 +1)/3 = 4/3 = 1.33`

Order of bond lengths

The greater the bond length, the shorter the bond. Thus, the order of bond
lengths is

`color(white)(mmll)"CO < CO"_2 < "CO"_3^"2-"`
`"BO":color(white)(m)3color(white)(mmll)2color(white)(mmll)1.33`