Question

In ammoniacal solution, dichromate in presence of H202 forms ${{\text{(N}{{\text{H}}_{\text{3}}}\text{)}}_{\text{3}}}\text{Cr}{{\text{O}}_{\text{4}}}$ in which $\text{Cr}$ is in $\text{IV}$ the state. The structure is a pentagonal bipyramid. State true or false.
A) True
B) False

Answer 1

This ${{\text{(N}{{\text{H}}_{\text{3}}}\text{)}}_{\text{3}}}\text{Cr}{{\text{O}}_{\text{4}}}$ is a pentagonal bipyramidal structure. It involves the peroxo linkages. The chromium is tetravalent. It is synthesized from the dichromate in presence of $30{\scriptstyle{}^{0}/{}_{0}}$ ${{\text{H}}_{\text{2}}}{{\text{O}}_{\text{2}}}$.

Complete answer:
It always mentioned that only a considerable number of peroxo-chromate complexes are known. These compounds are more or less stable and these peroxo compounds decompose slowly with the evolution of oxygen. Some of these peroxo compounds are explosive.
This ${{\text{(N}{{\text{H}}_{\text{3}}}\text{)}}_{\text{3}}}\text{Cr}{{\text{O}}_{\text{4}}}$ is synthesized by the treatment of alkaline dichromate solutions $30{\scriptstyle{}^{0}/{}_{0}}$ ${{\text{H}}_{\text{2}}}{{\text{O}}_{\text{2}}}$. This forms the peroxo chromate$\text{(CrO}_{\text{8}}^{\text{3-}}\text{)}$. They are paramagnetic with the once unoccupied electron per formula unit.
We can say that in $\text{(CrO}_{\text{8}}^{\text{3-}}\text{)}$ the chromium is pentavalent.
When the mixture used for the preparation of ${{\text{(N}{{\text{H}}_{\text{4}}}\text{)}}_{\text{3}}}\text{Cr}{{\text{O}}_{\text{8}}}$ is heated at the ${{50}^{0}}C$ and cooled down to ${{0}^{0}}C$ a brown crystal of ${{\text{(N}{{\text{H}}_{4}}\text{)}}_{\text{3}}}\text{Cr}{{\text{O}}_{\text{4}}}$ are obtained-ray studies has revealed the structure of ${{\text{(N}{{\text{H}}_{\text{3}}}\text{)}}_{\text{3}}}\text{Cr}{{\text{O}}_{\text{4}}}$ as shown below:

The ${{\text{(N}{{\text{H}}_{\text{3}}}\text{)}}_{\text{3}}}\text{Cr}{{\text{O}}_{\text{4}}}$ has the central metal atom as the chromium. The central chromium ion is surrounded by a total of seven ligands around it. The three are ammonia groups $\text{(N}{{\text{H}}_{\text{3}}}\text{)}$ and four of the oxygen are arranged around the central ion. The two superoxide ligands are bonded to the chromium. There are a total of five coordinating sites. For five ligands the possible structure is pentagonal bipyramidal since each oxygen of superoxide is considered as a single ligand.
Here, the oxidation state of chromium is a low valence state. The divalent chromium is coordinated to the two superoxide $\text{(-O-O-)}$ ions. The compound contains the two unpaired electrons and thus chromium is diamagnetic. In other words, we can naturally consider a compound containing the tetravalent chromium $\text{IV}$ which is coordinated peroxide ions.
Hence, when the ammoniacal solution, dichromate in presence of ${{\text{H}}_{\text{2}}}{{\text{O}}_{\text{2}}}$ forms ${{\text{(N}{{\text{H}}_{\text{3}}}\text{)}}_{\text{3}}}\text{Cr}{{\text{O}}_{\text{4}}}$ in which $\text{Cr}$ Is in $\text{IV}$ the state. The structure is a pentagonal bipyramid.

Hence, the statement is true. Correct option (A).

Note:
There are various peroxy –chromate for example deep blue chromium peroxide $\text{(Cr}{{\text{O}}_{\text{5}}}\text{)}$, the blue peroxy-chromate $\text{C}{{\text{r}}_{\text{2}}}\text{O}_{\text{12}}^{\text{2-}}$ , and the red peroxy-chromate $\text{(CrO}_{\text{8}}^{\text{3-}})$ .