Chapter 22, Problem 22.91QP

(a)

Interpretation Introduction

Interpretation:

Name of the complex ion for the diamagnetic complex ions ${\left[Co{\left(N{H}_3\right)}_6\right]}^{3+}$ and paramagnetic complex ion ${\left[Fe{\left(H_{2}O\right)}_6\right]}^{2+}$ have to be determined.

Concept Introduction:

Complex compounds are primarily formed by transition elements which are d-block elements.  A co-ordination compound is known as a complex compound is made of metal atom/ion and ligand(s).  Ligands are considered as Lewis bases and the central metal atom is Lewis acid.  Ligands donate a pair of electron to metal ion and establishes bonding with metal ion which is known as co-ordinate bond and hence these compounds are named as co-ordination compounds.  The ligands represented inside the square brackets adjacent to the central metal ion, which is termed as co-ordination sphere.

IUPAC has devised rules to name a co-ordination compound in chemical terminology.  The rules are stated below -

• Name of the cation must be written before the name of the anion.
• Metal ions are named first followed by ligand names.
• If the ligand is negatively charged, the name of the ligand carries suffix 'o'.  example - $CN$ is a negative ligand and it must be named as 'cyano' not cyanide.
• Number of simple ligands present in a co-ordination compound is denoted by prefixes - mono-, di-, tetra-, etc., so on.
• The prefixes bis-, tris-, tetrakis-, etc are used to denote the number of complicated ligands.
• Oxidation state of the central metal ion must be mentioned using a Roman numeral in parentheses.
• The ligands are named in alphabetical order if a complex contains more than one ligand.
• The suffix 'ate' is added to the name of the central metal atom in case of negatively charged complex ion.  While naming some of the metals, their Latin name is preferred.

Answer to Problem 22.91QP

• Name of the complex ion ${\left[Co{\left(N{H}_3\right)}_6\right]}^{3+}$ is hexaamminecobalt(III) ion.
• Name of the complex ion ${\left[Fe{\left(H_{2}O\right)}_6\right]}^{2+}$ is hexaaquairon(II) ion.

Explanation of Solution

Oxidation state of Cobalt in the complex ${\left[Co{\left(N{H}_3\right)}_6\right]}^{3+}$,

$\begin{array}{l}oxidation\text{ state of Co}\text{= charge on complex - charge of ligands}\\ \text{= +3-6}\left(0\right)\text{ = +3}\end{array}$

In the complex ion ${\left[Co{\left(N{H}_3\right)}_6\right]}^{3+}$ six ammine ligands are co-ordinated to $C{o}^{3+}$ ion that the term ''hexa'' has to be used to name the ligand.  Cobalt is in $+3$ oxidation state and the complex is ionic and so the name of the complex according to IUPAC conventions is hexaamminecobalt(III) ion.

Oxidation state of Iron in the complex ${\left[Fe{\left(H_{2}O\right)}_6\right]}^{2+}$,

$\begin{array}{l}oxidation\text{ state of Fe}\text{= charge on complex - charge of ligands}\\ \text{= +2-6}\left(0\right)\text{ = +2}\end{array}$

In the complex ion ${\left[Fe{\left(H_{2}O\right)}_6\right]}^{2+}$ six ammine ligands are co-ordinated to $F{e}^{2+}$ ion that the term ''hexa'' has to be used to name the ligand.  Iron is in $+2$ oxidation state and the complex is ionic and so the name of the complex according to IUPAC conventions is hexaaquairon(II) ion.

(b)

Interpretation Introduction

Interpretation:

Geometry of the diamagnetic complex ion ${\left[Co{\left(N{H}_3\right)}_6\right]}^{3+}$ and paramagnetic complex ion ${\left[Fe{\left(H_{2}O\right)}_6\right]}^{2+}$ have to be determined.

Concept Introduction:

Complex compounds are primarily formed by transition elements which are d-block elements.  A co-ordination compound is known as a complex compound is made of metal atom/ion and ligand(s).  Ligands are considered as Lewis bases and the central metal atom is Lewis acid.  Ligands donate a pair of electron to metal ion and establishes bonding with metal ion which is known as co-ordinate bond and hence these compounds are named as co-ordination compounds.  The ligands represented inside the square brackets.

Complex compounds exist in following geometries - tetrahedral, square planar, octahedral etc.

Denticity of a ligand correlates to number of donor groups or atoms available in that ligand to co-ordinate with a metal ion.  A ligand having one donor atom available to form co-ordinate covalent bond is termed as monodentate or uni-dentate ligand.   A ligand having two donor atoms or groups available to form co-ordinate covalent bond with metal ion is termed as bi-dentate ligand.   A ligand having more than two donor atoms or groups available to form co-ordinate covalent bond with metal ion is termed as polydentate ligand.

Answer to Problem 22.91QP

• Geometry of the complex ion ${\left[Co{\left(N{H}_3\right)}_6\right]}^{3+}$ is octahedral.
• Geometry of the complex ion ${\left[Fe{\left(H_{2}O\right)}_6\right]}^{2+}$ is octahedral.

Explanation of Solution

Both the complex ions have six unidentate ligands each. Thus the geometry of these complex ions would undoubtedly be octahedral.

(c)

Interpretation Introduction

Interpretation:

Among the two complex ions ${\left[Co{\left(N{H}_3\right)}_6\right]}^{3+}$ and paramagnetic complex ion ${\left[Fe{\left(H_{2}O\right)}_6\right]}^{2+}$ that has greater crystal field splitting energy has to be determined.

Concept Introduction:

• Spectrochemical series refers to the arrangement of ligands with respect to crystal field splitting they cause in the d-orbitals of a metal ion. 
• The ligands are arranged in increasing order of crystal field splitting they produce.

  $\begin{array}{l}{I}^{-}<B{r}^{-}<S^{2-}<SC{N}^{-}<C{l}^{-}<N{O}_3{}^{-}<N_3{}^{-}<F^{-}<O{H}^{-}<C_2{O}_4{}^{2-}<H_{2}O<NC{S}^{-}\\ <C{H}_{3}CN<py<N{H}_3<en<bipy<phen<N{O}_2{}^{-}<PP{h}_3<C{N}^{-}<CO\end{array}$

• The ligand that causes larger crystal field splitting is strong ligand and has higher crystal field splitting energy.  The ligand that causes smaller crystal field splitting is termed as weak ligand and has lower crystal field splitting energy.

Answer to Problem 22.91QP

Among the two complex ions the complex ion ${\left[Co{\left(N{H}_3\right)}_6\right]}^{3+}$ has higher crystal field splitting energy.

Explanation of Solution

According to spectrochemical series, ammine ligands are stronger than aqua ligands.  Thus the complex ion in which metal co-ordinated to ammine ligands have higher crystal field splitting energy.