Question

(a) In each of the following complexes, determine the overall charge, \(n,\) which may be positive or negative: \\[ \begin{array}{l} {\left[\mathrm{Fe}^{\mathrm{II}}(\mathrm{bpy})_{3}\right]^{n},\left[\mathrm{Cr}^{\mathrm{III}}(\mathrm{ox})_{3}\right]^{\prime \prime},\left[\mathrm{Cr}^{\mathrm{III}} \mathrm{F}_{6}\right]^{n},\left[\mathrm{Ni}^{\mathrm{II}}(\mathrm{en})_{3}\right]^{\prime \prime},} \\ {\left[\mathrm{Mn}^{11}(\mathrm{ox})_{2}\left(\mathrm{OH}_{2}\right)_{2}\right]^{\prime \prime},\left[\mathrm{Zn}^{11}(\mathrm{py})_{4}\right]^{n},\left[\mathrm{Co}^{111} \mathrm{Cl}_{2}(\mathrm{en})_{2}\right]^{n}}\end{array}\\] (b) If the bonding in \(\left[\mathrm{MnO}_{4}\right]^{-}\) were \(100 \%\) ionic, what would be the charges on the Mn and \(\mathrm{O}\) atoms? Is this model realistic? By applying Pauling's electroneutrality principle, redistribute the charge in \(\left[\mathrm{MnO}_{4}\right]^{-}\) so that \(\mathrm{Mn}\) has a resultant charge of +1 What are the charges on each 0 atom? What does this charge distribution tell you about the degree of covalent character in the Mn-O bonds?

Short answer

(a) Overall charges: \([\mathrm{Fe(bpy)_3]}^{2+}, [\mathrm{Cr(ox)_3]}^{3-}, [\mathrm{CrF_6]}^{3-}, [\mathrm{Ni(en)_3]}^{2+}, [\mathrm{Mn(ox)_2(OH_2)_2]}^{2-}, [\mathrm{Zn(py)_4]}^{2+}, [\mathrm{CoCl_2(en)_2]}^{+1}\). (b) Realistic model shows partial covalent character in \([\mathrm{MnO_4}]^-\).

Step-by-step solution

Determine charge on [Fe(bpy)3]

The complex is \([\mathrm{Fe}^{\mathrm{II}}(\mathrm{bpy})_{3}]^n\). The charge on the metal, \(\mathrm{Fe}^{\mathrm{II}}\), is +2. Each \(\mathrm{bpy}\) (2,2'-bipyridine) is a neutral ligand with 0 charge. Thus, the overall charge on the complex is \(+2\).

Determine charge on [Cr(ox)3]

The complex is \([\mathrm{Cr}^{\mathrm{III}}(\mathrm{ox})_{3}]^{n}\). The charge on \(\mathrm{Cr}^{\mathrm{III}}\) is +3. Oxalate \((\mathrm{ox}^{2-})\) has a charge of \(-2\). With three \(\mathrm{ox}\) ligands, the total charge is \(3 \times (-2) = -6\). The overall charge is \(+3 - 6 = -3\).

Determine charge on [CrF6]

The complex is \([\mathrm{Cr}^{\mathrm{III}}\mathrm{F}_{6}]^{n}\). \(\mathrm{Cr}^{\mathrm{III}}\) has a charge of +3. Each fluoride ion \((\mathrm{F}^-)\) has a charge of \(-1\). With six fluoride ions, the total negative charge is \(-6\). The overall charge is \(+3 - 6 = -3\).

Determine charge on [Ni(en)3]

The complex is \([\mathrm{Ni}^{\mathrm{II}}(\mathrm{en})_{3}]^n\). \(\mathrm{Ni}^{\mathrm{II}}\) has a charge of +2. Ethylenediamine \((\mathrm{en})\) is a neutral ligand. Thus, the complex has no additional charge from the ligands, resulting in a total charge of \(+2\).

Determine charge on [Mn(ox)2(OH2)2]

The complex is \([\mathrm{Mn}^{\mathrm{II}}(\mathrm{ox})_{2}(\mathrm{OH}_{2})_{2}]^n\). \(\mathrm{Mn}^{\mathrm{II}}\) has a charge of +2. Each \(\mathrm{ox}^{2-}\) contributes \(-2\), so \(2 \times (-2) = -4\). Water is neutral, so it does not affect the charge. Overall charge is \(+2 - 4 = -2\).

Determine charge on [Zn(py)4]

The complex is \([\mathrm{Zn}^{\mathrm{II}}(\mathrm{py})_{4}]^n\). Zinc \(\mathrm{Zn}^{\mathrm{II}}\) has a charge of +2. Pyridine \((\mathrm{py})\) is a neutral ligand. Thus, the complex has an overall charge of \(+2\).

Determine charge on [CoCl2(en)2]

The complex is \([\mathrm{Co}^{\mathrm{III}}\mathrm{Cl}_{2}(\mathrm{en})_{2}]^n\). \(\mathrm{Co}^{\mathrm{III}}\) has a +3 charge. Chloride \((\mathrm{Cl}^-)\) has \(-1\) per ion, resulting in \(-2\) for two. \(\mathrm{en}\) is neutral, leaving a charge of \(+3 - 2 = +1\).

Charge distribution in [MnO4]^- assuming 100% ionic

When assuming 100% ionic character in \([\mathrm{MnO}_{4}]^-\), \(\mathrm{Mn}\) would be \(+7\) because it balances with four oxygen atoms, each \(-2\). The charge of the entire molecule is -1, hence it doesn't align well with 100% ionic nature.

Charge distribution by Pauling's electroneutrality principle

To make the \(\mathrm{Mn}\) in \([\mathrm{MnO}_{4}]^-\) have a charge of +1, oxygen must be negatively charged enough to balance. Since the overall charge is \(-1\), if \(\mathrm{Mn}\) is +1, each oxygen should be \(-2 - 1 = -1.75\). This suggests a covalent character as it deviates from -2.

Key concepts

Coordination chemistry

Coordination chemistry is a fascinating and expansive field of chemistry that explores the behavior and properties of coordination compounds. These compounds consist of a central metal atom or ion surrounded by molecules or anions known as ligands. Understanding the nature of these complexes is essential for determining their chemical properties and behavior. Coordination chemistry is fundamental in fields like biochemistry, medicine, and materials science.

• In coordination complexes, the central metal usually acts as a Lewis acid by accepting electron pairs from the ligands, which are considered Lewis bases.
• The number of ligands attached to the central metal ion is called its coordination number.
• Ligands can be neutral molecules like water or anions like chloride (Cl^-), each contributing differently to the stability and properties of the complex.

Coordination compounds can exhibit color, magnetic properties, and catalysis, making them interesting subjects for study and application. Understanding the interactions within these complexes helps in the design of new molecules for specific purposes, such as drug development.

Ligand charge calculation

In coordination complexes, determining the overall charge is a step-by-step process that involves calculating the contributions from the central metal ion and the surrounding ligands. This charge calculation is crucial for predicting the behavior of the complex in chemical reactions and applications.

• The first step involves identifying the charge on the central metal ion. For instance, Fe(II) has a charge of +2, while Cr(III) is +3.
• Next, consider the ligands. Neutral ligands, like water or ethylenediamine, do not contribute any charge. Anionic ligands, such as chloride (Cl^-), subtract from the overall charge.
• For each ligand, multiply its charge by its stoichiometry in the complex and sum these values with the metal's charge for the total charge.

Understanding ligand charge calculation helps chemists manipulate the charge of complexes to favor certain reactions or stability conditions, which is essential in catalysis and synthesis.

Pauling's electroneutrality principle

Pauling's electroneutrality principle describes how charge is distributed amongst atoms in a molecule to minimize energy and maintain a neutral character overall. In coordination chemistry, this principle is vital for explaining the observed bond character in complexes.

• This principle suggests that the charges on individual atoms in a molecule should be as small as possible, deviating minimally from neutrality.
• Electroneutrality indicates that substantial charge distribution can result in molecules deviating from purely ionic behavior, hinting at covalent character.
• In the case of \([\mathrm{MnO}_{4}]^{-}\), for instance, applying Pauling's principle helps assign a charge of +1 to manganese, a result that balances charge distribution among the oxygen atoms to create a more realistic molecular picture.

This principle can provide insights into bond lengths, strengths, and magnetic properties, revealing the true nature of bonds in coordination compounds.

Covalent character in ionic bonds

Even in seemingly ionic compounds, some degree of covalent character can exist. This covalent character arises from the sharing of electrons between the central metal ion and the ligands, contrary to a fully ionic view where electron transfer is complete.

• Covalent character can manifest through various effects like polarizability, where electron clouds become distorted, affecting charge distribution.
• Elements with smaller radii and higher charges, such as transition metals in coordination complexes, often feature significant covalent character.
• In \([\mathrm{MnO}_{4}]^{-}\), the incomplete transfer of electrons suggested by a calculated charge different from the fully ionic nature implies covalent bonding influences.

Examining the covalent character in coordination compounds helps chemists reveal subtle electronic effects, influencing the compounds' reactivity, color, and magnetic properties. Understanding these nuances leads to better predictions and control of their chemical behavior.