Question

Indicate the likely coordination number of the metal in each of the following complexes: (a) \(\left[\mathrm{Ru}(\text { bipy })_{3}\right]\left(\mathrm{NO}_{3}\right)_{2}\) (b) \(\operatorname{Re}(\text { o-phen })_{2}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{2}\) (c) \(\mathrm{Pd}(\mathrm{PPh} 3)_{3} \mathrm{Cl}\) (d) \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{Mn}(\mathrm{EDTA})\)

Short answer

The likely coordination numbers of the metal ions in the given complexes are: (a) Ru: 3, (b) Re: 4, (c) Pd: 4, (d) Mn: 6.

Step-by-step solution

a) Coordination number of Ru in \([\mathrm{Ru}(\text { bipy })_{3}]\left(\mathrm{NO}_{3}\right)_{2}\)

First, identify the metal ion: This complex contains the metal ion Ru. The surrounding ligands are bipy (2,2'-bipyridine) and there are three of them as indicated by the subscript 3. Since the complex has three bipy ligands, the coordination number of Ru is 3.

b) Coordination number of Re in \(\operatorname{Re}(\text { o-phen })_{2}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)_{2}\)

First, identify the metal ion: This complex contains the metal ion Re. The surrounding ligands are o-phen (ortho-phenanthroline) and oxalate ion \((\mathrm{C}_{2}\mathrm{O}_{4})\). There are two o-phen ligands and two oxalate ligands. Since there are total of four ligands, the coordination number of Re is 4.

c) Coordination number of Pd in \(\mathrm{Pd}(\mathrm{PPh} 3)_{3} \mathrm{Cl}\)

First, identify the metal ion: This complex contains the metal ion Pd. The surrounding ligands are PPh3 (triphenylphosphine) and a Cl ion. There are three PPh3 ligands and one Cl ligand. Since there are total of four ligands, the coordination number of Pd is 4.

d) Coordination number of Mn in \((\mathrm{NH}_{4})_{2} \mathrm{Mn}(\mathrm{EDTA})\)

First, identify the metal ion: This complex contains the metal ion Mn. The surrounding ligand is EDTA (ethylenediaminetetraacetic acid). There is one EDTA ligand, but it can bond to the metal ion in six distinct places. Since there is one ligand (EDTA) with six bonding sites, the coordination number of Mn is 6. In conclusion, the likely coordination numbers of the metal ions in the complexes are: (a) Ru: 3 (b) Re: 4 (c) Pd: 4 (d) Mn: 6

Key concepts

Coordination Number

Coordination number is a fundamental concept in coordination chemistry, referring to the number of ligand atoms bonded to the central metal ion in a complex. These ligands are essentially molecules or ions that donate their electrons to the metal. The coordination number plays an essential role in determining the geometry and stability of the complex.
The coordination number depends on a few factors:

• Size of the central metal ion: Larger ions can accommodate more ligands.
• Size and charge of the ligands: Bulky ligands or those with high charge might limit the number of ligands that can surround the metal.
• Metal-ligand bonding interactions: Stronger interactions can support higher coordination numbers.

Understanding the coordination number helps predict the formation and structure of the metal complex. For instance, in the complex \([ ext{Ru}( ext{bipy})_{3}]\), with each bipy offering two points of attachment, the coordination number is 6, not 3, because each bipy is a bidentate ligand. Thus, an initial misinterpretation might occur if the multidendate nature of the ligands is not properly considered.

Transition Metals

Transition metals are the d-block elements in the periodic table known for their remarkable ability to form complex ions. These elements have partially filled d-orbitals, allowing them to form multiple oxidation states and serve as excellent central metal ions in coordination complexes. The distinct properties of transition metals are due to:

• Variable oxidation states: Contributing to diverse chemical reactions and bonding scenarios.
• Ability to form colored complexes: Due to electronic transitions between d-orbitals within the metal ion.
• Magnetic properties: Resulting from unpaired electrons in these d-orbitals.

For instance, the transition metal Rhenium (Re) in \( ext{Re}( ext{o-phen})_{2}( ext{C}_2 ext{O}_4)_2\) can use its d-electrons to bond with a variety of ligands, demonstrating its typical behavior as a transition metal. As such, the complex's stability and properties are influenced significantly by the metal's electron configuration.

Ligands

Ligands are atoms, ions, or molecules that donate a pair of electrons to a central metal atom or ion to form a coordination complex. They can be classified based on the number of donor atoms they have:

• Monodentate ligands: Bind to the metal ion through a single atom. An example is Cl in \( ext{Pd}( ext{PPh}_3)_3 ext{Cl}\).
• Bidentate ligands: Attach to the metal at two points, like o-phenanthroline (o-phen) in \( ext{Re}( ext{o-phen})_{2}( ext{C}_2 ext{O}_4)_{2}\).
• Polydentate ligands: Can form several bonds to the metal. EDTA, a hexadentate ligand, does this superbly, wrapping around metal ions like Mn in \(\text{Mn}( ext{EDTA})\).

The nature of these ligands—whether they are neutral molecules or anions—also critically influences the stability, solubility, and reactivity of the complexes they form. Properly understanding ligand types is essential for predicting the number of connections to a metal ion and therefore the coordination number.

Complex Ions

Complex ions consist of a central metal ion bonded to one or more ligands, creating a charged entity. They play a crucial role in coordination chemistry and are represented in chemical formulas enclosed by square brackets, indicating the combinations of metal ions and surrounding ligands.

• Form: Comprised of a central transition metal and surrounding ligands, which can be indicated with brackets, such as \([ ext{Ru}( ext{bipy})_3]^{2+}\).
• Charge: Determined by the metal's oxidation state and the ligand charges; this influences their chemical behavior and interaction.Modern-day uses include their application in catalysis, medicine, and photography due to their varied chemical properties.

Complex ion formation stabilizes the metal's oxidation states and increases the solubility of some metal ions in aqueous solutions. Understanding the structure of complex ions is integral to the study of reaction mechanisms and properties in chemistry, particularly how these complexes interact with other substances in varied environments.