Question

(a) What is the meaning of the term coordination number as it applies to metal complexes? (b) Give an example of a ligand that is neutral and one that is negatively charged. (c) Would you expect ligands that are positively charged to be common? Explain. (d) What type of chemical bonding is characteristic of coordination compounds? Illustrate with the compound \(\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6} \mathrm{Cl}_{3}\) (e) What are the most common coordination numbers for metal complexes?

Short answer

Coordination number refers to the number of ligand atoms directly bonded to the central metal atom/ion in a coordination compound through coordinate covalent bonds. An example of a neutral ligand is ammonia (\(NH_{3}\)), and a negatively charged ligand is chloride ion (\(Cl^-\)). Positively charged ligands are less common due to unfavorable electrostatic attraction with positively charged metal ions. Coordination compounds' chemical bonding is characterized by coordinate covalent bonding. The most common coordination numbers are 4 (square planar or tetrahedral) and 6 (octahedral).

Step-by-step solution

(a) Meaning of Coordination Number

Coordination number is the number of ligand atoms directly bonded to the central metal atom/ion in a coordination compound. It indicates the number of electron pairs that the central metal atom/ion accepts from the ligands, leading to the formation of coordinate covalent bonds.

(b) Neutral and Negatively Charged Ligands

A neutral ligand is a ligand that does not carry any charge and forms a coordinate covalent bond with the central metal atom/ion. An example of a neutral ligand is ammonia (\(NH_{3}\)). A negatively charged ligand is a species that carries a negative charge and forms a coordinate covalent bond with the central metal atom/ion. An example of a negatively charged ligand is chloride ion (\(Cl^-\)).

(c) Positively Charged Ligands

Positively charged ligands are not as common as neutral or negatively charged ligands. This is because metal ions (central atom in coordination complexes) generally tend to be positively charged, and the electrostatic attraction between the central metal and the positively charged ligand would be unfavorable, making it difficult for them to form coordinate covalent bonds.

(d) Chemical Bonding in Coordination Compounds

The chemical bonding in coordination compounds is characterized by coordinate covalent bonding, which involves the donation of an electron pair from the ligand to the central metal atom/ion. In the given compound, \(\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6} \mathrm{Cl}_{3}\), the cobalt ion (\(Co^{3+}\)) acts as the central metal ion which forms coordinate covalent bonds with six ammonia ligands (\(NH_{3}\)).

(e) Common Coordination Numbers

The most common coordination numbers for metal complexes are 4, which represents a square planar or tetrahedral geometry, and 6, which represents an octahedral geometry. Other coordination numbers such as 2, 5, and 7 can also be observed, but they are relatively less common.

Key concepts

Coordination Number

Coordination number is a fundamental concept in the study of coordination compounds. It refers to the total number of ligand "connections" that a central metal atom or ion makes with its surrounding ligands.
This number is crucial because it defines how many electron pairs the metal receives from the ligands, resulting in the formation of coordinate covalent bonds. Therefore, the coordination number can help predict the structure and properties of the compound.
Some key points about coordination numbers include:

• The coordination number is primarily determined by the size, charge, and electron configuration of the metal ion as well as the size and electron donation ability of the ligands.
• Common coordination numbers include 4 and 6, although various other numbers can occur depending on the specific complex.

Understanding the coordination number gives insight into the geometry and reactivity of the metal complex.

Neutral Ligands

In coordination chemistry, ligands are atoms, ions, or molecules that donate electron pairs to a central metal ion, forming coordinate covalent bonds. Neutral ligands are those that do not possess any electrical charge.
They can still interact strongly with metal atoms due to their available lone pair of electrons.
Consider the following aspects of neutral ligands:

• One common example of a neutral ligand is ammonia (\(NH_{3}\)), where the nitrogen atom donates its lone pair to the metal ion.
• Neutral ligands are crucial in forming stable complexes because they minimize charge repulsion, which benefits the stability and solubility of the coordination compound.
• By not carrying a charge, neutral ligands help to stabilize the overall charge of the complex, avoiding drastic changes in compound polarity.

Their ability to provide electron pairs without additional charge makes neutral ligands versatile and widely used in synthesizing coordination compounds.

Coordinate Covalent Bonding

Coordinate covalent bonding, also known as dative covalent bonding, is a unique type of chemical bond found in coordination compounds. It occurs when a single species, the ligand, donates a pair of electrons to the central metal atom or ion, which does not contribute any electrons to the bond formation.
This bond formation is significant for the following reasons:

• It allows for the formation of stable complexes, such as \(\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6} \mathrm{Cl}_{3}\), where six ammonia molecules donate lone pairs to the cobalt ion.
• Coordinate covalent bonds give coordination compounds unique properties, including varied geometries and distinctive reactivity patterns compared to other compounds.

The stability and nature of these bonds are influenced by the donor ability of the ligands and the acceptor capacity of the central metal, showcasing the specific interaction within these specialized compounds.

Common Coordination Numbers

Metal complexes can have various structural formations, often guided by their coordination numbers, which denote the number of ligand attachments to the central metal atom.
Understanding typical coordination numbers provides valuable insights into the possible shapes and stabilities of coordination compounds.
Commonly observed coordination numbers include:

• Coordination number 4: This usually results in square planar or tetrahedral geometries, as often seen in complexes like \(\text{[PtCl}_4]^{2-}\) (square planar) or \(\text{[Ni(CO}_4]\) (tetrahedral).
• Coordination number 6: Octahedral geometry is typical for this coordination number, as exhibited by complexes such as \(\text{[Co(NH}_3\text{)}_6]\text{Cl}_3\).

Other coordination numbers like 2, 5, and 7 can also occur, but are less frequent. These numbers play a crucial role in determining the spatial arrangement and electronic properties of the resulting metal complexes.