Question

What is the coordination number of the metal in the following complexes? (a) \(\left[\mathrm{Co}(e n)_{2}(\mathrm{SCN}) \mathrm{Cl}^{+}\right]\) (b) \(\left[\mathrm{Ni}(\mathrm{CN})_{4}^{2-}\right]\) (c) \(\left[\mathrm{Zn}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{2}^{2-}\right]\) (d) \(\left[\mathrm{Ag}\left(\mathrm{NH}_{3}\right) \mathrm{Cl}\right]\)

Short answer

Question: Determine the coordination number of the metal in each of the following complexes: (a) [Co(en)₂(SCN)Cl⁺], (b) [Ni(CN)₄²⁻], (c) [Zn(C₂O₄)₂²⁻], and (d) [Ag(NH₃)Cl]. Answer: The coordination numbers for the metal atoms in each complex are (a) Co: 6, (b) Ni: 4, (c) Zn: 4, and (d) Ag: 2.

Step-by-step solution

Identify the metal in each complex and the corresponding ligands

The central metal atoms and ligands in each complex are: (a) Metal: Co, Ligands: (en)\(_2\), SCN, Cl (b) Metal: Ni, Ligands: (CN)\(_4\) (c) Metal: Zn, Ligands: (C2O4)\(_2\) (d) Metal: Ag, Ligands: NH3, Cl

Determine the coordination number in each complex

Now we will count the number of ligand atoms bonded to the metal atom in each complex. (a) Co is bonded to 2 en ligands, 1 SCN ligand, and 1 Cl ligand. Each en (ethylenediamine) ligand has 2 nitrogen atoms that can bond to the Co, so the coordination number of Co is 2 * 2 (for the 2 en ligands) + 1 (for the SCN ligand) + 1 (for the Cl ligand) = 6. (b) Ni is bonded to 4 CN ligands. Each CN ligand has 1 nitrogen atom that can bond to the Ni, so the coordination number of Ni is 4 * 1 = 4. (c) Zn is bonded to 2 C2O4 ligands. Each C2O4 (oxalate) ligand has 2 oxygen atoms that can bond to the Zn, so the coordination number of Zn is 2 * 2 (for the 2 C2O4 ligands) = 4. (d) Ag is bonded to 1 NH3 ligand and 1 Cl ligand. Both NH3 and Cl each have 1 atom that can bond to the Ag (nitrogen in NH3 and chlorine in Cl), so the coordination number of Ag is 1 (for the NH3 ligand) + 1 (for the Cl ligand) = 2.

State the coordination number for each metal in the given complexes

The coordination numbers for the metal atoms in each complex are: (a) Co: 6 (b) Ni: 4 (c) Zn: 4 (d) Ag: 2

Key concepts

Chemical Complexes

Chemical complexes, also known as coordination compounds, consist of a central metal atom or ion surrounded by molecules or anions known as ligands. These ligands donate electron pairs to the metal, forming coordinate covalent bonds. The arrangement of these ligands around the central atom is defined by the coordination number, which is essentially the count of attachments points the ligand forms with the central metal. The nature and geometry of chemical complexes are critically important in fields like biochemistry, catalysis, and materials science.

Let's take a look at some examples from the exercise to illustrate this concept. A complex like \[\mathrm{Ni}(\mathrm{CN})_{4}^{2-}\], would be described as tetracyanonickelate(II) and has a coordination number of 4. This means nickel is surrounded and bound by four cyanide ligands. Coordination complexes are not just necessary for understanding chemical bonding, but they're also crucial in applications such as the coloring of materials and medicinal chemistry, like the use of cisplatin as a chemotherapy drug.

Ligands Bonding

Ligands are ion or molecules that bond to the central metal atom of a complex through coordinate covalent bonds. This type of bond involves a ligand offering both electrons for the bond with the metal, which accepts the electrons. Ligands can be classified by their 'denticity', or the number of bonding sites they possess. Monodentate ligands, such as Cl\(^-\), bond through a single site; bidentate ligands, like ethylenediamine, bond through two sites.

For instance, in the exercise given, ethylenediamine (en) is a bidentate ligand that forms two coordinate bonds with cobalt in the complex \[\mathrm{Co}(en)_{2}(\mathrm{SCN})\mathrm{Cl}^{+}\]. This multi-point attachment increases the stability of the complex, which is a phenomenon referred to as the chelate effect. Understanding how ligands bond to metals is crucial when predicting the properties and reactivity of chemical complexes in practical applications like catalysis or drug design.

Transition Metals

Transition metals are elements found in the central block of the periodic table (groups 3 to 12) and are characterized by their ability to form variable oxidation states and complex ions with ligands. They have d orbitals that can accommodate electrons from ligands forming chemical complexes. Noteworthy for their versatile chemical behavior, these metals can exhibit various colors when forming complexes due to the d-d electron transitions influenced by the ligand field.

In our exercise examples, cobalt is a transition metal that forms a complex with coordination number 6. Transition metals like cobalt, nickel, zinc, and silver often form the central ion or atoms in coordination compounds, and their complexes play vital roles in biological systems, industrial catalysts, and more. For instance, the hemoglobin in our blood is a complex of iron, another transition metal, which is essential for oxygen transport.