Question

Indicate the likely coordination number of the metal in each of the following complexes: (a) \(\left[\mathrm{Rh}(\text { bipy })_{3}\right]\left(\mathrm{NO}_{3}\right)_{3}\) (b) \(\mathrm{Na}_{3}\left[\mathrm{Co}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{2} \mathrm{Cl}_{2}\right]\) (c) \(\left[\mathrm{Cr}(o \text { -phen })_{3}\right]\left(\mathrm{CH}_{3} \mathrm{COO}\right)_{3}\) (d) \(\mathrm{Na}_{2}[\mathrm{Co}(\mathrm{EDTA}) \mathrm{Br}]\)

Short answer

The coordination numbers for the metals in the given complexes are: (a) Rh: 6, (b) Co: 6, (c) Cr: 6, and (d) Co: 7.

Step-by-step solution

Identify the metal and ligands in each complex

For each complex, identify the central metal atom and the surrounding ligands that are bonded to the metal. (a) In \(\left[\mathrm{Rh}(\text{bipy})_{3}\right]\left(\mathrm{NO}_{3}\right)_{3}\), the metal is Rh and the ligands are bipy (bipyridine), which has a coordination number of 2 per ligand. (b) In \(\mathrm{Na}_{3}\left[\mathrm{Co}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)_{2} \mathrm{Cl}_{2}\right]\), the metal is Co and the ligands are \(\mathrm{C}_{2}\mathrm{O}_{4}\) (oxalate) with a coordination number of 2 per ligand, and \(\mathrm{Cl}\) with a coordination number of 1. (c) In \(\left[\mathrm{Cr}(o\text{-phen})_{3}\right]\left(\mathrm{CH}_{3}\mathrm{COO}\right)_{3}\), the metal is Cr and the ligands are o-phen (ortho-phenanthroline), which has a coordination number of 2 per ligand. (d) In \(\mathrm{Na}_{2}[\mathrm{Co}(\mathrm{EDTA}) \mathrm{Br}]\), the metal is Co, and the ligands are EDTA (ethylenediaminetetraacetic acid) with a coordination number of 6, and Br with a coordination number of 1.

Calculate the coordination number for each metal

Multiply the coordination number of each ligand with the number of times the ligand is bound to the metal and add them together to find the overall coordination number of the metal. (a) For \(\left[\mathrm{Rh}(\text{bipy})_{3}\right]\left(\mathrm{NO}_{3}\right)_{3}\): Rh has 3 bipy ligands, each contributing 2. \\ Coordination number = 3 × 2 = 6 (b) For \(\mathrm{Na}_{3}\left[\mathrm{Co}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)_{2} \mathrm{Cl}_{2}\right]\): Co has 2 \(\mathrm{C}_{2}\mathrm{O}_{4}\) ligands, each contributing 2, and 2 \(\mathrm{Cl}\) ligands, each contributing 1. \\ Coordination number = (2 × 2) + (2 × 1) = 6 (c) For \(\left[\mathrm{Cr}(o\text{-phen})_{3}\right]\left(\mathrm{CH}_{3}\mathrm{COO}\right)_{3}\): Cr has 3 o-phen ligands, each contributing 2. \\ Coordination number = 3 × 2 = 6 (d) For \(\mathrm{Na}_{2}[\mathrm{Co}(\mathrm{EDTA}) \mathrm{Br}]\): Co has 1 EDTA ligand, contributing 6, and 1 Br ligand, contributing 1. \\ Coordination number = 6 + 1 = 7 So, the coordination numbers for the metals in each complex are: (a) Rh: 6 (b) Co: 6 (c) Cr: 6 (d) Co: 7

Key concepts

Coordination Number

In coordination chemistry, the coordination number of a metal in a complex is the total number of ligand "attachments" around the central metal atom. A ligand is a molecule or ion that binds to a metal atom via coordinate bonds. The coordination number is crucial because it affects the entire complex's geometry. For example, a coordination number of 6 typically results in an octahedral geometry, while a coordination number of 4 might lead to either a square planar or tetrahedral geometry. Determining the coordination number in complex formations helps us predict the shape and potential reactivity of the molecule. In the given exercise, the coordination numbers were 6 for parts (a), (b), and (c), while part (d) had a coordination number of 7. Knowing the coordination number, we can understand a lot about the stability and bonding nature of metal complexes.

Ligands

Ligands are a key part in forming coordination complexes. They are ions or molecules capable of donating electron pairs to metals to form coordinate covalent bonds. Ligands play a pivotal role in defining the structure and reactivity of coordination complexes. Ligands can be classified based on how many electron pairs they donate to the metal:

• Monodentate: Ligands that donate one pair of electrons (e.g., Cl).
• Bidentate: Ligands donating two pairs of electrons from two atoms, such as oxalate (C₂O₄).
• Polydentate: Ligands like EDTA that can donate multiple pairs of electrons from different atoms, often forming more than two bonds with the metal.

Understanding how ligands interact with metals aids in predicting the metal's coordination number and the complex's overall geometry and properties. In the exercise, bipyridine (bipy) and o-phenanthroline (o-phen) acted as bidentate ligands, while EDTA was a polydentate ligand.

Complexes

Coordination complexes consist of a central metal atom or ion bonded to surrounding ligands. These formations are crucial in many areas of chemistry, including catalysis and biological processes. The formation of coordination complexes involves the donation of electron pairs from ligands to unfilled orbitals on the metal. This donation forms stable coordinate covalent bonds. The properties of these complexes, such as color, magnetism, and solubility, largely depend on their coordination geometry and the ligands involved. In the examples from the original exercise, different metals like Rh, Co, and Cr were used, each forming distinct coordination complexes. These complexes were stabilized through specific metal-ligand interactions, leading to distinct coordination numbers and geometries.

Metal-Ligand Bonding

Metal-ligand bonding is a central concept in coordination compounds, involving the formation of a coordinate covalent bond between the metal and the ligands. This type of bonding occurs when a ligand donates an electron pair to the metal. One interesting aspect of metal-ligand bonding is how it affects the metal's oxidation state and the overall complex's electronics. Strong-field ligands can cause splitting of the metal's d-orbitals, leading to changes in magnetic properties and colors. The strength, length, and angle of the bonds can affect a complex's properties, such as its reactivity or how it interacts with other substances. In coordination chemistry, understanding these interactions is vital. The examples in the exercise illustrate how different ligands and metals combine to form stable complexes, each with unique properties based on their metal-ligand interactions.