The 18 and 16 Electron Rule: Videos & Practice Problems

The 18 and 16 electron rules are essential concepts in understanding the stability and reactivity of transition metals compared to main group elements. Main group elements, which include groups 1A to 8A, typically adhere to the octet rule, aiming for a stable electron configuration similar to that of noble gases, which is characterized by having eight electrons in their outer shell. This is particularly true for elements like hydrogen and helium, with helium being an exception due to its complete electron configuration.

In contrast, transition metals utilize the 18 electron rule as a guideline for their stability. The 18 electron rule suggests that the most stable transition metal complexes often have a total of 18 electrons, which is the sum of the maximum electrons in the s, p, and d orbitals: 2 from the s orbital, 10 from the d orbital, and 6 from the p orbital. This can be expressed mathematically as:

\[\text{Total Electrons} = \text{s (2)} + \text{d (10)} + \text{p (6)} = 18\]

Transition metals typically have valence electrons derived from their s and d orbitals, with groups 3 to 12 being particularly relevant. However, there are exceptions to the 18 electron rule, especially among transition metals in groups 8 to 11, which may stabilize with only 16 electrons. This phenomenon is referred to as the 16 electron rule. Notably, palladium and nickel are two transition metals that frequently exhibit this behavior, participating in reactions where they are stable with 16 electrons.

In summary, while main group elements strive for an octet of 8 electrons, transition metals generally aim for 18 electrons for optimal stability, with some exceptions where 16 electrons can suffice. Understanding these rules is crucial for predicting the behavior and reactivity of transition metal complexes in various chemical reactions.

To determine the electron count of the neutral transition metal complex BR₂PdCO₂, we start by identifying the transition metal, which is palladium (Pd). The electron configuration of palladium is unique; it is represented as [Kr] 4d¹⁰. This indicates that palladium has 10 electrons in its d orbitals and no electrons in the s orbitals, as the s electrons are promoted to fill the d orbitals.

The valence electron count for palladium can be calculated using the formula:

Valence Electrons = d Electrons - Charge + Number of X Ligands + 2 × Number of L Ligands

In this case, the complex is neutral, so the charge is 0. The complex contains:

• 2 bromine (Br) ligands, which are classified as X ligands.
• 2 carbonyl (CO) ligands, which are classified as L ligands.

Substituting these values into the formula gives:

Valence Electrons = 10 - 0 + 2 + 2 × 2

Valence Electrons = 10 + 2 + 4 = 16

This indicates that palladium in this complex follows the 16-electron rule, which is a guideline used to predict the stability of transition metal complexes. The 16-electron rule suggests that a stable complex typically has a total of 16 valence electrons, which allows for a filled d subshell and contributes to the overall stability of the complex.

To determine the electron count of the neutral transition metal complex of palladium with ethylenediamine (en), we first need to understand the nature of the ligand and the metal involved. Ethylenediamine is a bidentate ligand, meaning it has two sites capable of donating a pair of electrons, represented by its structure NH₂CH₂CH₂NH₂.

The electron count can be calculated using the formula:

Electron Count = Valence Electrons of Metal + (2 × Number of L-Type Ligands)

For palladium (Pd), the number of valence electrons is 10. Since the complex is neutral, the charge is 0. Ethylenediamine is classified as an L-type ligand, which contributes to the electron count based on its electron-donating sites. In this case, each ethylenediamine ligand donates 2 electrons, and since we have 2 bidentate ligands, the total contribution from the ligands is:

2 × 2 (for each bidentate ligand) = 4

Thus, the total contribution from the ligands is:

2 × 4 = 8

Now, substituting these values into the electron count formula gives:

Electron Count = 10 (valence electrons of Pd) + 8 (from ligands) = 18

This calculation highlights the importance of recognizing the type of ligand and its electron-donating capabilities when determining the electron count for transition metal complexes. In summary, for the palladium complex with ethylenediamine, the total electron count is 18, which is essential for understanding the complex's bonding and reactivity.