The ethylenediamine (en) ligand has the formula H2N-CH2-CH2-NH2: a.) Analyze the structure of en using a Lewis electron-dot struture and the valence-bond approach. Identify the presence of lone pairs and/or formal charges, and then draw a three-dimensional structure. b.) Which bonds along the N-C-C-N framework are capable of free (low barrier) internal rotation? Why? c.) Predict which part(s) of the en molecule will form bonds with a transition metal ion, such as Fe2+. (Think of ligand-metal interaction as a Lewis acid/Lewis Base interaction.)
The Correct Answer and Explanation is:
Correct Answer
a.) Structure Analysis: The Lewis structure of ethylenediamine (en) shows each nitrogen atom with one lone pair of electrons and no formal charges on any atom. Using the valence-bond approach, both carbon and both nitrogen atoms are sp³ hybridized. This results in a tetrahedral electron geometry around all four central atoms. The molecular geometry around the carbons is tetrahedral, while the geometry around the nitrogens is trigonal pyramidal.
b.) Bond Rotation: All single bonds along the N-C-C-N framework—the N-C bond, the C-C bond, and the second C-N bond—are capable of free internal rotation. This is because they are all sigma (σ) bonds.
c.) Metal Ion Bonding: The two nitrogen atoms will form bonds with a transition metal ion like Fe²⁺. The interaction is a Lewis acid-base reaction where the metal ion (Fe²⁺) acts as the Lewis acid (electron-pair acceptor) and the nitrogen atoms of the ethylenediamine ligand act as Lewis bases (electron-pair donors), donating their lone pairs to form coordinate covalent bonds.
Explanation
Ethylenediamine (H₂N-CH₂-CH₂-NH₂) is a common bidentate ligand in coordination chemistry. Its structure and bonding properties are key to its function.
a.) Structure and Hybridization:
A detailed analysis begins with its Lewis structure. With 26 total valence electrons (2 N x 5 + 2 C x 4 + 8 H x 1), the structure is drawn with single bonds connecting the N-C-C-N backbone and the attached hydrogen atoms. After forming these 11 single bonds (using 22 electrons), the remaining four electrons are placed as one lone pair on each nitrogen atom. This satisfies the octet rule for all carbon and nitrogen atoms. A calculation of formal charges confirms that all atoms in the neutral molecule have a formal charge of zero.
Using the valence-bond model, the hybridization of each central atom is determined by its number of electron domains (bonds + lone pairs). Each carbon atom forms four single bonds, indicating four electron domains and thus sp³ hybridization with tetrahedral geometry (109.5° bond angles). Each nitrogen atom forms three single bonds and possesses one lone pair, also corresponding to four electron domains. This results in sp³ hybridization and a tetrahedral electron geometry. However, the molecular geometry around each nitrogen is trigona_l_ pyramidal due to the lone pair’s influence. The 3D structure is therefore a flexible chain with tetrahedral carbons and trigonal pyramidal nitrogen ends.
b.) Internal Bond Rotation:
The N-C-C-N framework is constructed entirely of single bonds. In valence-bond theory, single bonds are identified as sigma (σ) bonds, formed by the direct, head-on overlap of hybrid orbitals (e.g., sp³-sp³ or sp³-s). This orbital overlap is cylindrically symmetrical along the bond axis. Consequently, rotation of the atoms around this axis can occur with a very low energy barrier, as it does not disrupt the orbital overlap. Therefore, the N-C, C-C, and C-N bonds all exhibit free internal rotation, allowing the ethylenediamine molecule to adopt various conformations.
c.) Ligand-Metal Interaction:
The formation of a coordination complex between a ligand and a metal ion is a classic example of a Lewis acid-base interaction. A Lewis acid is an electron-pair acceptor, while a Lewis base is an electron-pair donor. Transition metal cations, such as Fe²⁺, have vacant or partially filled d-orbitals and are electron-deficient, making them potent Lewis acids. The ethylenediamine molecule, as established in the Lewis structure, has two distinct sites of high electron density: the lone pairs on the two nitrogen atoms. These lone pairs are available for donation.
Therefore, when ethylenediamine acts as a ligand, it uses the lone pairs on its two nitrogen atoms to form two separate coordinate covalent bonds with a single metal ion. Because it binds through two donor atoms, it is classified as a bidentate ligand.
