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Difficulty: Very hardCovalent and Dative (Coordinate) Bonding

Complete the following statement describing the bonding and structural change during the dimerization of aluminium chloride.

Answer:In the formation of the dimer Al2Cl6Al_2Cl_6 from monomeric AlCl3AlCl_3, a chlorine atom from one molecule donates a lone pair of electrons to the electron-deficient aluminium atom of another, resulting in a total of 【2】 coordinate bonds in the dimer. This coordination causes the electron pair geometry around each aluminium atom to become 【tetrahedral】.

Answer

Blank 1: 2 (or two); Blank 2: tetrahedral
In the dimerization of AlCl3AlCl_3 into Al2Cl6Al_2Cl_6, two bridging chlorine atoms act as Lewis bases by donating lone pairs to the electron-deficient aluminium atoms, creating 2 coordinate bonds. Because each aluminium atom now shares 4 electron pairs, the hybridization changes from sp2sp^2 to sp3sp^3, resulting in a tetrahedral geometry around each aluminium atom.

Step-by-Step Solution

1
Analyze the electron configuration and geometry of monomeric AlCl3AlCl_3.
Aluminium has 3 valence electrons and forms 3 single covalent bonds with three chlorine atoms, leaving aluminium with 6 valence electrons (an incomplete octet) and a trigonal planar geometry with sp2sp^2 hybridization.
Identifying the electron deficiency in monomeric AlCl3AlCl_3 explains why it undergoes dimerization.
2
Determine the number of coordinate (dative) bonds formed in the Al2Cl6Al_2Cl_6 dimer.
To complete its octet, each aluminium atom accepts one lone pair of electrons from a chlorine atom belonging to the neighboring AlCl3AlCl_3 molecule. Since there are two aluminium atoms being bridged, exactly 2 coordinate covalent bonds are formed in Al2Cl6Al_2Cl_6.
Two bridging chlorine atoms each contribute a lone pair to form two separate dative bonds.
3
Determine the resulting molecular geometry around each aluminium atom in Al2Cl6Al_2Cl_6.
Each aluminium atom becomes surrounded by 4 electron pair bonds (3 single covalent bonds + 1 coordinate bond), shifting its hybridization from sp2sp^2 to sp3sp^3 and yielding a tetrahedral geometry.
According to VSEPR theory, four bonding pairs arrange themselves to minimize repulsion in a 3D tetrahedral orientation.

Key Concept

Dative Covalent Bonding and Geometric Transition in Dimeric Aluminium Chloride (Al2Cl6Al_2Cl_6)
Estimated Time:1m 30s
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Topics

Covalent and Dative (Coordinate) Bonding

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