Aluminium chloride (AlCl3) is a covalent compoundWhen ammonia (NH3) is mixed with aluminium chloride, a compound (H3N-AlCl3) is formed.a) Show the Co-ordinate bonding in the final product,b)What do you expect the orientation of the electron pairs in Al?
I am going to use hybridization concepts which are not the correct description (!)AlCl3 is in fact a dimer (Cl2Al(μ-Cl2)AlCl2) with two Al-Cl-Al bridges, but let's say in forms the monomer AlCl3 to some extent (BCl3 is a monomer)Al is Group 13 and has three valence e? s with which it forms the three Al:Cl σ bonds ; there no lone pairs and it is an AX3 system: trigonal planar; the 3pz AO on Al is empty (the LUMO)H3N: is trigonal pyramidal with a lone pair occupying the fourth site of sp^3 hybridization (the HOMO)On combination NH3 donates the lone pair to the 3pz AO on Al and a coordinate covalent (dative) bond is formed: H3N:→AlCl3Both centers are now AX4 systems and hence tetrahedralThis is an excellent example of Fukui's frontier orbital concept [G] (a reaction will have a low activation energy if there is a HOMO-LUMO match in terms of energy and symmetry for the reactants).
