A Molecular Orbital Study of the Stereochemistry of Pentacoordinated Aluminium

Abstract

An attempt has been made to characterize the stereochemical requirements of an aluminium atom coordinated by five hydroxyl groups (AlO₅H₅^2-) within the framework of CNDO/2 molecular orbital formalism. Total energy surfaces calculated for this system as the valence angles are varied within the constraints of C_2v symmetry clearly show the 'reaction coordinate' for the so-called Berry pseudo- rotation mechanism. With all Al-O and O-H bond lengths fixed at 1.84 and 1.00 Ǻ respectively, the activation energy for trigonal-bipyramidal/square-pyramidal interconversion is 7.1 kJ mol^-1. In addition, the minimum energy O(axial)-Al-O(basal) angle for the square-pyramidal group is estimated to be c. 103.5°. When one bond is lengthened relative to the other four the trigonal-bipyramidal configuration is further stabilized, but approaches tetrahedral geometry. When one bond is shortened square-pyramidal geometry is stabilized and the minimum energy O(ax)-Al-O(bs) angle increases. With two bonds lengthened (or shortened) the favoured trigonal-bipyramidal configuration is the one with long bonds directed towards the axial positions and at no stage is there a stabilization of square-pyramidal geometry. These results agree with trends in calculated Mulliken bond overlap populations and with observed ratios of axial-to-basal bond lengths in suggesting that for trigonal- bipyramidal stereochemistry the axial bonds are weaker, whereas for square-pyramidal geometry the axial bond is stronger. Moreover, consideration of the total energy for various isomers of Al(OH)₄F^2- indicates that more electronegative substituents prefer to enter the apical and basal positions in a trigonal bipyramid and a square pyramld respectively. For a cluster Al₂(OH)₈^2-, consisting of two aluminium trigonal-bipyramidal groups sharing a common edge of variable length, an energy minimum (62.2 kJ mol^-1 lower than the undistorted case) is found at a shared edge length of 2.246 A, 13 % shorter than for the undistorted polyhedron. The corresponding observed distance for a similar dimer in the mineral andalusite, Al₂SiO₅, is 2.247(7) Ǻ. Plots of minimum energy shared edge length against mean bond length for various clusters M₂(OH)₈^n-, where M = Al or Mg, clearly demonstrate an essentially linear relationship between the two parameters consistent wlth trends observed for natural systems in which M = Co, Cu, Mn and Zn.