Coordination Compounds of Aluminum

The simplest and best known coordination compound of aluminum is probably (CH3)3NA1(CH3)3. The gas-phase dissociation energy according to 

The dative N Al bond in (CH3)3NA1(CH3)3, 210 pm, is 20 pm longer than the Al-N bond in crystalline AIN and 28 pm longer than the normal covalent N-Al bond distance predicted by the modified Schomaker-Stevenson (MSS) rule see Sect. 6. The dative P— A1 bond in (CH3)3PA1(CH3)3 is about 23 pm longer than the covalent bond distance predicted by the MSS rule. 

Comparison of the structures of the two complexes with that of the free acceptor shows that the Al-C bonds are elongated and pushed away from the electrOTi donor as the complex is formed. These changes may be rationalized within the framework of the VSEPR model as the result of repulsion between incoming dative N— A1 

The A1 atom in the 2 1 complex is surrounded by five electron pairs in the valence shell, and the trigonal bipyramidal structure is in accordance with the 

VSEPR model. The axial positions of the donor molecules are also in accordance with the VSEPR model, particularly if it is assumed that an accepted bond pair requires less space at the acceptor atom than a covalent bond pair. Addition of a sectmd donor molecule increases the covalent Al-Cl distance by 6 pm and the dative N— A1 bond distance by 21 pm. 

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G. H. Robinson, Editor, Coordination Compounds of Aluminum, VCH New York, 1993, Chapter 2. 

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