Weakly Electronegative Ligands

If the electronegativity of the ligands X is much less than the electronegativity of the central atom A, the electrons in the valence shell of A are not well localized into pairs and therefore have a small or zero effect on the geometry. In such molecules the bonds are very ionic in the sense A X+, and the central atom A is essentially an anion with a spherical electron density distribution. In this case the VSEPR model is not valid, and the geometry of the molecule is determined by ligand-ligand repulsions. 

For example, the LiOH and Li20 molecules are linear, not angular like H20, because the Li ligand is not sufficiently electronegative to localize the eight electrons in the valence 

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In the earliest authentic halocarbon complex (1982), o-diiodobenzene was found to chelate to cationic Ir(III) as shown in diagram (5). An earlier proposed example proved to be misidentified when the crystal structure was carried out. To be stable, any such complex must resist oxidative addition, hence the use of an oxidation state, Ir(III), that is only oxidized with difficulty. The normally rather weakly basic halocarbon lone pairs are often reluctant to bind, but chelation and involvement of the least electronegative halide, iodine, favor binding as does the cationic character of the complex. A series of such complexes was soon found, including complexes of Ir(I) and a series of weakly bound dichloromethane complexes for certain systems. These solvento complexes can be very labile and so find use as precursors for binding of other weakly basic ligands. Even fluorocarbon complexes proved viable.A review of the area is available. It now seems... 

The stability of the coordinative bond in the borane complexes is determined by the electrophilicity of the borane, by the nucleophilicity of the ligand and by steric factors. The electrophilicity of the borane, in turn, depends on the electronegativity of the groups bonded to the boron and on the overlap between the unoccupied p orbital of the boron and the occupied p orbitals of the atoms directly bonded to it (p -p interaction). The predominance of the latter is reflected by the fact that the relative acidities of halogenoboranes increase in a sense opposite to that expected from electronegativity considerations and is supported by the very weak Lewis acidity of the alkoxy- and amino-boranes. The importance of steric factors is pointed out by Brown and co-workers.13-15... 

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