Ligand around /-metal ions, steric

For the aqua metal ions having pKa around 7, this mechanism is virtually unperturbed by the nature of the metal ion and steric and electronic factors of its ligand envelope as evident from a linear isokinetic plot (A H = AG(f + (10AS, (10 is the isokinetic temperature)... 

Steric bulk. The ligands are packed around the metal ion in such a way as to minimize interligand repulsion, and the coordination number is determined... 

Interestingly, the reactivity pattern in rare-earth metal-catalyzed hydroamination/cyclization reactions of aminoalkynes with respect to ionic radius size and steric demand of the ancillary ligand follows the opposite trend to that observed for aminoalkenes, namely decreasing rates of cyclization with increasing ionic radius of the rare-earth metal and more open coordination sphere around the metal. This phenomenon can be explained by a negligible sterical sensitivity of a sterically less encumbered triple bond, as sterically less open complexes and smaller metal ions provide more efficient reagent approach distances and charge buildup patterns in the transition state [110]. 

This behaviour is depicted in Fig. 1. As mentioned previously, it applies only, if the multidentate ligand is so flexible, that its conformation presents no steric hindrance for any metal ion in the alkali cation series. This means that the ligand binding groups can position themselves as freely around the metal ion as can the solvent molecules. 

Piets first attempt to prepare organolanthanide compounds marked the dawn of a new area of chemistry (2). Since then, a wide variety of complexes of f-block elements have been synthesized and their structures have been satisfactorily explained in terms of "steric congestion" of ligands around f-metal ions (3,4,5,6). However, the question of whether, the 4f valence orbitals participate in bonding has not as yet been answered. 

The number and the geometric distribution of solvent molecules around a metal ion in solution are often of major importance for the chemical behavior of the cation. For transition elements, the geometry of the coordination sphere is usually fixed by orbitals of the metal which form bonds with the solvent molecules and/or the ligands in complexation reactions. By contrast, the strongly ionic nature of the bonding of the f-block elements minimizes the importance of the metal orbitals and the geometry of the coordination sphere reflects an optimal balance of minimization of steric hindrance and maximum strength of electrostatic forces. 

The coordination number of a compound is defined as the number of attachment sites of the various ligands to the metal center. The valence-shell electron-pair repulsion (VSEPR) model does not work well for transition compounds having partially filled d-subshells. The Kepert model is sometimes used instead. As with the VSEPR model, the metal ion is assumed to be spherical with the ligands lying along the surface of the sphere. The ligands will repel one another for either electronic or steric reasons and will tend to distribute themselves around the sphere so as to avoid each other. In the Kepert model, the lone pair electrons (which are the low-lying d-electrons in the... 

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