Steric saturation

In addition to the decreased polarizability of the heavier metals, their larger radii require higher metal coordination numbers to achieve steric saturation. As a result, extensive aggregation, frequently coinciding with rather limited solubility in non-donor solvents, and occasionally even in donor solvents, complicates the characterization of these species in solution and the solid state. In fact, several structural characterizations of organoalkali species have relied on recent advances in powder diffraction techniques using synchrotron radiation.1 ... 

This effect is maximal for dimethylamine. It is less important with trimethylamine because of the steric saturation and of its solvation limiting the inductive effect (Fig. 3.24). 

In some cases the rate of substitution of alkoxide ligands drops oif dramatically before total substitution has occurred. This clearly reflects both the electronic and steric saturation of the metal coordination sphere by the bidentate acac ligands. Hence for titanium it is difficult to substitute the last alkoxide, 9 while for Nb and Ta the reaction stops at the eight-coordinate tris-substitution products (equation 64).240,241... 

The most interesting structural feature is the extremely short terminal Nd-N distance of 2167(2) A, which resembles Ln-O bond lengths [18]. This cannot be explained in terms of a double bond . However, as a common feature in all aforementioned Ln-NiPr2 complexes, close Ln---C interactions play a crucial part on steric saturation (Table 3). In Nd(NiPr)2(Me)(AlMe3)2 a Ln - C contact of 2934(2) A could be observed. For comparison M(NiPr2)3 (M = Al, Cr) do not show close Ln---C contacts [85]. 

Several alkali metal (M) derived ate complexes have been prepared (Table 8). Again, the formation of such ternary systems is preferred. Even Sm(II) derivatives were readily available [89], A common feature of the observed molecular structures are close inter- and intramolecular M - C as well as close Ln-- Si van der Waals contacts which ensure steric saturation of the metal centers. Depending on the size of the alkali metal, different types of solid state structures are generated (Fig. 15). A comparison to the ammonia derived species in Sect. 2.1 can be drawn. 

Sterically demanding alkyl or trimethylsilyl substituents provide the necessary steric saturation around the large lanthanide ions and assure a high solubility of the products in nonpolar organic solvents. 

The importance of the chelate effect combined with the construction of multidentate ligands is well known in lanthanide chemistry. This is expressed in the rich coordination chemistry of / -diketonates [88] or complexes with Schiff bases [89] and macrocyclic polyethers [90] where lanthanide cations achieve steric saturation by high coordination numbers. Entrapment of the cation in a macrocyclic cavity results in greater complex stability. However, simply functionalized ligands such as ethanolamines can also supply a suitable ligand sphere [91-93],... 

Use of a flexible aminosubstituted cyclopentadienyl ligand afforded a mononuclear species with the bulky silox ligand as alkoxide component. Only one donor arm is required to achieve a sterically saturated 8-coordinated neodymium center (Fig. 30, Table 14) [207]. 

The solid-state structure of the heteroleptic compound [Tp Bal (HMPA)2] is shown in Fig. 21.111 The steric bulk of the poly(pyrazolyl)bo-rate ligand is apparent in this figure, effectively blocking oligomerization from occurring. This is even more apparent in the bis(3,5-dimethylpoly (pyrazolyl)borate) complex Ba[HB(3,5-Me2pz)3]2 shown in Fig. 22, where two poly(3,5-dimethylpyrazolylborate) ligands have sterically saturated the coordination sphere of the barium center. The shortest intermolecular distance is 5.23 A.112... 

R = Me, Et, -Bu, CH2SiMc3, Ph). They feature short Cr-Cr distances ( 2.26A), low effective magnetic moments, and attenuated reactivity, all consistent with strong metal-metal bonding. Steric saturation of chromium with a sterically encumbered tris(pyrazolyl)borate ligand (i. e. =... 

There are two low-valent oxidation states available to the lanthanides under normal conditions the +2 oxidation state and the formally zero oxidation state found in the elemental metals. The zero oxidation state is available to all the lanthanides, but only three members of the series have +2 oxidation states accessible under common organometallic reaction conditions Eu (4/ ), Yb (4/ ), and Sm (4/ ). The Ln VLn" reduction potentials [vs. normal hydrogen electrode (NHE)] (12), - 0.34 V for Eu, - 1.04 V for Yb and - 1.50 V for Sm, indicate that Eu is the most stable and Sm the most reactive of these divalent ions. Sm is also the most reactive based on radial size considerations, since it is the largest and most difficult to stabilize by steric saturation. 

---