Coordination complexes, from Lewis acid-base arrangements

From the valence bond point of view, formation of a complex involves reaction between Lewis bases (ligands) and a Lewis acid (metal or metal ion) with the formation of coordinate covalent (or dative) bonds between them. The model utilizes hybridization of metal s, p, and d valence orbitals to account for the observed structures and magnetic properties of complexes. For example, complexes of Pd(ll) and Pt(Il) are usually four-coordinate, square planar, and diamagnetic, and this arrangement is often found for Ni(II) complexes as well. Inasmuch as the free ion in the ground state in each case is paramagnetic (d, F), the bonding picture has to... 

Numerous investigations of reactions of allyfic stannanes with aldehydes have demonstrated that changes in steric interactions and electronic effects are introduced by the coordination of Lewis acids. In fact, the oxocarbenium species serves as the reactive partner in the allylation process, and deficiencies of detailed knowledge regarding the structure, reactivity, and steric requirements of oxocarbenium complexes leads to some uncertainties in providing a simple model to forecast stereoselectivity. Based upon a body of results accumulated from a variety of investigators, the pre-complexation with a specific Lewis acid and aldehyde may sustain the inherent attributes of the synclinal transition state, or the coordination complex may present features that override this tendency in favor of antiperiplanar arrangements. 

When each ion maintains its own primary solvation shell, the new chemical species is a solvent separated ion-pair (SSIP). If a single solvent layer is shared by ion partners, the species is a solvent shared ion-pair (SIP). If the cation and the anion are in contact and no solvent molecules are present between them, the form is contact ion-pair (CIP) or intimate ion-pair. Figure 2.1 illustrates multistep ion-pair formation equilibrium. What sets ion-pairing apart from complex formation is the absence of directional covalent coordinative bonds resulting from a Lewis base-acid interaction and a special geometrical arrangement. 

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