Ligands hapticity

The effects of one-electron redox proeesses on various bond strengths is often substantial and can often be qualitatively understood by appropriate consideration of pertinent Lewis struetures for each side of the redox couples. Significant pXa or BDE changes can be ascribed to changes in metal-ligand hapticities, metal-ligand bond orders, or C-C bond orders. 

Alkyl, Halide, and Amide Precursors for Complexes of Higher Ligand Hapticity 763... 

Sample 6( La)/ppm line width (Hz) solvent total ligand hapticity Ref. 

What are the ligand hapticities in the complex [Cr(CpCH2CH2Cp)(CO)2] Draw the three-dimentional formula. 

A change in hapticity of the Cp ligand in 1 from q3/5 to ri1 should lead to a drastic change in the character of the molecule whereas the 7t-bonded species (I) can be regarded as a hyper-coordinated, electronically over-saturated and thus nucleophilic silylene, the a-bonded species (II) represents an example of an electron deficient and thus electrophilic silylene. 

The geometries in Figs. 4.86 and 4.87 suggest an important distinction in the multicenter hapticity character of ligand attachment to the metal atom. Hapticity refers to the number of atoms in a ligand that are coordinated to the metal. In the Ir+ diammine complex (Fig. 4.86(a)), the metal attaches to each of two nN donor lone pairs in simple monohapto (one-center, q1) fashion. However, in the Ir+ complexes with HCCH or CML the metal attaches to the face of the pi bond or three-center allylic pi system in dihapto (two-center, r 2) or trihapto (three-center, q3) fashion, respectively. The hapticity label q" therefore conveniently denotes the delocalized n -center character of the donated electron pair(s) and the geometry of the resulting coordination complex. 

Whereas changes in hapticity are normally required for changes in the number of coordinated electron pairs, acetylene and other triple-bonded ligands exhibit the... 

The (M)... L symbol represents a set of bonding interactions between the metal atom M and 1 + adjacent atoms in the ligand L. So it features one n-conjugated r interaction, and not the global hapticity of the ligand that results from the combination of all o-separated such interactions. 

In the simpler cases, such as ethylene, the n-electron pair can be donated to the metal just like a lone pair on, say, the nitrogen of ammonia. This results in a contribution of unity to the coordination number, but two carbon atoms are bound to the central atom. The hapticity of the ethylene is defined as two and is denoted formally by the symbol rf. In general, the hapticity of a ligand is the number of ligating atoms, n, in the ligand that bind to the metal, and is represented by the symbol t ". 

The usefulness of such a series is twofold (I) If two different unsaturated ligands are found in the same complex, one can predict which ligand will react, and (2) it is possible to eslimaie how activating a metal fragment must be in order to cause a reaction to occur. Notice that hydrocarbons of even hapticity are more reactive than those with odd hapticity. In addition, acyclic ligands are more reactive than cyclic ones. 

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