7r-bonding model

In the 7r-bonding model, diamides and triamides are expected to show longer M—N distances than their monoamido counterparts, as there are two or three nitrogen lone pairs competing for just one empty p-orbital... 

Pyrrole also acts as a -electron donor in association with tri-halomethanes to form weak 1 1 CH-7T complexes (20).141 Similarly pyrrole appears to form (pyrrole) 7T-H-0 bonds with phenol instead of an NH-O hydrogen bond or NH-v (phenol) bonds.112 The association between pyrrole and nitriles has been described both as NH-N132 (21) and as NH-n-146,156 (22) hydrogen-bonded complexes. No definite conclusion has been reached but the NH frequency difference between the bonded and nonbonded vibration is more compatible with a NH-7r-bonded model (Table IV). 

In the so-called 7r-bonded model, the C2H4 molecule lies flat on the surface and a <7-bond is formed by the overlap of the Ni d orbital and the C2H4 fc3u(ff)-... 

An important feature of the (p-d) 7r-bonding model is the lack of conformational requirements. The degree of (p-d) n overlap is constant, regardless of rotation around the Si—X bond, as a consequence of the symmetry of the five 3d orbitals on silicon. 

Octaethylbilindione (H3OEB) is a convenient model for the bile pigment biliverdin IXa. Key redox states of this ligand as observed in its complexes are shown in Figure 14. The redox behavior of the palladium complex of octaethylbilindione was examined in order to determine the generality of the redox behavior of this group of transition metal complexes.202 A preliminary report on the novel tetrameric Pd4(OEB)2, which contains palladium) ) ions 7r-bonded to C=C bonds of the tetrapyrrole ligand, and of Pdn(OEB ) has appeared.203... 

Ru < Os throughout. (The porphodimethene spectra are different and cannot yet be interpreted in the sense given here (55).) Concomitant with this hypsochromic shift, a bathochromic shift of the CO-stretching frequency and an increase in the shielding of the porphyrin methine protons indicate enhanced 7r-bonding, thus corroborating the w-bonding model (Sect. 4.2). 

By contrast, the Dewar resonance energy represents solely the contribution coming from the cyclic electron (bond) delocalization since the model reference structure is represented not by a system of isolated 7r-bonds, but by a hypothetical cyclic polyene with the number of tr- and tr-bonds equal to that in a given molecule. Making use of the additivity of bond energies in acyclic polyenes (65JA692), one may calculate the total energy... 

The Huckel 7r-electron model uses information about what is bonded to what (i.e. the connectivity) but does not cater for molecular geometries. 

C. Ab Initio Cl Models of Simple Double, Charged, and Dative 7r-Bonds... 

Using a transition state model for enolate formation and a database search, a thiourea with a pendant amine has been designed as a catalyst, and its ability to hydrogen bond the enolate of acetone explored.287 Both in-plane and out-of-plane hydrogen bonds, to a lone a pair and the carbonyl 7r-bond, respectively, were considered. 

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