Chelation coordination modes

In order to formally insert CH2, the growing alkyl chain must attain a situation of metal-to-carbon bonding that favors CH2 insertion over coupling of two CHR groups (R=H, alkyl).29 A (C, H)-chelating coordination mode characterized by agostic M-H-C interaction30 would meet this requirement. 

Owing to their conformational flexibility, bridging and/or chelating coordination modes are typical for polychalcogenide ligands. As a result, very few... 

In this example, significant chiral induction can only be realized if silver salts are added to trap the iodide ions, and thus a chelating coordination mode of the chiral bidentate ligand is facilitated during coordination of the olefin (Scheme 2, cationic pathway). Even better results are obtained from substrates with non-coordinating leaving groups such as vinyl triflates. 

Within the variety of the designed ligands, bulky heteroallylic compounds have been described. Such complexes usually contain the benzamidinate fragment in a chelating coordination mode. 

It is of interest that each Pt in [Pt4(CH3COO)4(en)4] + (15) is in a chiral environment (Fig. 13). However, the compound is optically inactive as a whole, because chelate ring configurations around the four Pt atoms are of meso(AAAA) form. Optically active Pt4 clusters [Pt4(CH3COO)4(L-aa)4] have been prepared by introducing optically active a-amino acid (aa) at the in-plane sites (L-aa = L-alanine, L-proline, L-hydroxyproline, L-phenylalanine, L-valine) (37, 38). As described in the previous section, these a-amino acids take the chelating coordination mode in Pt4 clusters. Therefore, as far as the chelate ring... 

Fig. 11. The adoption of both mono- and bis-chelating coordination modes by the squarate ion to Ce(III) in the polymeric structure [Ce(H20)2J2(0404)3 n (57).

Diaminomethane, a molecule with two amine groups. Once the first is coordinated, the second lone pair from the other amine can be oriented in a direction more appropriate for bonding than is the case for two lone pairs on a single atom, with limited bond angle distortion permitting both to coordinate, illustrated at right, in a chelated coordination mode. 

In their pioneering work, West and Niu assumed that the squarate complexes of divalent cations Mg ", Ca, Mn, Fe, Co ", Ni ", Cu " and Zn form an isostructural series (Figure 1). They suggested a bis-chelating coordination mode for the squarate dianion in all the squarate complexes, but their proposed structure does not correspond to the actual coordination mode of squarate. 

The NMR spectra of the compounds la-d clearly indicate a chelating coordination mode of the amidinate ligands. Both R groups show only one set of signals in the temperature range from +30 to -90 C ( h, C NMR), indicative for a fast pseudo-rotation of the molecular skeleton in each case, whereas the Si NMR resonances confirm the presence of a pentacoordinate silicon centers. The... 

Only a small number of metal trithiophosphonate complexes have been structurally characterised, most of which are monometallic and contain the ligand in a bidentate chelating coordination mode In these complexes... 

FIGURE 5.2 Chelating coordination mode between chiral metal complexes and pymvate or glyoxylate derivatives. 

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