Dangling ligand

Mechanisms for such reactions have been viewed by the effective coordination number of the transition state. Bond rupture processes proceed via five-coordinate transition states for which idealized square-pyramidal (SP) and trigonal-bipyramidal (TBP) geometries with the dangling ligand axial or equatorial have been considered. Twist mechanisms are considered to proceed via six-coordinate transition states with idealized trigonal prismatic (TP) geometry. Twists of the chelate rings about the real or pseudo C3 axis in the case of the cis or trans isomer, respectively, and about the... 

To illustrate the approach, let us consider some of the data and deductions for the system Co[CH3COCHCOCH(CH3)2]3, measured in C6H5C1. It was found that both the isomerization and the racemization are intramolecular processes, which occur at approximately the same rate and with activation energies that are identical within experimental error. It thus appears likely that the two processes have the same transition state. This excludes a twist mechanism as the principal pathway for racemization. Moreover, it was found that isomerization occurs mainly with inversion of configuration. This imposes a considerable restriction on the acceptable pathways. Detailed consideration of the stereochemical consequences of the various dissociative pathways, and combinations thereof, leads to the conclusion that for this system the major pathway is through a tbp intermediate with the dangling ligand in an axial position as in Fig. l-12(c). 

The simplest dissociative pathways, in which one end of one ligand becomes detached from the metal atom are shown as (c)-(f) in Fig. 21-16. The intermediate may be five-coordinate, with either tbp or spy geometry, and the dangling ligand may occupy either an axial or an equatorial (or basal) position. In the case of the spy intermediates, it is possible that a solvent molecule might temporarily occupy a position in the coordination shell. 

Exemplificative of a third type of oxygen-bonded acetylacetonate derivative is the compound formed from the reaction trimethylchloro-silane and acetylacetone (77). The product from this reaction is 2-tri-methylsiloxy-2-pentene-4-one. It is to be noticed that this silicon compound contains a dangling ligand even though several chelated acetyl-acetonates of silicon are known. Dangling complexation of potential... 

Breaking one bond in an ethylenediamine complex leaves the en ligand dangling but tethered to the metal by the second nitrogen atom. (Each carbon atom bonds to two hydrogen atoms that are not shown.)... 

Figure 1. Ligands containing oxidizable dangling phenols. Their coordination chemistry is described in (3-13).

Figure 10. Coordination chemistry of ligands containing dangling phenols (138—141).

Chain length may inter alia leave a potential ligand site dangling ,191 where the stability of the chelate is determined by its intrinsic strain (equation 48). Chelating ligands properties may also be described in terms of torsion angles. 

Polydentate ligands do not necessarily need to bind a metal at all possible locations. In some cases, only one site is bound to the metal and the other one or more are left dangling. Such complexes are rare, and it has been necessary to develop special synthetic methods to prepare them. Explain in terms of thermodynamic arguments why polydentate ligands will almost always form chelating complexes rather than leave dangling arms. 

---