Ligands shape

The ligand shape has changed and it has inflated on encapsulating the cation. The structures of a large number of [2.2.2]cryptand alkali and alkaline earth metal complexes have been studied by X-ray crystallography.372 Using the criterion that the optimum complexation occurs when the cation and cavity radii are similar, it can be predicted that for this ligand potassium is... 

Fig. 2. The molecular structures of Co4(CO)12 and Ir4(CO)12 showing the different ligand shapes...

Macropolycycles, as distinct from the macromonocycles mentioned above, are also well explored. This can be exemphfied by the macrobicyclic N3S3 donor (150), with a ligand shape that is well suited to hexacoordination of... 

Chen QM/MM (2a) and (4a) Ligand shape (phosphine vs. NHC) is linked to rate of dissociation of trans phosphine. Phosphine dissociation is always slow step for (4a) while metallocyclobutane formation is the energetic barrier for (2a)... 

Many-Armed Monsters - Introducing Ligand Shape... 

The process of complexation of polydentate ligands involves an aspect of wrapping around the metal ions, usually with gross rearrangement of the ligand shape. 

Ligand shape directing complex shape in five-coordination. 

Immediately above, we have introduced some of the issues that arise when a polydentate ligand is bound rather than simple monodentate or didentate ligands. Even stepping from two to three donors increases the options in terms of ligand shape (or topology), and this shape will affect the way a molecule may bind to a metal ion. Some shapes for potentially tridentate ligands appear in Figure 4.36. 

Ligand shape has an impact on the resultant complex shape, the number of possible isomers that could form, and the thermodynamic stability of complexes formed. 

How a pre-defined appropriate ligand shape (pre-organization) provides chelation at less energy cost than in circumstances where energy-demanding ligand re-arrangement must accompany chelation. 

Fig. 2. Facial selectivity of dihydrogen reduction controlled by phosphine ligand shape.

Systematic variation of a metal complex is not routine. A particular metal center prefers a particular set of donors in order to be stable under physiological conditions. The label must be stable and biologically inactive or be cleaved from the vector in a predictable pharmacokinetic way, and then rapidly excreted for the sake of good imaging. These conditions point towards organometalhc complexes or moieties for radiopharmaceutical application. Radiopharmaceutical drug development is only possible if the ligand shape can be systematically altered... 

Comput. Methodol, 3, 595 (1990). 3D Database Searching on the Basis of Ligand Shape Using the SPERM Prototype Method. 

V. J. van Geerestein, N. C. Perry, P. D. J. Grootenhuis, and C. A. G. Haasnoot, Tetrahedron Comput. MethodoL, 3, 595 (1992). 3D Database Searching on the Basis of Ligand Shape Using the SPERM Prototype Method. 

---