Tetrahedral enforcer ligand

The hydridotris(pyrazolyl)borate (Tp) ligand has become well known and well established as a formal analogue of the cyclopentadienyl (Cp) ligand (43). Unlike Cp, however, when appropriately substituted, Tp can become what has been described (44, 45) as a tetrahedral enforcer ligand, producing complexes that are constrained to be tetrahedral even when other factors might allow octahedral coordination. 

Bulky Tp ligands, sometimes considered tetrahedral enforcers, strongly promote a tetrahedral geometry even when this results in a highly unusual structure, such as 15e paramagnetic Co(ll) complex 5.40. 

The metallation should proceed via the formation of a chelated tetrahedral magnesium enolate complex, with a (Z)-geometry. The conformational rigidity would be enforced by chelation of both the imide enolate and bis(sulfonamide) ligand to the tetrahedral magnesium ion. 

The ligand L3 was initially intended to provide enforced tetrahedral coordination at the metal. Moore and co-workers (30,31) have successfully used the pendant arm macrocycle L5, also a derivative of [12]aneNs, for the same purpose (although in view of the foregoing discussion, enforcing the geometry may not have been necessary). The three macrocyclic nitrogen donors form the base of a tetrahedron and the pendant amine fills the apical site. If the pendant arm is shorter. 

At both active centers, the growing polymer chain and the vacant site for binding and activation of a monomer require cis positions. In d° metallocene complexes, the cis position is automatically arranged by the pseudo-tetrahedral coordination environment of the metal cation. In the square-planar d palladium complexes, cis coordination is enforced by the use of the neutral biden-tate ligands. 

Fig. 8.9. Tripodal ligands in which X, Z, and Y donor groups are attached to a tetrahedral boron or carbon center enforce the facial binding. ...

If one cuts the macrocycle of porphyrins by oxidation, helical structures become dominant. The most spectacular case has been found with zinc octaethylformyl-biliverdinate. At neutral pH, the central zinc ion is hydrated and a helix is formed with a disturbed square planar ligand field and one axial water molecule. Upon acidification, however, the water molecule is removed and the zinc ion enforces a tetrahedral ligand field by binding to two different formyl-biliverdinate molecules, rearranging to form a double-helix. Upon neutralization, hydration takes place again and the planar, monomeric monohydrate is reformed in quantitative yield. Both the helical monomer and dimer structures have been solved by single crystal analysis (Fig. 6.2.16). 

Ligands which enforce a tetrahedral coordination sphere, or one which approaches this geometry, on the central cation. 

Similarly, the ligands in the complex [CuL °2] (H) enforce a tetrahedral, or a distorted tetrahedral, coordination sphere, and therefore, a relatively large is observed for this complex (Fig. 7). 

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