Fac geometry

Structures of l-hydroxy-2-pyridinonate and 3-hydroxy-2-p5rridino-nate complexes may be compared with those of the more extensively studied 3-hydroxy-4-pyridinonates through the published crystal structures of their tris-ligand iron(III) complexes. The iron(III) tris-ligand complex of l-n-butyl-3-hydroxy-2-pyridinone has fac geometry it crystallizes as a trihydrate 135). 

There appears to be a relatively fine balance between mer and fac geometries for octahedral trichloroiron(III) complexes. Thus it was established some time ago that [Fe(phen)(solvent)Cl3] adopted the fac geometry for solvent = H20 or MeOH/ but mer for solvent = DMF. The geometric and steric demands of terdentate ligands such as 1,3,5-trimethyltriazacyclohexane, ... 

In modem NHC chemistry these two patterns are realised by two carbene entities substituted in the 2,6-position of a pyridine or in the 1,3-position of a benzene ring for the mer or pincer geometry and a tripod arrangement of three NHC moieties on a boron or an arene scaffold for the fac geometry, respectively (see Figure 2.9). 

Both ligand classes are modelled on long established ligands, the pincer ligands with amino, phosphane or ether sidechains on a pyridine or benzene base for the mer geometry [64-66] and the scorpionate ligands (tris-pyrazolylborates) for the fac geometry [67-69]. 

The tris-carbene ligand family with fac geometry points its three wingtip groups downwards around the metal shielding it effectively from the approach of any but small substrates. Its main application is therefore the activation of small molecules, including the activation of dioxygen and proton coupled electron transfer (PCET), a reaction normally performed by certain enzymes [70,71],... 

Identify element M. Propose and draw structures for the compounds A, B and C, showing your reasoning in detail. Why does D have a fac geometry. (University of Leicester). 

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