Square pyramidal coordination

Mixed 6),Af-donor ligands such as Schiff bases are of interest in that they provide examples not only of square-planar coordination but also, in the solid state, examples of square-pyramidal coordination by dimerization (Fig. 28.6(b)). A similar situation occurs in the bis-dimethylglyoximato complex, which dimerizes by sharing oxygen atoms, though the 4 coplanar donor atoms are all nitrogen atoms. Copper(II) carboxylates are easily obtained by crystallization from aqueous solution or, in the case of the higher carboxylates, by precipitation with the appropriate acid from ethanolic solutions... 

The structural analyses reveal distorted square pyramidal coordination with a nearly linear Cl( 1)—Ru—Cl(2) angle (168.62 and 170.42 for 12 and 13,... 

To mimic the square-pyramidal coordination of iron bleomycin, a series of iron (Il)complexes with pyridine-containing macrocycles 4 was synthesized and used for the epoxidation of alkenes with H2O2 (Scheme 4) [35]. These macrocycles bear an aminopropyl pendant arm and in presence of poorly coordinating acids like triflic acid a reversible dissociation of the arm is possible and the catalytic active species is formed. These complexes perform well in alkene epoxidations (66-89% yield with 90-98% selectivity in 5 min at room temperature). Furthermore, recyclable terpyridines 5 lead to highly active Fe -complexes, which show good to excellent results (up to 96% yield) for the epoxidation with oxone at room temperature (Scheme 4) [36]. 

Like their Fe analogs, Co porphyrins are aerobically stable. Co , being a if ion, favors a square pyramidal coordination sphere, and simple Co porph5Tins t)q)ically have fairly low affinity to axial hgands. As a result, many simple Co porphyrins are aerobically quite stable in the sohd state, but oxidize slowly if dissolved in a coordinating solvent. Simple Co porphyrins are typically stronger oxidants than the Fe ... 

V0)2+ ion in a square pyramidal coordination. On alumina only the latter configuration is stable. Tanaka and Matsumoto IBS) attributed a poorly resolved spectrum on silica gel to the formation of small crystallites of a V02+ salt. Upon dilution with an alkali salt the resolution increased. They also observed much better resolution on alumina, indicating that the vanadyl ion is more uniformly dispersed on this surface. 

As shown by ab initio studies with the anionic model species 49, the energy difference between the trigonal-bipyramidal (C2 symmetry) and the alternative square-pyramidal structure (C2v symmetry) is rather small (6.9 kJ mor1).40 From this result one may conclude that related zwitterionic A5S7-silicates, such as 47 and 48, should also have a chance to exist with a square-pyramidal /-coordination polyhedron in the solid state. The square-... 

For this reason, it is not even appropriate to state that the square pyramidal nature of the oxometal(IV) porphyrins, (e.g., TiO(OEP) ([22a], 75, 76), VO(OEP) (55), or MoO(OEP) (55)) is caused by the very strong tram effect of the multiply bound oxygen. In TiO(OEP) [23a, the TiIV ion is exoplanar by 53 pm (75), and any addition of a tram ligand L will produce elongated and therefore weak Ti-L bonds in this system. The preference of square pyramidal coordination is therefore thought to be of steric origin. 

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