Metal ligands around

Other imaging techniques such as magnetic resonance and ultrasound have opened up avenues of tremendous potential for contrast medium enhancement (123). Ultrasound contrast media developments have centered around encapsulated air micro-bubbles. Magnetic resonance contrast agents iavolve metal—ligand complexes and have evolved from ionic to nonionic species, much as radiopaques have. 

The physical and chemical properties of complex ions and of the coordination compounds they form depend on the spatial orientation of ligands around the central metal atom. Here we consider the geometries associated with the coordination numbers 2,4, and 6. With that background, we then examine the phenomenon of geometric isomerism, in which two or more complex ions have the same chemical formula but different properties because of their different geometries. 

Two or more species with different physical and chemical properties but the same formula are said to be isomers of one another. Complex ions can show many different kinds of isomerism, only one of which we will consider. Geometric isomers are ones that differ only in the spatial orientation of ligands around the central metal atom. Geometric isomerism is found in square planar and octahedral complexes. It cannot occur in tetrahedral complexes where all four positions are equivalent... 

Coordination isomerism refers to cases where there are different ways to arrange several ligands around two metal centers. For example, there are several ways to arrange six CN- ions and six NH3 molecules around two metal ions that total +6 in oxidation number. One way is [Co(NH3)6][Co(CN)s], but... 

M[pz(A4)] A = S2ML2. The octakis(.V-R)porphyra/,ines reported by Schramm and Hoffman (2), M[pz(S-R)8 (M = Ni, Cu), (60), can be converted to the octathiolate M[pz(S )g] (Scheme 11) via reductive cleavage of the sulfur-carbon bond when R = benzyl (Bn), and this tetra-bis(dithiolate) can then be peripherally capped with metal-ligand systems to yield peripherally tetrametalated star porphyrazines. The benzyl dinitrile 57 can be macrocyclized around magnesium butoxide to form [Mg[pz(S-Bn)8] (58) (35-40%), which can then be demetalated with trifluoroacetic acid to form 59 (90%), which is subsequently remetalated with nickel or copper acetate to form 60a (95%) and 60b (70%) (Scheme 11) (3, 23, 24). Deprotection of 60a or 60b with sodium in ammonia yields the Ni or Cu tetra-enedithiolates, 61a or 61b to which addition of di-ferf-butyl or n-butyl tin dinitrate produces the peripherally metalated star porphyrazines 62a (37%), 62b (80%), and 62c (41%). 

As we have seen earlier in this chapter, metal catalysts may be made soluble in water by careful design of the ligands around them. Metal catalysed reactions may also be conducted under phase transfer conditions. Here, by contrast, the metal catalyst usually resides in the organic phase and not the aqueous phase. The use of a phase transfer catalyst in these systems may be to transfer a water-soluble metal catalyst into the organic layer, or else to ensure a supply of water-soluble substrate or reagent to a catalyst already resident in the organic phase. An example of the former is the use of methyltrioctylammonium chloride to extract aqueous RhCL... 

Ligand effects are extremely important in homogeneous catalysis by metal complexes. One metal can give a variety of products from one single substrate simply by changing the ligands around the metal centre see Figure... 

Information concerning the symmetry of the electric field at the metal nucleus can be found from this latter parameter, AEq, which can also be measured directly by nuclear quadrupole resonance techniques. Additional information concerning the symmetry of the ligand around the metal can be deduced from x-ray, infrared, and nuclear magnetic resonance data. 

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