Five-coordinate system

Attaching the five-coordinate system to a rigid support (i.e., silica gel), thereby reducing its mobility and ability to add a sixth ligand. 

The development of the AOM in the mid-1960 s coincided with an explosion of activity in the study of five-coordinate systems unfortunately, many of these have symmetries which are rather lower than the idealised D3h (trigonal bipyramid) or C4v (square pyramid). Even so, some useful information has arisen from the AOM. 

The reaction of a 1,10-phenanthroline complex of iridium, [Ir(cod)-(phen)]+, with dioxygen in methanol solution has been studied (38). When the anion for this cationic complex is chloride, no anion-cation interaction occurs, and the iridium system remains four-coordinate. However, when either iodide or thiocyanate is present due to the addition of their sodium salts (or in the presence of added triphenylphos-phine when the anion is chloride), the iridium system becomes five-coordinate because of the interaction between I", SCN", or PPh3 and the iridium center. These five-coordinate systems react more rapidly with dioxygen than did the four-coordinate system at both normal and elevated pressures. An end-on oxidative addition of the dioxygen moiety, with displacement of the , SCN, or PPh3 ligands, was postulated. 

The Jt interaction from the oxygen atoms in this five-coordinate system is much larger than that in the four-coordinate one. e i(0) this system 1500 cm"1 and ewi(0) tetrahedral system 800 cm"1. 

Chang and Traylor argued that in heme-peptide models the side chains containing the imidazole had either too few or too many atoms to achieve a strain-free iron-imidazole bond On the other hand it was argued that condensation of l-(3-aminopropyl) imidazole 21 with the acid chloride of pyrroporphyrin XV 22 followed by insertion of iron would give a strain-free five-coordinate-system 23 (Scheme 11). This chelated heme was capable of binding dioxygen in a reversible manner in the solid state or when... 

The fifth and the sixth parts will be devoted to recent work based on templating metal centers other than copper(I). Five-coordinated systems constructed around Zn + will be considered in the fifth section, whereas the use of octahedral M(diimine)3 + as templates will be the topic of the sixth and last chapter. 

It was proposed that species B is a five-coordinate distorted trigonal bipyramidal, and C is a distorted square pyramidal species. Both of them are either high- or intermediate-spin Fe" +, although valence assignment in these systems with significant covalent character is rather dubious. The two species are separated by a stereochemical barrier, and there is a dynamic equilibrium between them that shifts toward C at higher temperatures. Such stereochemical nonrigidity in five-coordinate systems has been well known [5]. 

Most of the effort in the area of five-coordinate systems has been on trigonal bipyramid systems, such as PFj and Fe(CO)j. The interconversion is between the axial, a, and equatorial, e, positions. Studies on Fe(CO)j are described in section 4.5.1. 

The most direct evidence in support of an SnICB mechanism would be to establish the formation of a five-coordinated intermediate. This intermediate would be extremely reactive and therefore very difficult to detect. Experiments in water have not been successful because water is such a good coordinating solvent that it immediately enters the five-coordinated system. However, experiments in the solvent dimethyl sulfoxide do rule out an Sn2 mechanism in this solvent and support the SnICB process. This was demonstrated by experiments showing that the reaction of []Co(NH3)5Cl]p+ with NOa is catalyzed by hydroxide ion, Eqs. (31) and (32). 

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