Five-membered rings metal complexes

Fan angle, 1019 Ferribalamins, 883 Ferrichromes, 970 calcium complexes, 972 Ferridoxins, 773 Ferrioxamines, 970,971 Ferritin, 772 structure, 975 Ferroverdin, 797 structure, 272 Five-membered rings metal complexes, 76 substituents metal complexes, 82 Fluorine... 

In 1973 Hafner prepared the first metal complex of unsubstituted pentalene 46 (Fig. 4) by reaction of the [2 + 2] dimer of pentalene 2 with Fe2C09 [39]. Several other dinuclear complexes of 2 have been prepared from the pentalenediide 45 as reviewed previously by Paquette [1]. All these structures contain a planar pentalene ligand, in which each five-membered ring is complexed to a different metal atom as in bis(pentamethylcyclopentadienylnickel)pentalene 47 obtained by reaction of 45 with Cp Ni(acac) [40]. 

Ti -Cyclopentadienyl(triphenylphosphine)cobalt reacts with phosphites and forms complexes of 1-alkoxyphosphole oxides 251 (R = Me, Et, Ph) through a step involving (ri -cyclopentadienyl)(phosphite)cobalt (80JA4363). (ri -Cp)Co(PF3)2 reacts with hexafluorobut-2-yne and 252 is formed, which hydrolyzes into 253 (X = OH) [73JCS(CC)583 75JCS(D)197]. The five-member ring has the envelope conformation, in which the carbon atoms are coplanar, and the phosphorus atom deviates from this plane in the direction opposite to the cobalt atom. The heterocycle is a four-electron donor relative to the metal center. 

When, however, the ligand molecule or ion has two atoms, each of which has a lone pair of electrons, then the molecule has two donor atoms and it may be possible to form two coordinate bonds with the same metal ion such a ligand is said to be bidentate and may be exemplified by consideration of the tris(ethylenediamine)cobalt(III) complex, [Co(en)3]3+. In this six-coordinate octahedral complex of cobalt(III), each of the bidentate ethylenediamine molecules is bound to the metal ion through the lone pair electrons of the two nitrogen atoms. This results in the formation of three five-membered rings, each including the metal ion the process of ring formation is called chelation. 

However, EDTA has the widest general application in analysis because of its powerful complexing action and commercial availability. The spatial structure of its anion, which has six donor atoms, enables it to satisfy the coordination number of six frequently encountered among the metal ions and to form strainless five-membered rings on chelation. The resulting complexes have similar structures but differ from one another in the charge they carry. 

The problem of tr-facial differentiation, i.e. diastereomer formation, encountered in the metal complexation of the above mentioned annulated cyclopentadienyl ligands is avoided when C2-symmetrical ligands [153] are utilized. Since in such ligands both sides of the five-membered rings are homotopic, only one isomer is... 

A more elaborate representation of an EDTA-metal complex (10.13), which gives some indication of the three-dimensional aspects of the structure, shows a complex of five five-membered rings [18]. A similar representation of a DTPA-metal complex shows a system of eight five-membered rings. 

Y3Fe50i2 [7], These complexes have been obtained at 110-120°C, isolated, and studied in the crystalline form and in an aqueous solution. IR spectra suggest that these are monodentate coordinated to the metal ions. NMR data are in accord with an assumption that hydroxy and carboxylate groups attached to the central carbon atom of the citric acid anion are coordinated to the metal cations forming a stable five-membered ring. However, these important data are obtained at conditions that do not correspond to the conditions of the synthesis of precursors. 

Bimetallic biconnective bridging can also be symmetric or can display a variable degree of unsymmetry (Scheme 4). This coordination mode occurs in dinuclear complexes and in supramolecular chain-like arrays. When a metal-metal bond is present five-membered rings are formed. 

Figure 12. Hypothetical fullereneyne C18o, which can be stabilized in principle by complexation at the five-membered rings with metal-ligand moieties.

While the preponderance of data thus indicates ionic bonding, there has been some suggestion of covalent interaction. An ABX pattern for the pmr spectrum of the five-membered ring portion of Sm(C9H7)3 THF was interpreted as evidence of covalent bonding, since such a pattern is also found for covalently-bound transition metal indenide complexes 66). However, it has been pointed out in some detail that no conclusions of this type should be drawn from the simple pmr spectra 127). 

---