Hepta-coordinated complexes

Transition Metal Complexes with Bis(Hydrazone) Ligands of 2, 6-Diacetylpyridine. Hepta-Coordination of 3d Metals... 

A route for designing Gd(HI) complexes whose relaxivity depends on the presence of lactate, is provided by the ability shown by some hexa- or hepta-coordinate chelates to form ternary complexes with a wide array of anionic species (154-161). The interaction between the coordinatively unsatured metal complex and lactate involves the displacement of two water molecules coordinated to Gd(III) ion with the two donor atoms of the substrate, thus leading to a marked decrease in the relaxivity. Lactate is a good ligand for Gd(IH) ion because it can form a stable 5-membered ring by using the hydroxo and carboxylic oxygen donor atoms (Fig. 19). 

In a more recent report, a novel dinuclear N,N,0-donor Schiff base-calcium complex [(DAIP)2Ca]2 where DAIP = 2-[2(-dimethylamino-ethylimino)methyl] phenol 76 (Fig. 12) was shown to initiate ROP of L-lactide in a controlled fashion in the presence of benzyl alcohol giving a 96% conversion within 30-60 min at room temperature, yielding polymers with high molecular weight and low PDIs [89]. Taking into account the dimeric molecular structures of hexa- and hepta-coordinated Ca" in the solid state and the kinetic results gained from this study (first order dependency on [LA] a first order dependency on [BnOH] with d[LA]/ df = prop[LA] [BnOH] and a p op constant of 1.95 s ), a coordinatio-... 

TRANSITION METAL COMPLEXES WITH BIS(HYDRAZONE) LIGANDS OF 2,6-DIACETYLPYRIDINE. HEPTA-COORDINATION OF 3d METALS... 

The remaining two hepta-coordinate structures of Fe(III) are p-oxo dimers. The first of them [(H20)BFe-0-FeB(H20)](C104)4 includes a macrocyclic ligand (Table I), and the other one [Fe20(N03)4(bpy)2] (bpy = 2,2 -bipyridine), bidentate ligands (100,101). The PBP geometry of the p-oxo-diiron(III) complexes is unusual since these complexes have square-pyramidal or octahedral structures with the exception of the above-mentioned two complexes. 

Following Co(II) complexes, the most frequently found hepta-coordinate structures among 3d metals are those of Mn(II) complexes. Based on literature data, 21 X-ray analyses of Mn(II) complexes with coordination number seven have been reported so far. They include... 

The present volume is a non-thematic issue and includes seven contributions. The first chapter byAndreja Bakac presents a detailed account of the activation of dioxygen by transition metal complexes and the important role of atom transfer and free radical chemistry in aqueous solution. The second contribution comes from Jose Olabe, an expert in the field of pentacyanoferrate complexes, in which he describes the redox reactivity of coordinated ligands in such complexes. The third chapter deals with the activation of carbon dioxide and carbonato complexes as models for carbonic anhydrase, and comes from Anadi Dash and collaborators. This is followed by a contribution from Sasha Ryabov on the transition metal chemistry of glucose oxidase, horseradish peroxidase and related enzymes. In chapter five Alexandra Masarwa and Dan Meyerstein present a detailed report on the properties of transition metal complexes containing metal-carbon bonds in aqueous solution. Ivana Ivanovic and Katarina Andjelkovic describe the importance of hepta-coordination in complexes of 3d transition metals in the subsequent contribution. The final chapter by Sally Brooker and co-workers is devoted to the application of lanthanide complexes as luminescent biolabels, an exciting new area of development. 

The exchange of labeled cyanide ion with II proceeds via either a penta- or a hepta-coordinated transition state complex as found for other cyanide complexes (I, 29). The latter hypothesis is favored, since preliminary results indicate that an electron-withdrawing substituent, X, in [ (p-XC6H4CH2NC)5FeCN]HS04, which apparently reduces the charge density of the iron, facilitates this exchange. 

There are some data indicating the possible existence of anionic silicon complexes with coordination number 7 and 8 The assumption of hepta-coordination... 

In the N,iV, N"-trimethyl-l,4,7-triazacyclononane complex, the molybdenum atom probably assumes a 4 3 piano stool hepta-coordination, similar to that found in the X-ray structure of [Mo([9]aneN3)(CO)3Br] [61]. 

Hepta-coordinate 3d metal complexes are relatively rare and solution reactions of such complexes have not been investigated systematieally. Iron (III) forms a seven-coordinate complex with 2,6-diacetylpyridine-bis-(semioxamazide) (H2dapsox). 

Tc-EDTA chemistry studies demonstrated at least two types of stable complexes, one containing a Tc(V)Oj core in a hepta-coordinated environment (Davison and Jones 1982), and the other is a complicated dimer in which technetium can be present as Tc(IV) or as Tc(III) (Linder 1986 Noll et al. 1980 Seifert et al. 1982). The crystal structure of a Tc(IV) complex has been reported (Burgi et al. 1981). 

In the compounds so far described in this subsection the metal center is tetracoordinate, if cyclopentadienyl ligands are counted as occupying one coordination site. In the preceding part dealing with ether-functionalized zirconium and hafnium complexes, we have already seen that these metals can extend their coordination sphere and are easily able to accommodate more than four ligands. Not surprisingly, therefore, a number of alkoxo-linked cyclopentadienyl species which contain hexa- or even hepta-coordinate zirconium(IV) have been reported. 

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