Seven coordinate

With Lammerstma and Simonetta in 1982, we studied the parent six-coordinate diprotonated methane (CH/ ), which has two 2e-3c bonding interactions in its minimum-energy structure (Cid- On the basis of ab initio calculations, with Rasul we more recently found that the seven-coordinate triprotonated methane (CHy ) is also an energy minimum and has three 2e-3c bonding interactions in its minimum-energy structure 3 ). These results indicate the general importance of 2e-3c bonding in protonated alkanes. 

Divalent molybdenum compounds occur in mononuclear, dinuclear, and hexanuclear forms. Selected examples are shown in Figure 6. The mononuclear compounds are mostiy in the realm of organometaUic chemistry (30—32). Seven-coordinate complexes are common and include MoX2(CO)2(PR3)2, where X = Cl, Br, and I, and R = alkyl MoCl2(P(CH3)3)4, heptakis(isonitrile) complexes of the form Mo(CNR) 2 (Fig. 6d), and their chloro-substituted derivatives, eg, Mo(CNR)3CR. The latter undergo reductive coupling to form C—C bonds in the molybdenum coordination sphere (33). 

Seven-coordinate pentagonal-bipyramidal In has been found in the chloroindium complex of 1,4,7-triazacyclononanetriacetic acid... 

A concerted [2 + 2] cycloaddition pathway in which an oxametallocycle intermediate is generated upon reaction of the substrate olefin with the Mn(V)oxo salen complex 8 has also been proposed (Scheme 1.4.5). Indeed, early computational calculations coupled with initial results from radical clock experiments supported the notion.More recently, however, experimental and computational evidence dismissing the oxametallocycle as a viable intermediate have emerged. In addition, epoxidation of highly substituted olefins in the presence of an axial ligand would require a seven-coordinate Mn(salen) intermediate, which, in turn, would incur severe steric interactions. " The presence of an oxametallocycle intermediate would also require an extra bond breaking and bond making step to rationalize the observation of trans-epoxides from dy-olefms (Scheme 1.4.5). 

Seven coordination in molybdenum chemistry analysis of oligonuclear structures. M. Melnik and P. Sharrock, Coord. Chem. Rev., 1985, 65, 49 (91). 

A Rh—Si bond cleavage has been demonstrated, but an addition-elimination mechanism is unlikely as this would require a seven-coordinate Rh(V) intermediate coordination of HCl through the chlorine atom was postulated [Eq. (64)] 135). 

A mononuclear diastereopure high-spin Fe alkylperoxo complex with a pen-tadentate N,N,N,0,0-ligand 33 (Scheme 17) was reported by Klein Gebbink and coworkers [109, 110]. The complex is characterized by unusual seven-coordinate geometry. However, in the oxidation of ethylbenzene the iron complex with 33 and TBHP yielded with large excess of substrate only low TON s (4) and low ee (6.5%) of 1-phenylethanol. 

These observations are, indeed, consistent with an associative activation, generation of a seven-coordinated intermediate (easier in the case of Mo and W than for Cr because their larger sizes produce less steric hindrance) by attack taking place directly upon the metal atom, that is, with an A or a limiting 5 2 mechanism, accompanied by a reaction sequence involving dissociative activation similar to scheme (24) above, viz. 

A// 2 is significantly higher for Cr than for either Mo or W, and this is in agreement with the generally observed order of stability for those group VI compounds which are seven-coordinated. 

Indelli and Guaraldi propose the acid-independent path to involve eight-coordinate iron (six CN plus two I ligands) and the acid-catalysed path to involve seven-coordinate iron, viz. 

Treatment via chelation has been observed for 2-acetylpyridine thiosemi-carbazone derivatives, which have been found to possess inhibitory activity for the RNA-polymerases of the influenza virus [133]. The iron(III) complexes were shown to be 3 to 6 times more active as inhibitors of partially purified ribonucleotide reductase (no added iron) compared to uncomplexed thiosemi-carbazone [128]. Raina and Srivastava [134] prepared and characterized low spin iron(III) complexes of 2-acetylpyridine thiosemicarbazone, [Fe(8-H)2A] (A = NO3, OH, Cl, N3, NCS or NO2), which were proposed as being seven-coordinate. However, all but the azide complex are 1 1 electrolytes in DMF and their solid ESR spectra are rhombic with the g-values being about 2.20,2.15 and 2.00. Of the six complexes, the azide ion seems to interact ihost strongly with the iron(III) center. 

Ti(R2 fc)3Cl is a seven coordinated monomeric species, as is suggested from the spectral data and the non-electrolytic character and confirmed by an X-ray diffraction study (6). Apart from the chlorine compounds the bromine analogues are reported (9). 

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