Seven-coordinated complexes

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). 

The results described above can be accounted for by the sequence of reactions outlined in Scheme I. Insertion into the C-D bonds of C D presumably yields the seven-coordinate complex 1. This reaction must be readily reversible since the NMR and IR experiments detailed above indicate that it leads to H/D exchange to produce [ReD ppe ] and C D H. The ready reversibility of this reaction presumably obtains because of severe steric crowding in 1 in which there are nine phenyl groups located around the central Re atom. [ReH ppe ] also undergoes... 

C. Acyclic and Rigid Seven-Coordinate Complexes as SOD Mimetics 73... 

The interchange character of the water-exchange mechanism of the studied seven-coordinate complexes can be a reason why there is no clear correlation between their rates for the exchange process and the energies required for the dissociation of the coordinated water molecule. AE is also not possible to correlate with the catalytic rate constants published in the literature. 

The unique seven-coordinate complex [TcO(EDTA)] is obtained by reaction of [TcOCy with H4EDTA in anhydrous dmso. The X-ray crystal structure confirmed pentagonal-bipyrami-dal geometry, with the 0x0 group and the two nitrogen donors bound in the equatorial plane. " All other structurally characterized technetium complexes with polyamino-polycarboxylates are in oxidation state IV. 

Another type of complex in which Tc C bonds predominate is the seven-coordinate complex [Tc(CNR)gX] + (398) (R = variety of organic groups X = C1, Br). The compounds are easily prepared in good yield by reaction of the Tc complex [Tc(CNR)6] " with Br2 or CI2 in acetonitrile. Attempts to introduce a bipy ligand as well led to dealkylation of the isocyanide to give the seven-coordinate complex [Tc(CNR)5(CN)X]+ (399). Reductive coupling of the coordinated isocyanides was attempted, but the reduction of Tc to Tc was the only reaction." ... 

ReCl3(PPh3)(benzil)] reacts with bipy and related ligands or terpy to form a number of rhe-nium(III) and rhenium(II) compounds which are useful precursors for the synthesis of lower-valent rhenium complexes. " Thus, reduction of [Re(bipy)3][PF6]2 with zinc amalgam results in the rhenium(I) compound [Re(bipy)3][PF6] in excellent yields. The corresponding terpyridyl bis-chelate [Re(terpy)2][PF6] has been prepared in a similar manner. " The electrochemistry of the products provides a convenient measure of the chemical reactivity associated with the redox processes. Thus, the one-electron oxidation of [Re(bipy)3]" is reversible at -0.33 V, whereas the Re"/Re" redox couple is irreversible and occurs at relatively low potentials (-1-0.61 V) which is consistent with the instability of [Re(bipy)3] + in solution. However, in the presence of a small coordinating molecule such as CNBu, oxidation to the rhenium(III) state is readily available by the formation of seven-coordinate complexes of the composition [Re(bipy)3(L)]. " ... 

The very useful lanthanide shift reagents, which facilitate analysis of molecular stereochemistry because of their line-broadening characteristics in NMR spectra, were studied when bound as a chelate complex to thietanes. X-Ray analysis of the adduct 3,3-dimethylthietane 1-oxide with tris(dipivalo-methanato)europium(III) [Eu(dpm)3] revealed the structure of a seven-coordinate complex (271). ... 

Reduction of Re (terpy)2Cl" is irreversible and re-forms Re (terpy)2 in a two-electron reaction. Other seven-coordinate complexes [e.g. Re (bpy)3 (CNBu ) +] are generated in this manner and exhibit similar reactivity [31]. 

In the following example, the electrochemical cleavage of the Mo—Cl bond of the seven-coordinate complex [MoCl(dtc)2(dppe)]+ (dtc = 5 2CNR2 R = Me or Ft dppe = Ph2PCH2CH2PPh2)... 

Seven-coordinate complexes that have been analyzed so far show structures consistent with the pentagonal bipyramid, with hydrogen atoms in equatorial positions. The structures of H3Re(DPPE)(PPh3)2 (31), H3Re(DPPE)2 (see Figure... 

Although many vanadium(III) complexes are unstable towards air, there are quite a few compounds in this oxidation state, most with octahedral geometry. However, remarkable seven-coordinate complexes were also characterized. 

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