Eight-coordinated complexes

Octacoordination is now recognized to be common for many lanthanide and actinide complexes. Among the < -transition metal ions, however octacoordination is less frequent. The d, d, d and d configurations are known to produce eight coordinated complexes (97). In an excellent review Lvppard (92) surveyed the chemistry of the octa coordinated complexes, especially stereochemical and bonding aspects, and included most of the published material upto 1967. 

Chromium(IV) does not have any aqueous solution chemistry except for the formation of intermediates in the reduction of CrVI to Crm. Chromium(IV) compounds tend to disproportionate into Cr111 and CrVI species (equation 78) and the metal ion in this oxidation state is powerfully oxidizing towards organic compounds. An eight-coordinate complex [CrH4(dmpe)4] is known (Section 35.3.4.1). 

Another series of eight-coordinate complexes has been prepared with picolinates, AH eight-coordinate complexes of the type W(L-L)4 to date are diamagnetic, with the two electrons of the d2 configuration paired in the low-lying level of the Du dodecahedron.245 The IR electronic spectra consist of intense low energy bands (e > 104), attributed to d—nt charge transfer transitions. 

Eight-coordinate complexes with other bidentate ligands 1343... 

Figure 12-8 Seven-coordinate geometry of Fe(EDTA)(H20). Other metal ions that form seven-coordinate EDTA complexes include Fe2. Mg2, Cd2, Co2. Mn2, Ru3, Cr3+. Co3, V3, Ti3+, In3, Sn4, Os4, and Ti4. Some of these same ions also form six-coordinate EDTA complexes. Eight-coordinate complexes are formed by Ca2, Er3 , Yb3r, and Zr4. p Mizuta.

There are extensive possibilities for the formation of geometric and optical isomers in eight-coordinate complexes. Thus far. apparently only one pair has been completely characterized The diglyme [= di(2-methoxyethyl)etherl adduct of samarium iodide. Sml rO(CH2CH2OCH3)2]2. has been isolated in both cis and irons forms. The trans complex (Fig. 12.39a) has a center of symmetry Thus, the I—Sm—1 angle is exactly 180. and the molecule is a bicapped trigonal antiprism. The cis isomer (Fig. (2.39b) has ihe lower symmetry of a distorted dodecahedron with I—Sm—I angles of 92 > ... 

There are no known eight-coordinate complexes in which the lone pair occupies a coordination site, even in compounds containing bidentate ligands. 

These unique eight-coordinate complexes of the first-row transition metal ions maintain the four-membered chelate ring even in solution, where they dissociate into the tris-complex species.94 97... 

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