Chromium complexes tetranuclear

Chromatography cyclophosphazenes, 21 46, 59 technetium, 11 48-49 Chromites, as spinel structures, 2 30 Chromium, see Tetranuclear d-block metal complexes, chromium acetylene complexes of, 4 104 alkoxides, 26 276-283 bimetallics, 26 328 dimeric cyclopentdienyl, 26 282-283 divalent complexes, 26 282 nitrosyls, 26 280-281 trivalent complexes, 26 276-280 adamantoxides, 26 320 di(/ >rt-butyl)methoxides, 26 321-325 electronic spectra, 26 277-279 isocyanate insertion, 26 280 substitution reactions, 26 278-279 [9]aneS, complexes, 35 11 atom... 

Metal chloride complexes (59) with structures analogous to that of the samarium complex (50) have been obtained by the reaction of (57) with titanium or zirconium tetrachlorides via elimination of LiCl. Equimolar reaction of (57) and CrCl2(thf)2 has been shown to yield binuclear chromium complex (60), whilst reaction with a twofold excess of CrCl2(thf)2 affords a partially substituted tetranuclear complex (61). ... 

A specific example is provided by the tetranuclear chromium(III) complex ion [Cr4(en)6(OH)6]+6. To a first approximation each Cr atom... 

The three different tetranuclear structures which have been observed in the crystalline state are the two compact structures 6 and 8 and the chain structure 7a. Structure 6 is found in [Co4(NH3),2(OH)6]C16-8H20 and its amine analogs (52 59). The analogous ammonia and ethylenediamine chromium(III) complexes Cr4(NH3)12(OH)66+ and Cr4(en)6(0H)66+ have been characterized quite recently (40, 41, 42, 60). Structure 7a has so far been observed (42) only in a chromium(III) amine complex, Cr4(en)6(OH)66+, but, as discussed in Section IV, both structures 7b and 7c are possible structures for the tetranuclear aqua chromium(III) species. Structure 8 is known from the so-called rhodoso complex, Cr4N12(OH)66+ [N12 = (NH3)12 or (en6] (61, 62). 

Only a very few polynuclear complexes containing more than two chromium(III) centers have been studied so far. However, magnetochemical and inelastic neutron scattering studies, heat capacity measurements, and emission spectroscopy have been reported for various tetranuclear species (40,142 151). Two review articles dealing with the spectroscopic and magnetic properties of chromium(III) oligomers have recently appeared (127, 128). 

The circular dichroism (CD) spectra of optically active di-, tri-, and tetranuclear complexes of chromium(III) and cobalt(III) have been reported and used to establish the complexes absolute configurations (55 59, 111, 115, 116, 152-157). The changes in circular dichroism resulting from ion pairing have been studied for the tetranuclear hexol Co j(OH)2Co(NH,)4J, h+ and have been shown to be attributable to the vicinal effect of the chiral oxygen centers produced stereospecifically by the ion-pair formation (56). For a series of trinuclear cobalt (III) amine complexes, cis-Co(CN)2[(OH)2Co(N4)2 J3 +, it was shown that the main CD contributions due to the two chiral Co(OH)4(CN)2 and Co(N4)(OH)2 centers are additive (155). In the case of the related tetranuclear complex Co((OH)2Co(en)2J,< + this postulate of additivity of CD spectra proved unsatisfactory (57). 

There is no evidence for any toxic effects of chromium(III), which is an essential trace element in mammals (required daily intake 50-200 /tg) and participates in glucose and lipid metabolism. In the low-molecular-weight Cr binding substance (LMWCr), an oligopeptide, a tetranuclear Crm carboxylate complex may be present.44... 

---