Chromium mononuclear complexes

Non-ionic thiourea derivatives have been used as ligands for metal complexes [63,64] as well as anionic thioureas and, in both cases, coordination in metal clusters has also been described [65,66]. Examples of mononuclear complexes of simple alkyl- or aryl-substituted thiourea monoanions, containing N,S-chelating ligands (Scheme 11), have been reported for rhodium(III) [67,68], iridium and many other transition metals, such as chromium(III), technetium(III), rhenium(V), aluminium, ruthenium, osmium, platinum [69] and palladium [70]. Many complexes with N,S-chelating monothioureas were prepared with two triphenylphosphines as substituents. 

G.A. Ozin, University of Toronto In our Cr/CO matrix cocondensation experiments (Angew. Chem., Int. Ed. Eng. 1975, 14, 292), we reported evidence for the facile formation of a binuclear chromium carbonyl complex Cr2(CO)i0 or Cr2 (CCOi x which could be described as square pyramidal Cr(CO)5 weakly interacting with either a Cr(CO)5 or Cr(CO)6 moiety in the vacant (sixth) site. As a result, the infrared spectrum of this "weakly-coupled" binuclear species closely resembled that of the mononuclear fragment Cr(CO)5. I would like to ask you, whether or not you have any evidence for the existence of such a binuclear species in your Cr(CO)6 /Xe cryogenic solutions following various photolysis treatments. 

The electronic301 and magnetic properties of mononuclear chromium(III) complexes are quite well understood however there is a distinct tendency for octahedral symmetry to be invoked in cases where the true symmetry is much lower. Chromium(III) is a hard Lewis acid and many stable complexes are formed with oxygen donors. In particular hydroxide complexes are readily formed in aqueous solution, and this may be a problem in synthesis. Substitution at chromium(III) centres is slow302,303 and may well have some associative character in many cases. The kinetic inertness of chromium(III) has led to the resolution of many optically active complexes this work has been extensively reviewed.304... 

Mononuclear and binuclear chromium(III) complexes containing bispicen and related tetradentate ligands have been prepared. The isomers cis-a- (118) and -/3- (119) [CrCl2(bispicen)]+ are obtained by the methods outlined in Scheme 58495-496 but complexes with the trans geometry are unknown. 

Dinuclear dihydroxo-bridged complexes can often be obtained from the parent mononuclear complexes by the solid-state reaction Eq. (7). This was first reported by Werner (7, 11) and Dubsky (18), and it is generally the most convenient method for the preparation of dihydroxo-bridged complexes of Cr(III), Co(III), Rh(III), and Ir(III) with L4 = (NH3)4 or (en)2 [and (tn)2 in the case of chromium(III)] (67, 131, 133, 214 219). With the exception of the ammonia chromium(III) complex, these reactions are essentially quantitative and the rate of reaction follows the order chromium(III) > cobalt(III) > rhodium(III) >... 

Hydrolysis of polynuclear hydroxo-bridged chromium (III) complexes in concentrated solutions of strong acid yields the corresponding mononuclear species. Such cleavage reactions are fast in comparison with the hydrolysis in dilute acid and proceed with retention of configuration of the mononuclear entities. A few representative examples are shown in Eqs. (46)-(49) (40, 42,161, 252). 

The cleavage of polynuclear hydroxo-bridged rhodium(III) and iridium(III) complexes into the corresponding mononuclear fragments has been reported in only a few instances, but the well-established tendency of mononuclear complexes of these metal ions to undergo substitution reactions with retention of configuration indicates the possibility of analytical and synthetic applications such as described above for chromium (III). 

A comparison of the activation parameters Af/ ( j) for the ammonia and ethylenedi amine (meso isomer) complexes shows that Af/ (NH3) > Af/l(en) for both chromium(III) and rhodium(III), and that Af/ (Ir) > AHt(Rh) > AHt(Cr) for both NH3 and en. These trends are in keeping with the data for substitution reactions of the corresponding mononuclear complexes (364). 

Finally, it should be noted that this chemistry may have biological relevance. Several metalloenzymes are believed to contain more than one metal ion bound at the active site. One relevant example is the glucose tolerance factor (GTF) which is important for the metabolic degradation of glucose (398-401). GTF is a low-molecular-weight protein which contains chromium(III). Its structure is not known, but it has been suggested that the active site contains a dinuclear chromium(III) complex (401). The fact that hydroxo-bridged dinuclear chromium(III) complexes exhibit reactions which are often very fast compared with those observed for the parent mononuclear species seems to support such a proposal. 

Bis(adamantylimido) compounds, with monomeric chromium(VI) complexes, 5, 348 Bis(alkene) complexes conjugated, Rh complexes, 7, 214 mononuclear Ru and Os compounds, 6, 401 -02 in Ru and Os half-sandwich rj6-arenes, 6, 538 with tungsten carbonyls and isocyanides, 5, 685 Bis(u-alkenylcyclopentadienyl) complexes, with Ti(II), 4, 254 Bis(alkoxide) nitrogen-donor complexes, with Zr(IV), 4, 805 Bis(alkoxide) titanium alkynes, in cross-coupling, 4, 276 Bis(alkoxo) complexes, with bis-Cp Ti(IV), 4, 588 Bis[alkoxy(alkylamino)carbene]gold complexes, preparation, 2, 288... 

Mononuclear chromium(III) complexes, with halides, alkyls,... 

Excited States of Mononuclear and Dinuclear Chromium till) Complexes... 

Large numbers of mononuclear, octahedral Cr(III) complexes are known with magnetic moments close to the spin-only value of 3.87 /tb (Table 20.7) the electronic spectra of octahedral d complexes contain three absorptions due to d-d transitions (see Figure 20.18). Selected examples of octahedral chromium(III) complexes are [Cr(acac)3], [Cr(ox)3] ,... 

The chromium(ii) acetate dimer has again been used as a reductant. Cannon and Stillman found two forms of kinetic behaviour, depending on the nature of the oxidant X, but consistent with a single mechanism [equation (26)]. This involves dissociation into mononuclear chromium(n) complexes which are the effective... 

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