Cr-C bond

Acute and Chronic Toxicity. Although chromium displays nine oxidation states, the low oxidation state compounds, -II to I, all require Special conditions for existence and have very short lifetimes in a normal environment. This is also tme for most organ ochromium compounds, ie, compounds containing Cr—C bonds. Chromium compounds that exhibit stabiUty under the usual ambient conditions are limited to oxidation states II, III, IV, V, and VI. Only Cr(III) and Cr(VI) compounds are produced in large quantities and are accessible to most of the population. Therefore, the toxicology of chromium compounds has been historically limited to these two states, and virtually all of the available information is about compounds of Cr(III) and/or Cr(VI) (59,104). However, there is some indication that Cr(V) may play a role in chromium toxicity (59,105—107). Reference 104 provides an overview and summary of the environmental, biological, and medical effects of chromium and chromium compounds as of the late 1980s. 

Chromium compounds as catalysts, 188 Chromium oxide in catalytic converter, 62 Chromium oxide catalysts, 175-184 formation of active component, 176,177 of Cr-C bonds, 177, 178 propagation centers formation of, 175-178 number of, 197, 198 change in, 183, 184 reduction of active component, 177 Clear Air Act of 1970, 59, 62 Cobalt oxide in catalytic converter, 62 Cocatalysts, 138-141, 152-154 Competitive reactions, 37-43 Copper chromite, oxidation of CO over, 86-88... 

Attempts at the carbonylation of CpCr(NO)2Me in hexane or THF at reflux resulted only in recovery of the nitrosylalkyl (105). A recent report of the reaction between CpCr(CO)3Me and L [L = PPhj, P(p-CjH40Me)3, and PMejPh] to yield trawi-CpCr(CO)2L(COMe) furnishes the first example of insertion of CO into Cr—C bonds (19b). 

In this case, the product is the fac isomer, in which all NH3 ligands are trans to the CO molecules. Ammonia does not form ty bonds to metals because it has no orbitals of suitable energy to accept electron density. Thus, the back donation from Cr in Cr(NH3)3(CO)3 goes to only three CO molecules, and the bond order is reduced even more than it is in Cr(CO)s, where back donation occurs equally to six CO molecules. There is, of course, an increase in Cr-C bond order and stretching frequency in Cr(NH3)3(CO)3 compared to Cr(CO)s. Based on the study of many mixed carbonyl complexes, it is possible to compare the ability of various ligands to accept back donation. When this is done, it is found that the ability to accept back donation decreases in the order... 

An alternative end-game has the CO insert into the Cr=C bond of the allylidenechromium compound to give a Cr complex of a ketene. Electrocyclic ring closing of the ketene would then give the product. 

Formula Cr(CO)6 MW 220.058 the CO group is bound to Cr atom through C atom Cr-C bond distance 1.909A. 

While the ultimate product of the reaction is the product of an O2 insertion into the Cr - C bond (Sect. 3.1), the first identifiable sfep is the binding of O2 to chromium. At - 45 °C, a color change of the solution from the brilliant blue of Tp Cr-Ph to a dark red indicated the formation of a new compound, which is stable at this low temperature. Monitoring the reaction by in-situ IR spectroscopy revealed the appearance of a new band at 1027 cm which shifted to 969 cm when 02 was used. These vibrational data are consistent with the formation of a chromium(lll) superoxide complex, namely Tp u.MeQy(Q2)pj (Scheme 3, top). 

Measuring the volumes of activation for several reactions of this type indicates that the An- ligand, usually in the trans position to R, weakens the Cr-C bond, thus shifting the mechanism from a pure I to an Id mechanism (105,109). For a large variety of anions AV (kA) = 10 cm3 mol-1 was measured. 

The reaction between chromium(III) alkyls and 02 is quite complicated (57) and may involve 02 insertion in the Cr—C bond to form an alkyl peroxo species, [(H20)5Cr02R]2+, analogous to [(H20)6Cr02H]2+, described in Section II,A. 

The Cr—C bond exerts a considerable (ca. 0.1 A) solid-state trans elongation (Table I), and the trans effect is implicated in the enhanced substitution lability of the trans aqua ligand. A variety of chelating systems can be used to form [L6CrR]2+ cations, including diamines (63), polyamines, macrocyclic N4 ligands, acac, bipy, and polycarboxylates such as nta, edta (64), and hedtra. 

Comparison between the half-wave potentials (equations 2 to 4) of [Cr(CNR)6](PF6)2, e.g. for R = Bu , -1.04, -0.28 and 0.84 V (versus SCE),22 with those for [Cr(CNPh)6](PF6)2, i.e. -0.35, 0.25 and 1.00 V,20 shows that alkyl and aryl isocyanides favour respectively the higher and the lower oxidation states as expected from the greater a-donor and weaker jr-acceptor capabilities of the alkyl over the aryl isocyanides. Similarly, the phosphines in the mixed ligand complexes (Table 3), 23 relative to isocyanide ligands, stabilize the Cr111 oxidation state. The great difference in the relative stabilities of Cr—C bonds in the cyano and phenyl isocyanide complexes is indicated by the magnitude of the shift (ca. 2.0 V) between the Cr(CN) "/Cr(CN)r (-1.130 V) and the Cr(CNPh)i+/Cr(CNPh)i+ reduction potentials.28... 

Fig. 11-2 Effect ol metal-ligand it bonding the Cr—C bond order is increased and the C—O bond order is decreased, (a) VB viewpoint it bond between d orbital on Cr atom and p orbital on C atom, (b) MO viewpoint it bond between d orbital on Cr atom and anlibonding orbital (jr ) on the CG ligand.

In a preceding section we saw crystallographic evidence that substitution of phosphorus ligands for carbon monoxide in Cr(C0)6 leads to a strengthening of Cr—C bonds, particularly those trans to the phosphorus groups, and this was interpreted in... 

Bond homoconjugation via a,cr-type overlap is identical with cr-conjugation, and if the latter occurs it cannot be distinguished from the former (Scheme 5). As such it is not reasonable to use the term cr,c-bond homoconjugation. 

More recently a benzylchromium complex has been found to insert C02, though in this case the benzyl ligand is also bound by an o-NMe2 linkage 143a). Thus, Cr(CH2C6H4-o-NMe2)3 inserts one C02 into a Cr—C bond to yield a product formulated with a bidendate carboxylate. 

The X-ray structure is of carboxonium ions 266560 and 267561 have been determined. The main features, a substantial lengthening of the C—O bonds and a shortening of the Cc=cr C bonds, were observed upon protonation of the starting ketones. 

The binding properties of the t74 2-5H-benzocycloheptene ligand resembles that of t 4 CH-1,3-cycIoheptadiene ligand in 39n. In place of the C—H—Cr bridge, a weak interaction with C-10 and C-ll is recognized. A comparison of the Cr—C bond lengths of 40 with those of 38o shows a surprising similarity for the coordinated diene unit, and for C-5 in 38o with C-10 in 40. 

The structures of 56a-56e and 56g have been elucidated by H- and 13C-NMR spectroscopy, and for 56a also by X-ray structure analysis (Fig. 8). The structural properties of 56a have similarities to those of the isomeric 46a. The diene portion is normally bound to the metal with Cr—C distances of 220 and 230 pm. For the isolated C=C double bond, Cr—C bond lengths of 241.8 and 271.6 pm were found. 

Although the active species containing a Cr C bond is difficult to determine, an oxidative addition of ethylene to low-valence coordinatively unsaturated chromium ions is proposed to produce such species [219] ... 

Another possible pathway for the formation of Cr-C bonds involves allyl species [233-235] ... 

The compounds Co(NH3)t>Cr(CN)8 and Cr(NH3)6Co(CN)6 are termed coordination isomers. The first contains Co— N and Cr—C bonds whereas the second contains Cr—N and Co—C bonds. There are four additional coordination isomers related to these two compounds (Exercise 2). Pairs of compounds such as PtCI2(NH3)2 and Pt(NH )4+PtCl are sometimes called polymerization isomers, but may just as logically be regarded as coordination isomers in which both centers of coordination hold the same atom. 

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