Low-Oxidation-State Compounds

Carbonyl and other Low Oxidation State Compounds.— The full report of the polaro-graphic study of redox phenomena in the vanadium-bipyridyl system has been pub- 

Agreement between experimental and calculated binding energies in V(CO)g has been obtained with a model in which the formal charge on V is + 0.660 and those on C and O are —0.693 and —0.583, respectively. Similar agreement for CpV(CO)4 necessitates the allocation of an anomolously high formal charge to the V atom.  

The formal charge on the vanadium in CpVC, , obtained in this work (-I-0.464) is a good deal lower than that reported previously (see also Vol. 4,p. 45). 

Electronic structural and photochemical studies of metal carbonyls have been extended to the relatively little explored area of the carbonylate anions [M(CO) ] (M = V, Nb, or Ta). (see also Vol. 4, p. 46 and Vol 5, p. 36). Overlap of the emission and absorption bands, and the energy dependence of these bands on the central metal atom support an - Tig t2g eg) assignment for the emission. 

Saji and S. Aoyagui, J. Electroanalyt. Chem. Intetfacial Electrochenu, 1975, 63, 405. 

Carbonyl and other Low Oxidation State Compounds.—Tris-(2,2 -bipyridyl)vana-dium(O) solutions in DMF exhibit three one-electron reduction waves corresponding to VL3 - VL3 (L = 2,2 -bipyridyI) VLJ - VL and VLf -VL, and two oxidation waves corresponding to VL3- VL3 and VLJ - VL3+ whose half-wave potentials differ by only 0.10 V.342 The difference for the isoelectronic chromium complexes is 0.49 V. An explanation has been offered for the small difference between the vanadium potentials in terms ol the possible diamagnetism of the d4VL3 species arising from a substantial splitting of the l2g orbitals. 

The spin-only value, 2.80 + 0.17 BM, for the magnetic moment of the V1 cation of V(mesitylene)+ cannot be reconciled with a ligand-field model, even allowing for very substantial distortions from Cmv symmetry.10 

341 Gmelin s Handbook of Inorganic Chemistry System No. 48, 49, 50, Index 8th edn. 

The last 25 years have seen the synthesis of a number of remarkable aluminum and gallium compounds containing the metals in the -K and -Ell oxidation states. Although some of these compounds do not contain true low-valent metal centers, others do contain 

For stable products, the R groups should ideally be sterically hindered aryl or alkyl groups examples follow in the discussion below. A typical example of an (RE) oligomer is the following  

Note that the six bonds linking the vertices of the tetrahedron are not normal two-electron bonds but are part of a delocalized bonding network the tetrahedron is held together by eight valence electrons, two from each group 13 element. 

The tetraorganodigallanes, Ga2R4, can be further reduced to digallane anion radicals, a Ga3 ring, and even a formal digallyne (Su, J. Li, X.-W. Crittendon, R. C. Robinson, G. H. J. Am. Chem. Soc. 1997,119, 5471-5472), as shown below  

For the last product, we have used the triple-bonded digallyne formulation simply to highlight the low coordination number of the gallium centers. An alternative formulation would be to have a lone pair on each gallium that is only partially involved in a -overlap. Quantum chemical studies also suggest a bond order closer to two than to three  

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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. 

In all the cluster compounds discussed above there are sufficient electrons to form 2-centre 2-electron bonds between each pair of adjacent atoms. Such is not the case, however, for the cationic bismuth species now to be discussed and these must be considered as electron deficient . The unparalleled ability of Bi/BiCb to form numerous low oxidation-state compounds in the presence of suitable complex anions has already been mentioned (p. 564) and the cationic species shown in Table 13.12 have been unequivocally identified. 

Replacement of ligands in C3H5MoCl(CO)2(NCMe)2 by isocyanides has given the substituted products C3H5MoC1(CO)2(CNR)2 (R = alkyl) and C3H5MoC1(CO)(CNBu )3, and the reduced products [MoC1(CNBu )4]2 and m-Mo(CO)2(CNR)4 (R = Me, Et). No rationale for the loss of allyl and allyl chloride in the latter two cases was proposed (206). These reactions are rare examples of the formation of low-oxidation state metal-isocyanide complexes via reductive elimination of allyl or allyl chloride from metal-allyl species. The potential applications of mono-, bis-, and tris-n-allylic systems as precursors to low-oxidation state compounds remain to be explored. Substitution and simultaneous reduction of Mo(SBu )4 also occurred on reaction with CNBu to give Mo(SBu )2(CNBu )4 (207) (see Section IV,D,2). 

Many low oxidation state compound of the heavy transition metals display metal-metal bonds... 

Ga and In form a number of mixed valent and low oxidation state compounds... 

Whereas discrete low oxidation state compounds of the transition metals are common in organometallic chemistry, the most typical series of examples being the transition metal carbonyl compounds see Carbonyl Compound), there are at present significantly fewer classes of compounds containing main group elements in low oxidation states. An... 

In compounds, the elements of group 15 (pnictogens, Pn) are typically found in either the -1-3 or - -5 oxidation state. Low oxidation state compounds are known for group 15 elements in the -1-2, - -1, 0, -1, and -3 oxidation states examples of compounds exhibiting common structmal types are shown in Figme 6. 

The Photochemistry of Transition-metal Organometallic Compounds, Carbonyls, and Low-oxidation-state Compounds... 

As in last year s Report, the photochemistry of each transition metal is treated systematically. Transition-metal organometallics, low oxidation-state compounds, and porphyrins are considered in later sections. 

T he expectation that, by analogy to phosphines, thioethers should function as tt acceptor ligands and thereby stabilize low oxidation state compounds, led several investigators to try to synthesize thioether complexes of rhodium (I). Walton (I) treated [Rh(DTH)Cl2]Cl (DTH = CH3SCH2CH2SCH3) with ethanolic potassium hydroxide, a reducing system developed by Chatt and Shaw (2), but he failed to obtain a complex of the expected type. Attempts to obtain rhodium(I) derivatives by reducing [Rh(DTH)2Cl.]Cl with sodium borohydride or by electrochemical methods were equally unsuccessful. 

Wang, Y. Robinson, G. Carbene Stabilization of Highly Reactive Main-Group Molecules, Inorg. Chem. 2011, 50, 12326-12337. A summary of an ingenious route to low-oxidation-state compounds pioneered by Robinson and his coworkers. 

Although it is advantageous to use metals in an oxidation state in the catalytic cycle, there can be problems with using low oxidation states. For example, the low oxidation state compound of palladium Pd(PPh3)4 is susceptible to oxidation. This problem can be overcome by using a more stable palladium(O) compound. 

One of the most interesting aspects of the work, as noted earlier, is that low oxidation state compounds are often produced by the dissolution of a metal anode. A list of some such syntheses is given in Table 1, and the appropriate papers should be consulted for details. The particular interest in Main Group metals in our laboratory has again lead us to concentrate on the elements Ga, In, Tl, Sn and Pb, although the early syntheses on copper species [25], of CrBr3 [15], and of Thl2 [28], show that these are examples of interest in transition and heavy element chemistry. 

The coverage of group 2 homobimetallics that is given here is largely restricted to the chemistry of isolable molecular compounds with metal-metal bonds. With that said, a brief summary of metal- metal bonded species that are only fleetingly stable in the gas phase, or at very low temperatures, is warranted and is addressed first. It should be noted that systems containing homonuclear group 2 metal-metal bonds are by definition low oxidation state compounds. A number of excellent recent reviews have covered the known chemistry in this area [70-75], and the reader should consult these for more detailed information. 

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