Valence compounds, stability

Valence Bond Isomers of Aromatic Compounds Stabilized by Trifluoromethyl Groups Kohayashi Y Kumadaki, I Acc Chem Res 14 76-82 46... 

Thiepin, as a seven-membered conjugated system with sulfur as heteroatom, is a member of the 8 7t-electron heteroannulenes which are antiaroinatic according to Hiickel s rule. In contrast to oxepin, thiepin is not stable at room temperature and no valence isomerism with an arene sulfide has been observed. Stable thiepins are obtained only when two bulky substituents, e.g. /ert-butyl, are introduced into positions 2 and 7. In benzothiepins the annellation effect of the aromatic rings contributes decisively to the stability of these compounds stability increases with an increasing number of fused benzene rings. 

M R 3 compounds of group 13 elements yielded mixed-valence compounds of the general type RM-M R 3,321 which are valence isomers of M2R4 (Section 3.07.4.1). To date, several homonuclear and heteronuclear species have been synthesized and structurally characterized and their stability was investigated by computational calculations (Table 2).322 322a 323... 

Valence Bond Isomers of Aromatic Compounds Stabilized by Trifluoromethyl Groups ... 

The alkaline-earth oxides MO are composed of M2+ and O2- stabilized by electrostatic forces. The electrostatic Coulombic energies are directly proportional to the product of the two ionic charges and inversely proportional to the interionic distance. The alkaline-earth oxides MO are 2 2 valence compounds and have the same electronic charge for M2+, but the radii of M2+ and the... 

Easy formation of Ag3F8 and its much greater stability relative to AgF3 indicate that the Ag(III) in this mixed valence compound is less electronegative than in AgF3. In... 

By further evaluation of the redox system SeBr4/SeBr2/Se/Br in aprotic solvents (Section III,A), the first examples of bromoselenates-(II,IV) containing selenium in both oxidation states +2 and +4 were prepared. Three dinuclear, trinuclear, and tetranuclear types of anions are reported in this novel class of mixed-valence compounds Se2Brg, SeaBrio , and Se4Bri2 . They are shown in Fig. 31. Similar to the bromoselenates(II), they were obtained by nucleophilic addition reactions of bromide ions within the complex reaction mixtures of selenium bromides. Evidently, the formation of Bra" is important for the stabilization of Se(II) in addition to Se(IV), shifting the equilibrium... 

We have begun a study of the stabilization of seml-conductlng and metallic oxides with other metal cations that will font covalent metal-O-Cu bonds and a two level electronic band structure. These materials will be essentially semiconductors where the conductivity arises from doping to produce mixed-valence compounds. Ue chose to begin our study with cations that adopt tetrahedral coordination and focus on how to create structures that Incorporate distorted octahedral, square pyramidal and square planar coordination of copper compatible with still other electropositive (Ionic) cations. The mixed valency Introduced by doping can then be accommodated on the copper metal and adjacent oxygen atom sites by an accompanying bond polarization around the cation with tetrahedral coordination. 

Eq. (2.56)] is determined solely by thermodynamic stability of intermediate species, which, in turn, is expressed by the equilibrium formation constant (or, alternatively, the disproportionation equilibrium constant) of the intermediate valence compound. 

Additional areas of lanthanide halide chemistry that have been reviewed include the synthesis, phase studies, and structures of complex lanthanide halides - compounds formed between one or more group 1 cation and lanthanide element halides (Meyer 1982). Halides in combination with lanthanide elements in the II, III, and IV oxidation states were considered with the chemistry of the heavier halides being emphasized. More recently the reduced ternary lanthanide halides (Meyer 1983) and the reduced halides of the lanthanide elements were reviewed (Meyer 1988). The latter review considered lanthanides in which the formal oxidation state of the cation was 2 and included hydride halides, oxide halides, mixed-valence ternary halides, and reduced halide clusters. Corbett et al. (1987) discussed the structures and some bonding aspects of highly reduced lanthanide halides and compounds stabilized by a second-period element bound within each cluster, e.g., SC7CIJ2B, SC5CI5B, YjCljC. 

When the scandium and lutetium compounds are triturated with toluene, one THE molecule is lost, resulting in [Ln(CH2Ph)3(THE)2] (Ln = Sc, Lu) of which the scandium compound has a trigonal bipyramidal structure. There is no evidence for [Yb(Bz)3(THF)3] when C6H5CH2K reacts with [Ybl3(THF)3], the mixed-valence compound [Yb°(CH2Ph)(THF)5]+ [Yb (CH2Ph)4(THF)2]- results As a family of a-alkyls obtainable for most lanthanides, the benzyls have promise as synthetic precursors, notably for phosphorus-stabilized carbenes. ... 

Ground-state electronic configuration is ls 2s 2p 3s 3p 3i 4s. Manganese compounds are known to exist in oxidation states ranging from —3 to +7 (Table 2). Both the lower and higher oxidation states are stabilized by complex formation. In its lower valence, manganese resembles its first row neighbors chromium and especially iron ia the Periodic Table. Commercially the most important valances are Mn, Mn ", or Mn ". ... 

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