Divalent state stabilization energy

Values for the important index of reactivity for silylenes, DSSE (= divalent state stabilization energy) defined as follows ... 

This article starts with a discussion of the parent hydrides, MH (M = C, Si, and Ge, n = 2-4), highlighting the major differences between the methane series and the silane and germane series. Some attention is paid to substituent effects on bond energies and geometries, and we introduce the important notion of the divalent state stabilization energy (DSSE). After laying this foundation, Si=C and Ge=C compounds are reviewed. This is followed by Si=Si, Ge=Ge, and Ge=Si doubly bonded structures and their isomers. Finally, triply bonded Si==C, Si=Si, and Ge=Ge compounds are briefly considered. 

A. Bond Dissociation Energies and Divalent State Stabilization Energies... 

Sequential Bond Dissociation Energies, D, and Divalent State Stabilization Energies, DSSE, of Monosubstituted Silanes and Methanes ... 

Preferred Values for Divalent State Stabilization Energies of RR M... 

Divalent state stabilization energies are not easy to come by, as they require knowledge of both the first and second BDE, and the reactive intermediates MR2 are not trivially characterized. Quantum mechanical studies are certainly ahead of experiment in this area, and we can combine the results of two separate studies, one by Coolidge and Borden (109) and the other by Luke et al. (88), to assemble a small list of DSSEs for monosubstituted carbenes and silylenes. Specifically, Coolidge and Borden determined the effects of substituents, X, on the stability of methyl and silyl radicals through determination of the heat of reaction... 

Fig. 3. Isomeric energy differences of ethane and derived species in terms of bond dissociation energies (D), n bond energies ( > ), and divalent state stabilization energies (DSSE).

The divalent state stabilization energy (DSSE) is seen to be a particularly useful concept. It arises naturally from the comparison of X2M=MX2 XjMMX energy differences and leads to simple and reasonably accurate approximation schemes for the determination of doublebond dissociation energies and isomeric energy differences. 

A more direct indication of the increased stability of the divalent state relative to the others derives from the patterns in the individual bond dissociation energies. Specifically, it is generally observed that the second BDE in an SiX4 compound is considerably smaller than the first. Walsh (99) was the first to point this out, and he has defined the difference between the first and second BDE to be the divalent state stabilization... 

When the f sub-shell is less than half-filled the stabilization of the divalent state (f"" " ) relative to the trivalent state is more rapid for the lanthanides than for the actinides. But the behaviour is reversed in the second half of the series. In part, this must be due to the low binding energy of the 6 d electrons compared to the 5 d. 

The situation is noticeably improved beyond Am, but the progressive stabilization of the divalent state soon introduces yet another complexity, beginning possibly already with Cf. In any case, an accurate number for the cohesive energy remains the single most important parameter that can be provided for both the experimentalist and theoretician. From this value the complete thermodynamics can be eventually derived, giving a firm data base from which to draw meaningful conclusions about the physics and chemistry observed. 

Because of the increased separation of the valence s and p orbital energies, the stability of the divalent state increases down the group. Thus, whereas divalent carbon (carbenes) and silicon (silylenes) compounds are generally unstable, there are many relatively stable divalent compounds of Ge, Sn, and Pb compounds. Divalent Ge and Sn compounds, however, are strong reducing agents. By contrast, divalent Pb... 

The divalent state is of major importance and has attracted the interest of many experimenters since 1967 when the appreciable stability of this state was first recognized in the actinides. The ni - ii reduction potential of einsteinium was first estimated to be - 1.6 V from the lowest-energy electron-transfer band [57]. A later estimate of - 1.21 V was given for chloroaluminate melts [58], as well as another estimate of the standard potential of —1.18 V [54]. Mikheev and co-workers identified Es(ii) from the co-crystallization of einsteinium tracer with SmCl in an ethanol solution [59]. Einsteinium was only partially reduced to the II state by SmCl2, which allowed them to conclude that the standard reduction potential ofEs " was close to that of Sm ", or —1.55 0.06 V [60]. An ionic radius of 0.105 nm was estimated from the radius of maximum electron density obtained in Hartree-Fock calculations, which was then corrected to obtain the crystalline radius by an empirical proportionality constant [61]. 

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