Effects of Substituents on Bonding and Structure

Transition Metal Metallocenes 1. Molecular Orbital Calculations and Spectra 

Ligand field parameters for [Fe(C5XH4)2] (X = H, CH3, Br, Cl) have been derived from photoabsorption spectra.88 The ligand field splittings of the Fe d orbitals follow those expected from the electron donating properties of the substituents e.g., chlorine or bromine substitution increases the A, ( 2g-alg) splitting (by —0.16 and —0.28 eV, respectively) and decreases the A2 (e]g- lg) splitting (by —0.22 and 0.32 eV) (see Fig. 

Transition Metal Metallocenes with Distorted Structures 

Metallocene M-C(ring) (avg., range) Displacement of /p.v carbons M-ring centroid M-ring normal Ref. 

Among lanthanide metallocenes, both [Eu(C5(SiMe3)2H3)2] and [Yb(C5 (SiMe3)2H3)2] are polymeric, despite the steric bulk of their cyclopentadie-nyl rings.110 Monomeric metallocenes of each metal are known with penta-methylcyclopentadienyl ligands, however.111 

Metal-Carbon Distances in Linear Main-Group Metallocenes 

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Interesting stabilization and structural trends have been noted using MP2/6-31G calculations on the effect of substituents on imines. The data below give AE for the isodesmic reaction and show that stabilization tends to increase with Xbe 1 gi (>up electronegativity of the substituent. The X—N=CH2 bond angle decreases with Xbe-... 

The effect of substituents on colour in substituted anthraquinones may be rationalised using the valence-bond (resonance) approach, in the same way as has been presented previously for a series of azo dyes (see Chapter 2 for details). For the purpose of explaining the colour of the dyes, it is assumed that the ground electronic state of the dye most closely resembles the most stable resonance forms, the normal Kekule-type structures, and that the first excited state of the dye more closely resembles the less stable, charge-separated forms. Some relevant resonance forms for anthraquinones 52, 52c, 52d and 52f are illustrated in Figure 4.3. The ground state of the parent compound 52 is assumed to resemble closely structures such as I, while charge-separated forms, such as structure II, are assumed to make a major contribution to the first excited state. Structure II is clearly unstable due to the carbocationic centre. In the case of aminoanthraquinones 52c and 52d, donation of the lone pair from the... 

Absolute rate constants for reaction of MeOH with a wide variety of other transient silenes have also been reported, but with the main goal of quantitatively defining the effects of substituents on the kinetic and/or thermodynamic stability of the Si=C bond in various structural situations. These data are discussed in Section III.C of this chapter. 

As indicated already, the optical transition energy are an extremely sensitive probe for the electronic and steric properties of the three-electron-bonded species and their respective relative contributions. However, the effect of substituents on the optical transitions becomes of much lesser importance in intramolecular radical cations derived from open-chain dithianes (type 7-9). Changing the terminal substituents in R-S-(CH2)3-S-R from methyl to isopropyl results in a just 15 nm change (440 vs. 455 nm), i.e., structure clearly appears to be the dominating parameter. This is fully corroborated by the pulse radiolysis results on 2-substituted-l,3-dithiacyclopentanes.l23 As mentioned already, the radical cation (11), derived from 1,3-dithiacyclopentane (12), is very unstable if formed at all ( niax > 650 nm). The analogous radical cation generated upon oxidation of l,3-dithia-2,2-dimethylcyclopentane (13), on the other hand, exhibits a pronounced and blue-shifted absorption at 610 nm as well as a considerable kinetic and thermodynamic stability. 

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