Bond elongation

Solvation effects on the molecular vibrations of 128 were studied by SCRF methods and by supermolecular approaches of 128 with one water molecule [97JPC(B) 10923, 98JPC(A)6010]. Correlations between the N—H (uracil) and O—H (water) bond elongations and the corresponding frequency shifts of the stretching vibrations are reported as... 

Six members of this series could be isolated in modest yields as highly air-sensitive, dark blue or dark purple crystalline solids for which analytical, spectroscopic, and single-crystal X-ray analyses were fully consistent with the side-on-biidged N2 structures shown in Scheme 102. These complexes show unusual structural features as well as a unique reactivity. An extreme degree of N = N bond elongation was manifested in rf(N-N) values of up to 1.64 A, and low barriers for N-atom functionalization allowed functionalization such as hydrogenation, hydrosilylation, and, for the first time, alkylation with alkyl bromides at ambient temperature. ... 

The attractive nature of XB causes D- -X distances shorter than the sum of van der Waals radii of involved atoms the stronger the interaction, the shorter the D- -X interaction lengths. Consistent with the rationalization of XB as an electron donation from D to the antibonding X-Y orbital [59], XB formation results in an elongation of the X-Y covalent bond. The D- -X interaction length is usually a more sensitive probe for XB strength than the X - Y covalent bond elongation (Table 3). 

The existence of a Pd° >Mo" dative bond elongates the quadruple Mo2 bond to some extent. This complex has two Pd° centers that can react with a variety of olefinic compounds, shown schematically in Figure 111 such reactions give Mo2Pd2(pyphos)4(L)2 [L = acrylonitrile, fumar-onitrile, tetracyanoethylene] in moderate yields. Unlike the previous compounds which have a strictly trans arrangement of the P-donor atoms, the last compound has a cis arrangement and thus the four metals are not linear.959... 

The structural comparison (Fig. 3) with both the twofold tris(trimethylsilyl)methyl substituted acetylene and 1,4-benzene derivatives (Fig. 1) as well as with the literature data [6a] for hexa-kis(rm.butyl)disilane [6b] containing a SiSi bond elongated to 270 pm ( ), for the linear ( ) hexa-kis(rm.butyl)disiloxane [6c] or for di(tris(trimethylsilyl)silyl)zinc [6d] is based advantageously on a model in which the two substituent half-shells are separated along their central C3 axes by spacers of different lengths. 

Table 5.1. Properties of binary B - -HA hydrogen-bonded complexes (see Fig. 5.1), showing the H-bond energy A b-h, H-bond length Rb-e, van der Waals contact distance f B---H(vdW), cind covalent-bond elongation A/sAn (relative to isolated monomer)...

Shortened M-Si bonds Long Si-X bonds Si-H contacts of 1.8-2.1A Small Si-M-Si angles in bis(silyl)hydride systems X and H substituents in the apical positions in respect to silicon atom Elongated M-H bonds Elongated M-Si bonds Normal Si-X bonds Si-H contacts of 1.7-1.8 A No regulations No regulations Normal M-H bonds... 

The aldehyde group in 197 bears a potential for C-C bond elongation reactions. For example, 198 was obtained in 89% yield by the reaction with a stabilized ylide as an E-isomer (> 95 5) (Figure 4.9). Additionaly Lewis-acid-mediated reactions are possible. The reaction of the aldehyde 197 with aUyltributyltin and TiCl4 gave the homoallylic alcohol 199 in 86% yield. 

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