Secondary bonding interactions

As proximity electrical effects are a function of the a and Or constants, and secondary bonding interactions when present may be a function of the Oi constants the effect of an ortho- or a ds-vinylene substituent may be represented by an equation including electrical and steric terms. The presence or absence of a steric effect may be ascertained in the following manner (72). There are four major cases of interest, (a) The steric effect obeys a linear free energy relationship. Then we may write the equation... 

There are two closely related structures, that is, [Zn (2-Me-5-Ph-pyr-azole)3BH (S2COMe)] (172) and [Zn (2-Me-5-(4-/-Pr-Ph)-pyrazole)3BH (S2. COEt)] (173), in which the putative Zn- -O secondary bond interactions alluded to above attain a higher level of significance. The molecular structure of the former molecule is shown in Fig. 94 from which it can be seen that three coordination sites about the zinc center are occupied by nitrogen atoms derived from the substituted pyrazole ligand. The xanthate ligand coordinates via the SI... 

Figure 143. The structure of Pb(S2COEt)2 (a) supramolecular association, and (b) highlighting the formation of the double chain along the b axis via Pb- -S secondary bond interactions.

Figure 144. Dimer formation between centrosymmetrically related pairs of Pb(S2COEt)3 anions, via Pb- -S secondary bond interactions, in the structure of [NEt4][Pb(S2COEt)3].

Figure 145. The structure of Pb(S2CO— i-Pr)2(py) showing (a) the immediate coordination geometry about the lead atom, and (b) the chain along the c axis that is stabilized via Pb S and Pb- O secondary bond interactions.

A, respectively, and Sb-S3 and Sb-S4 of 2.617(2) and 2.904(2) A, respectively. Also, unlike the phenyl analogue, significant supramolecular Sb- S secondary bond interactions are present in the structure so as to generate a chain motif shown in Fig. 154. The intermolecular Sb- -S4 distances are 3.353(2) A and the resultant double-chain is helical as it is disposed about a crystallographic screw (2j) axis. 

Figure 158. Supramolecular association, via Bi1 S secondary bond interactions along the b axis, in the structure of Ph2Bi(S2CO—r-Pr).

There are single examples of sulfur and selenium xanthates with the remaining structures to be described in this section featuring tellurium as the central element. There are a significant number of binary xanthates and, in common with these, their organotellurium xanthates feature extensive supra-molecular association, usually, but not exclusively, via Te- S secondary bond interactions. A rare example of mixed-ligand 1,1-dithiolate structure is available where the odd dithiolate ligand is a dithiocarbamate. 

Figure 159. Supramolecular structure of Se(S2COMe)2 showing the intermolecular Se-S secondary bond interactions along the a axis.

Figure 16E The structure of Te(S2CO—n-Pr)2 (a) showing supramolecular association via intermolecular Te- S secondary bond interactions giving rise to a helical chain along the b axis, and (b) viewed down the approximate axis of the chain.

Figure 167. Supramolecular structure of (4-MeOC6H4)Te(S2COMe) showing the intermolecular Te- -S secondary bond interactions along the c axis viewed (a) to emphasize the aggregated structure and (b) side on.

Figure 171. The structure of Me2Te(S2COMe)(S2CNEt2) showing dimer formation via inter-molecular Te- -S secondary bond interactions intramolecular Te- -S interactions are not shown for clarity so as to emphasize the eight-membered [TeSCS]2 supramolecular ring.

Figure 173. Dimeric self-assembly in the structure of Me2Te(S2COEt)Cl via intermolecular Te- -Cl secondary bond interactions.

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