Lone-electron pair, activity

The free selenite ion has a pyramidal shape (C3v symmetry) owing to the lone electron pair at the selenium atom. Thus, the Se032- ion can be treated as a pseudo-tetrahedral anion and the lone electron pair often acts as an invisible ligand within the crystal structures of selenites. This observation is called the stereochemical activity of the lone electron pair and it will turn out as one of the... 

Table VIII lists some of the more important physical properties of TeF4. In the orthorhombic crystals, each tellurium atom is surrounded by three terminal and two bridging fluorine atoms, arranged at the apices of a distorted square pyramid. The square-pyramidal units are linked by cis-bridging atoms into endless chains with a bridge angle of 159°. The nearest intermolecular contacts to the tellurium atom are 2.94 and 3.10 A, so that there are no other significant interactions. This geometry is in accordance with the steric activity of the lone electron pair at the tellurium atom. Figure 6 shows the atomic arrangement (54).

Fig. 27. Optimal hydrogen bond stereochemistry in the active site of carbonic anhydrase II. Lone electron pairs are represented by stippled dumbbells.

Water, in its reaction with the alkali- and alkaline-earth metals, resembles ammonia, but the complexes with the halides of the platinum metals are different. The water molecule has two lone pairs of electrons, but these pairs seem to be less active in complex formation. There are many cases in which from the magnetic moment it can be concluded that the hydrates are still ionic, whereas in the corresponding NH3 complex there is covalency, the NH3 molecules sharing their lone electron-pairs with the metal atom. 

The activity of the lone electron-pair decreases if the compound becomes more ionic, decreasing in the sequence... 

The chemistry of 1,3-diketone complexes of nonmetallic elements (B, Si, Ge, Sb, Te) has been reviewed.68 Condensation of acetylacetone with TeCl4 yields Te(CH2COCH2COCH2)Cl2 (8a), which may be reduced to Te(CH2COCH2COCFI2) (8b). The crystal structures of these compounds demonstrate an unusual coordination of tellurium to the terminal carbons of the diketonate.61 72 In the dichloride complex, Teiv exhibits a distorted trigonal bipyramidal geometry with a stereochemi-cally active lone electron pair in the equatorial plane. In S2(MeCOCHCOMe)2 the acetylacetonate methine carbons are bonded to a disulfur unit.73... 

In Sn(PhCOCHCOMe)2, divalent tin exhibits essentially a trigonal bipyramidal coordination geometry with the stereochemically active lone electron pair in an equatorial site.115 For tetravalent... 

Acetyl migration occurs in the reaction of Tl(MeCOCHCOMe) with Pms Pd(PhCN)2Cl2, which yields an A-acetyl-jS-ketoimine chelate (17).471 Mossbauer data indicate that Sn(MeCOCHCOMe)2 has a trigonal pyramidal geometry with one oxygen atom from each acetylacetonate ligand in an axial position.472 The equatorial lone electron pair is both stereochemically and chemically active. Complexes in which the metal acts as the donor rather than the acceptor are formed by photolysis of Sn(MeCOCHCOMe)2 and Group VI metal hexacarbonyls in THF. The products, Sn(MeCOCHCOMe)2M(CO)5 (M = Cr, Mo, W), evidently contain tin-metal bonds. The low... 

A conceptual question is whether simple octahedral or even tetrahedral complexes that have a dynamic Lewis-basic pendant group, donating a pair of electrons to the metal center, are suitable for the production of an elastomeric polypropylene. As shown in Scheme 7, a dynamic equilibrium may take place between a tetrahedral and an octahedral configuration (X=halide, E=donor group with a lone electron pair, R=C, N, P, or other anionic bridging group). (A plausible frans-octahedral complex, which can be formed in this type of dynamic process, is unable to perform the olefin insertion and has no catalytic activity [5,20,80,81].)... 

Amines (Table 2) are relatively strong Lewis bases and are known to coordinate to metals (see Ammonia N-donor Ligands) in a multitude of transition metal complexes. On an HDN catalyst, they are also likely to coordinate through the nitrogen lone electron pair to a metal site as discussed in Section 3.2, this mode of coordination can activate the hydrocarbon R group in R3N amines in ways that lead to C N bond cleavage. 

A recent study on the reactivity of the parent benzo-l,2-dithiolan-3-one 1-oxide 79a (R = R = H) and o- and /i-substituted derivatives 79b-g with -propyl thiol in acetone/water mixture (7/3) was prompted (Table 5) by the observation that the DNA-cleaving activity and antitumor activity of leinamycin 5 depends, in part <2005JOC6968>, on initial thiol attack on its l,3-dioxo-l,2-dithiolane functionality. Experimental results have proved that the presence of chlorine as an electron acceptor in the /i ra-position relative to the sulfmyl sulfur S-1 of precursor 79e and ortho-substituents with lone electron pairs in the case of precursors 79b and 79d are responsible for increased product formation of polysulfanes 80 and 81. A rationale in terms of substituent effects, operating through-space and through-bond of the intermediates a and b, respectively, was suggested. In other words, the reaction is favored by ortho-substituents with lone pair electrons next to the dithiolanone-oxide (S-1) reaction center or a decrease of the electron density at the /i ra-position. 

We proposed that the synthesis of 12 proceeded via the mechanistic sequence depicted in Scheme 3. A key step in this sequence was the abstraction of the proton in the 2 -position by the alkoxide ion derived from the 14-hydroxide of the dithiane derivative 10. Previous reports have described the participation of this 14-OH group in the formation of naltrexone derivatives [13]. In our scheme, the 14-OH group of naltrexone (1) is activated intramolecularly by the lone electron pair on the 17-nitrogen then the resulting 14-OH extracts the axial hydrogen from the 7-position of naltrexone (1). This facilitates enolization, and enol acetate is produced with Ac20 in pyridine at room temperature (Scheme 1). Consistent with this proposal, we found that the key intermediate 15 was isolated... 

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