2c-3e Bond Research Articles

Bond orders in three-centre bonds: Part 2. The role of d-orbital participation in sulfuranes and related systems

The simple analytical model of Mayer for the qualitative study of bond order in three-centre four-electron bonds, corresponding to the apical hypervalent bonds in sulfuranes has been extended by taking into account a d orbital on the central sulfur atom. This seems to be a useful extension of Mayer's results, which were based on the model of Rundle and Musher, using only an sp basis set. The maximal sulfur-apical ligand bond order attainable by an sp model in symmetric sulfuranes is only 0.5, whereas, according to our previous ab initio calculations, this bond order is sometimes close to the value of 1, characteristic of a usual single bond. It is shown by a simple extension of Mayer's considerations that, in the extreme case when one electron is promoted to a d orbital, the bond order due to d participation may reach the value of 0.5. At relatively small d orbital populations (0.05–0.3) the increase of the bond order has been found to be roughly equal to the d-orbital population.

Bond orders in three-centre bonds: an analytical investigation into the electronic structure of diborane and the three-centre four-electron bonds of hypervalent sulphur

Analytical calculations are performed for the bond orders in two- and four-electron three-centre model systems. It is shown that the presence of the two three-centre two-electron bonds in diborane leads to bond orders of about 0.5, not only between the “bridging” hydrogens and each boron atom, but also between the two borons; the corresponding attractive boron-boron exchange interaction stabilizes the diborane molecule against the dissociation into two BH 3 molecules. In accordance with the actual observations and Musher's theory, it is demonstrated that appreciable hypervalent bond order can be formed only with rather electronegative substituents withdrawing a considerable electron population from the central sulphur. It is concluded that the bond orders and bond lengths of such hypervalent bonds must be very sensitive to any changes in the group-electronegativity of the substituents; the increase in the electronegativity of one substituent leads to the strengthening of the hypervalent bond formed with the other, in full accord with the empirical observations.

Nuclear quadrupole resonances of α-(CH 3) 2 TeX 2 (X=Cl, Br, I) and (CH 3) 2 TeI 4

NQR spectra were observed for α-(CH 3) 2 TeX 2 (X=Cl, Br, I) and (CH 3) 2 TeI 4 at various temperatures. The two 81Br NQR lines were observed above 110 K in α-(CH 3) 2TeBr 2. The characteristic temperature dependence of the 127I NQR line in α-(CH 3) 2 TeI. can be explained by the 3c—4e bond of the linear ITeI group. The positive temperatures dependence of the lowest 127I NQR line in (CH 3) 2TeI 4 is discussed on the basis of the electron population calculated from Townes—Dailey treatment.

ヘテロ原子効果 考え方と応用

In order to rationalize and correlate a variety of chemistry exhibited by heteroatoms, “heteroatom effect”, a new term, is proposed and its meaning is defined with the emphasis on the role of an unshared electron pair. For the first row heteroatoms (N, O), heteroatom effect is confined to the effect of unshared electron pair and the latter is classified into the ability to stabilize carbocations due to stereoelectronic effect of Deslongchamps sense and the ability to coordinate with metal cations, especially with lithium ion, to effect ortho-lithiation and to generate pseudo-five membered ring intermediates for controlling stereoselectivity.For the 2nd, 3rd, and 4th row heteroatoms of IVB, VB, VIB and VIIB groups, heteroatom effect is classified into three effects on the basis of i) role of unshared electron pair, ii) role of valence expansion of the central atom, and iii) role of low lying orbital of σ*c-x. Importance of electron-rich and polarizable three-center four-electron bond is implicated to explain the geometry and reactivity of σ- and π- hypervalent molecules.

Tellurium-125 Mössbauer spectra of some tellurium subhalides

The 125Te Mossbauer spectra at 4.2 K for the crystalline tellurium subhalides Te3Cl2, Te2X (X = Br or I), Te2Br0.75- I0.25, β-Tel, and α-Tel show the presence of at least two different tellurium sites in each subhalide in accordance with their known crystal structures. The chemical isomer shifts δ(relative to Zn125mTe at 4.2 K) in four different types of co-ordination have been found to be 0.6 mm s–1 for trigonal bipyramidal tellurium atoms [Te(Te)2(Cl)2E], 0.8–1.0 mm s–1 for square-planar (pseudo-octahedral) tellurium atoms [Te(Te)2(X)2E2](X = Br or I), and 0.8 mm s–1 for two-co-ordinate (pseudo-tetrahedral) tellurium atoms [Te(Te)2E2]. The symbols in square brackets indicate the nearest-neighbour environment of the relevant tellurium atom, the symbol E being used to represent an essentially non-bonding pair of electrons. The corresponding quadrupole splittings for the four environments were 11.0, 12.1–13.7, 3.4–6.1, and 8.1 mm s–1 respectively. The bonding scheme for square-planar tellurium atoms in the tellurium subhalides is considered to involve two three-centre four-electron bonds and is thus similar to that in the isostructural and isoelectronic species [ICl4]– and XeCl4. The Mossbauer spectra at 4.2 K for the glassy phases Te2Br0.75I0.25 and TeBr2 were different from those of the corresponding crystalline compounds but again showed the existence of at least two different tellurium sites in each compound.