E-3 T-shaped

Unsaturated five membered 2-chalcogenons with two heteroatoms at 1,3-positions (6) will show versatile reactivities with acceptors, since E in 6 must be very electron rich with the formation of a stable cyclic 671 electron system (6A).27,28 A saturated ring system 7 is expected to show similar trend due to the stabilization by the allylic X-C-Y framework with 471 electrons (7A). Arduengo reported the TBP formation of 1,3-dimethyl-4-imidazoline-2-thione (8), a typical example of 6 (X = Y = NMe and E = S), with bromine (8 Br2), together with the reactions.36 

The TBP structures of 9 I2 and 11 2I2 are noteworthy, since iodine adducts of selenides are predicted to be MC based on the general rule (x(Se) (=2.48) xCO (=2.21)). This must be the reflection of the high ability of the imidazoline ring to donate electrons to Se by the formation of the stable cyclic 6ji system. It will decrease xeff of Se in 9 and 11 to give TBP with iodine. Since the electronic conditions in 10 must not be so different from those in 9 and 11, they should not be responsible for the different structures of the adducts. This working hypothesis is supported by DFT calculations.38 Therefore, the crystal packing effect must play a crucial role in determining the structures of the iodine adducts of 9-11. The reactions are followed by spectroscopic and conductometric methods.37 

The structure of 11 2IBr is (TBP, TBP), which represents the first example of a T-shaped adduct of a selenium compound with IBr.39a The anomalous bond distances in ll-2IBr (Se-Br (A)/Se-I (A) 2.897(2)/2.606(2) and 2.766(2)/ 2.629(2) for the two Br-Se-I units, respectively) have been explained on the basis of different polarities of the two bonds and on the basis of the involvement of Br in intramolecular hydrogen bondings. Reactions and/or structures are similarly investigated for A-methylthiazolidine-2(3H)-selone (12), N-methylbenzothiazole-2(3H)-selone (13), tV,7V -dimethylbenzoimidazole-2(3H)-thione (14), and -selone (15) with bromine,39b,c and more recently, for 14 and 15 with IBr.39d In this last case, the prediction of a 10-1-2 adduct for 14 IBr and a 10-Se-3 adduct for 15 IBr from the DFT calculations has been experimentally confirmed.39d 

Therefore, the donor ability of E in 16 must be larger than that in usual selenides. T-shaped structures 17 are observed for the bromine adducts of 16 (E = Se), where R, R, and R are NMe2, C6Hn, i-Pr, and /-Bu.40,41 18 (MC) are reported for 16 I2 (E = Se).42 Some II bonds in 16 I2 (E = Se) are elongated.15e,f The secondary bonds play an important role for the elongation. The results show that the structures of 16 X2 (E = Se, X = Br and I) are essentially the same as those predicted from the general rule, while 16A contributes to 16. 

T-shaped Y- E-X 3c-4e interactions occur by the attack of Y at E of polar E-X bonds. Such interactions are usually highly unsymmetric.17,43 However, the N-E-X bonds are close to symmetric in some cases. Singh and co-workers reported such interactions. Table 2 shows the bond distances and the deferences from the sum of covalent radii in 19 (E = Se), together with the angles.44 The N-E-X (E = Se) bonds are rather typical 3c-4e for X of halogens, especially for X = I. The Y-E- -X character shown in 19A contributes to the interactions in substantial amount, although the distances are affected by the nature of the Se-X bonds. Similar interactions are reported.45 48 

Therefore, the donor ability of E in 16 must be larger than that in usual selenides. T-shaped structures 17 are observed for the bromine adducts of 16 (E = Se), where R, R, and R are NMc2, CeHn, z-Pr, and 18 (MC) are reported 

---

FIGURE 3.5 Two lone pairs in an AX,E, molecule adopt equatorial positions and move away from each other slightly. As a result, the molecule is approximately T-shaped. 

For real molecules or ions, the stereochemical descriptor should be based on the nearest idealized geometry. However, some idealized geometries are closely related [e.g. square planar (SP-A), four-coordinate square pyramidal (SPY-A), see-saw (SS-4), and tetrahedral (T-4) T-shaped (TS-3), trigonal planar (TP-3), and trigonal pyramidal (TPY-3)] and care may therefore be required in making the choice. 

Only at the former structure does the lack of s orbital interaction allow them to be degenerate. The destabilisation energy due to s orbital interaction of these components increases as the distortion from D3 , increases, crudely speaking simply because they become less like the e species function at the geometry which has no s orbital interaction. Since the sum total s orbital destabilisation interaction with these flj combinations remains constant, if 2a and 4uj increase in s orbital destabilisation energy then 3 must decrease in energy to counterbalance this effect, s-p mixing of this type was a process which Walsh did not consider and thus the T shape molecule is markedly absent from his scheme.)... 

---