3 center 4 electrons bond

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1. 3 Center 4 Electrons Bond

2. The Valence Shell Electron Pair Repulsion (VSEPR) model has long been used successfully to predict the geometry of various main group molecules. H exa coordinated AB 6 -type main group molecules belong to an important class of compounds. This type of molecule possesses six electron pairs in the central atom . Similarly the penta coordinated AB 5 -type molecules possessing five electron pairs are predicted to have trigonal bipyramidal structure. IMPORTANCE

3. According VSEPR model hexa coordinated AB 6 -type molecules have Octahedral structure. Coordinated by six ligands. Two d orbitals are required to participate in bonding. As a result, two electrons in the s and p subshells are promoted to higher-level d orbitals forming six sp 3 d 2 hybrid orbitals in the central atom. Previous Knowledge About AB 6 -type Molecules

4. According VSEPR model penta coordinated AB 5 -type molecules possess Five electron pairs Trigonal bipyramidal structure Central atom coordinated by five ligands One d orbital is required to participate in bonding. As a result, one electron in the s or p subshell is promoted to a higher-level d orbital forming five sp 3 d hybrid orbitals in the central atom. Previous Knowledge About AB 5 -type Molecules

5. Two factors which disfavor the utilization of d orbitals and have made d-orbital participation in bonding in main group compounds a controversial issue. Energetically unfavorable s 2 p n d → s 1 p n-1 d 2 [n = 3(P), 4(S), or 5( Cl , Br. I)] excitation, which requires a large promotion energy. d orbitals in nonmetals are heavily shielded by the more penetrating s and p electrons and extremely diffuse. As a result, they have poor overlap with the orbitals of neighboring atoms. Factors which disfavor the utilization of d orbitals

6. SF 6 and PF 6 - anion are often chosen as representatives of AB 6 -type main group molecules. Six fluorine ligands approach to the central sulfur or phosphorus atom along the x, y, and z axes forming an octahedral structure. Bonding in the AB 6 -type Molecules

7. Structure of SF 6

8. Electronic Configuration of Sulphur is 1s 2 ,2s 2 ,2p 6 ,3s 2 ,3p x 2 ,3p y 1 ,3p z 1 Electronic Configuration of Phosphorus is 1s 2 ,2s 2 ,2p 6 ,3s 2 ,3p x 1 ,3p y 1 ,3p z 1 Electronic Configuration of Florine is 1s 2 ,2s 2 , 2p x 2 ,2p y 2 ,2p z 1 Electronic Configuration of S,P and F

9. Ligand’s p orbitals have effective overlap with all the orthogonal sulfur or phosphorus 3p (3p x , 3p y , 3p z ) orbitals in six orientations (±x, ±y, ±z). A filled 3p subshell in the central atom (achieved readily by s-p transition ) is required, because the entire S.F or P.F bond distances in SF 6 or PF 6 . are equal. Bonding in the AB 6 -type Molecules

10. Six S.F bonds in SF 6 result from the overlap of F(p 1 ).S(3p x 2 ).F(p 1 ) F(p 1 ).S(3p y 2 ).F(p 1 ) F(p 1 ).S(3p z 2 ).F(p 1 ) Six P.F bonds in PF 6 - are formed by the overlap of F(p 1 ).P(3p x 2 ).F(p 1 ) F(p 1 ).P(3p y 2 ).F(p 1 ) F(p 1 ).P(3p z 2 ).F(p 1 ) Bonding in the AB 6 -type Molecules

11. Bonding in the AB 6 -type Molecules

12. The halogen pentafluoride XF 5 (X = Cl , Br, or I) belongs to the AB 5 E type molecule. A square pyramidal structure with a lone pair in the central halogen atom. In all the XF 5 molecules the basal X.F b bonds (all are equivalent) are about 0.1 Å longer than the axial X.F a bond. This shows that bonding in basal and axial positions must be different. Bonding in the AB 5 E-type Molecules

13. Bonding in the AB 5 E-type Molecules

14. Electronic Configuration of Chlorine is 1s 2 ,2s 2 ,2p 6 ,3s 2 ,3p x 2 ,3p y 2 ,3p z 1 Two sp hybrid orbitals could form on the central halogen atom pointing toward the axial positions. The shorter axial X.F a bond is reasonably formed by the overlap of an sp orbital with the fluorine p orbital. T he lone pair resides in another sp orbital in the opposite axial position. Bonding in the AB 5 E-type Molecules

15. Two sets of the longer, linear basal F b .X.F b bonds are reasonably formed by the overlap of the unhybridized , orthogonal p x and p y orbitals, respectively, with the terminal fluorine p orbitals leading to a three-center, four-electron bond. Bonding in the AB 5 E-type Molecules

16. Xenon tetrafluoride XeF 4 is an example of AB 4 E 2 -type molecules. A square planar structure with two lone pairs in the central xenon atom. In XeF 4 two sp hybrid orbitals could reasonably form on Xe pointing toward the axial positions and holding a lone pair in each of them. The two sets of linear F.Xe.F bonds are formed as three-center, four-electron bonds, the same as formation of the basal F b .X.F b bonds in the above-discussed halogen pentafluorides XF 5 . Bonding in the AB 4 E 2 -type Molecules

17. Bonding in the AB 4 E 2 -type Molecules

18. An alternative model that does not involve d orbitals in bonding in the pentacoordinated AB 5 -type molecules such as phosphorus pentahalides PX 5 (X = F, Cl ). Instead , three sp 2 hybrid orbitals (each contains a single electron) are formed on the central atom overlapping with ligand orbitals in the equatorial positions Longer axial bonds are formed via the overlap of the unhybridized p orbital (containing two electrons) on the central atom and the two terminal ligand orbitals, resulting in a three-center, four-electron bond . Bonding in the AB 5 -type Molecules

19.      Bonding in the AB 5 -type Molecules

20. Bonding in the AB 5 -type Molecules