Back Bonding and its application in chemistry

This presentation tells about the back bonding and its application in the chemistry

1. BACK BONDING AND ITS APPLICATION PRESENTED BY LILUPTA KUAMR GOCHHAYAT CH/21/23 M.SC 3 RD SEMESTER DEPT. OF CHEMISTRY FAKIR MOHAN UNIVERSITY , BALASORE

2. C0NTENTS

3. What is back bonding ? Back bonding, also know as back donation, is a type of chemical bonding that occurs when electron move from one atom to another in a compound. It happens when one atom has lone pair of electrons and another atom has an empty orbital nearby.

4. Driving force of back bonding Completion of octate Delocalisation of charge Condition of back bonding Presence of electron doner atom Presence of electron acceptor atom Either doner or acceptor must be from second period

5. FACTORS AFFECTING BACK BONDING 1.Types of 0rbitals and Size Differences Generally the order of overlapping is 2p pi -2p pi >2p pi-3d pi >2p pi-3p pi 2p pi-3d pi overlapping is more stronger than 2p pi-3p pi,because in 2p pi-3d pi,due to slight bending d- orbital,overlapping becomes effective comparing to 3p pi-2p pi where both orbitals are parallel to each other. Again when involved d-orbital is compressed(for any reason) the size becomes smalland which in turn becomes less diffused,resulting better overlapping

6. For example here Ge atom attached with electronegative N-atom , which compressed the d-orbital of Ge atom . So that the overlapping becomes effective, resulting greater Back Bonding. 2. HOMO-LUMO Gap The gap between Highest Occupied Molecular Orbital and Lowest Unoccupied Moleculer Orbital influences the extent of back bonding. Generally the gap is inversely proportional to back bonding i.e greater the gap lesser will be the backbonding and vice-versa Let us consider the above statements by taking examples;

7. SiO 4 - PO 4 3- SO 4 2- ClO 4 - O.S: +4 +5 +6 +7 Si +4 P +5 S +6 Cl +7 O (HOMO) (LUMO) As the energy gap is lowest in case of ClO 4 - , So effective overlapping will take place resulting greater backbonding . Oxidation States also play an important role in deciding extent of back bonding. e.g compairing the extent of back bonding in MnO 4 - ,MnO 4 2- and MnO 4 3- , MnO 4 - Should have highest back bonding interaction. Because Mn in its highest oxidation state,resulting low HOMO-LUMO Gap.So in other word extent of back bonding is directly proportional to oxidation state.

8. 3. Electronegetivity Differences Electronegetivity differences also affect the extent of back bonding. Generally electronegativity differences inversely proportional to the extent of back bonding . B F B O B N ELECTRONEGETIVITY DIFFERENCE : > > STRENGTH OF PI BACK BONDING < < 4. Percentage of P-Character in Lone pair Increasing percentage p-character in lone pair back bonding also increase. When bulkier group attached with the cental atom the bond angle also increase. And bond angle directly proportional to the percentage s-character of the bond .

9. With increasing s- character in bond increases the back bonding overlapping Reason-1 As percentage s-character increases in bonding then percentage s-character decreases in lone pair which in turn increases the percentage p-character in lone pair . So that the back bond overlapping will be good . Reason-2 Due to the increase in s-character of bonds(as angle increases) atoms closeness also increases . That results maximum overlapping of lone pair and p/d-orbital of one atom takes place

10. APPLICATION OF BACK BONDING 1. EFFECT ON ACID BASE REACTION According to the effect of back bonding acid base reaction are generally of two types; Lewis acid base interaction Bronsted acid base interaction On Lewis Acid Base interaction Due to back bonding the electron deficiency at boron atom decreases,which in turn decreases the lewis acidity.So BF3 act as weaker lewis acid. Due to back bonding the electron or lone pair availability decreases at nitrogen atom as it going to the d-orbital of Si atom.So in other way we can say that lewis basicity decreases

11. In a nutshell we can say that when electron donation takes place from side atom to central atom,lewis acidity decreases, where as electon donation takes place from central atom to side atom lewis basicity decreases. Let us compare the following molecules BF 3 BCl 3 BBr 3 BACK BONDING 2p-2p > 2p-3p > 2p-4p LEWIS ACIDITY < < N(CH 3 ) 3 N(SiH 3 ) 3 BACK BONDING NO YES LEWIS BASICITY >

12. 0n Bronsted Acid Base Interaction (ACID) (CONGUGATE BASE) Acidity of a species depends upon the stabiliry of its conjugate base Let us consider the following molecules me 3 C-OH me 3 C-O - + H + Here we can see that me 3 C-O - is the conjugate base of the acid me 3 C-OH.Now me 3 C-O - is not stable due to its charged structure.So that unstable conjugate base,resulting weak acidity of me 3 C-OH . me 3 Si-OH me 3 Si-O - + H +

13. Here the conjugate base formed after the dissociation of H+ is stable only because of back bonding.Here back bonding possible due to the availability of d-orbital of Si to form 2p pi-3d pi overlapping which make conjugate base stable. Thus me3Si-0H act as acid. 2 . EFFECT ON B0ND ANGLE AND GEOMETRY/SHAPE Back bonding play a very important role in deciding the bond angle and geometry Let us consider the following cases NO BACK BONDING SP3 HYBRIDISED PYRAMIDAL BACK BONDONG PRESENT SP2 HYBRIDISED PLANER

14. 3. EFFECT ON ADDUCT FORMATION BOND ORDER >1 =1 BOND LENGTH Increases GEOMETRY Planer Tetrahedral BOND ANGLE(F-B-F) Decreases CHANGE AROUND BORON ATOM BOND LENGTH Decreases BOND ANGLE (C-N-C) Increases CHANGE AROUND NITROGEN ATOM

15. EFFECT ON STREATCHING FREQUENCY 4. We know stretching frequency directly proportional to bond order.And bond order directly proportional to p pi-d pi bonding. Let us consider the stretching frequency of following molecules STREATCHING FREQUENCY 1416.6 CM- 1912.9CM- 1297.9CM- Due to the attach of most electronegative F-atom , it withdraw the electron cloud of P-atom resulting the contraction of d-orbital .Thus p pi- d pi overlapping becomes very good.So greater bond order and greater streatching frequency.

16. 5. TENDENCY OF POLYMERISATION We know carbon show catenation property and it also form graphite like structure by forming (c-c) double bond, which are very strong and this type of catenation is called Homocatenation But Sillicon have no such catenation(homo) property because its bond energy is not high like Carbon and also not form graphite like structure But Si-O bond energy is very high . It is due to the p pi-d pi overlapping . So Si-O have some double bond character due to this back bonding. So it shows Heterocatenation with Oxygen.

17. 6. EFFECT ON BOND LENGTH AND BOND ORDER Bond length and bond order are inversely proportional to each other . Due to back bonding bond order further increases , So bond length decreases . BACK BONDING BOND ORDER 1 BOND LENGTH .

18. N-H sigma bond strong S-H sigma bond weak Sigma weakening No such pi bonding present but having 2p pi-3d pi back bonding Conventional pi- bonding is present Pi bonding strengthening but in very little N 4 S 4 H 4 7. APPLICATION TO STRUCTURE DETERMINATION IN INORGANIC CYCLIC RING SYSTEM Let us consider the preferred structure of the following molecule Less preferred More preferred

19. N-F STRONG SIGMA BOND S-F STRONG SIGMA BOND Sigma weakening very little PROPER DOUBLE BOND WITH CONTRACTED d-ORBITAL OF S-ATOM BY PRESENCE OF ELECTRONEGETIVE F-ATOM ATTACHED WITH S-ATOM Pi bond strengthening very large BACK BONDING IS TAKING PLACE BUT NOT EFFECTIVE N 4 S 4 F 4 LESS PREFERED STRUCTURE MORE PREFERED STRUCTURE

20. CONCLUSION Back bonding is a unique type of covalent bonding where electron are shared unequally between two atoms . The atom with lower electronegativity receives the shared electrons, forming a stronger bond and often a shorter bond length . This phenomena is common in molecules involving element with empty or partially filled orbitals such as boron ,carbon , and transition metals. Back bonding plays a crucial role in the stability ,structure and reactivity of many compounds , especially in inorganic and organometallic chemistry.