Structure And Bonding In Xenon Fluorides And Oxo Compounds

Structure And Bonding In Xenon Fluorides and Oxo Compounds

Structure of XeF₂ on the basis of VSEPR treatment

In XeF₂,the central Xe-atom is surrounded by five electron pairs.Out of the five electron pairs,two are bond pairs and three are lone pairs.Five electron pairs form trigonal bipyramoidal shape with three lone pairs occupied in equotorial positions.Due to the presence of three lone pairs,XeF₂ molecule assumes linear shape with F–Xe–F bond angle equal to 180°.

Structure And Bonding of XeF₂ molecule on the basis of valence bond treatment

In XeF₂,outer electronic configuration of the central Xe–atom is $$5s^25p^65d^0$$

Xe-atom in ground state=

Xe uses only two electrons in forming two Xe-F σ–bonds.Hence it is assumed that an electron from 5P level of Xe is promoted to 5d level in excited state.

Xe-atom in excited state=

Now,SP³d hybridization takes place and a new set of five hybridised orbitals are formed,out of which three orbitals arre filled by electron pairs and the remaining two orbitals are partially filled.The remaining two orbitals which are partially filled involved in the bond formation with two sets of orbitals of two F-atoms.The outer electronic configuration of Fluorine shows the following$$(2s^22P^5)$$

Hence,two singly filled SP³d hybrid orbitals of Xe-atom overlap with half filled 2P_z atomic orbital of two F-atoms giving XeF₂ molecule.The five electron pairs of the SP³d hybridisation point to the corner of trigonal bipyramid.Out of these,three are lone pairs and occupy the equatorial positions and two are bond pairs and occupy the apical positions.The atoms thus form distorted trigonal bipyramidal(linear)molecule.

Objection of Valence Bond Treatment

The difference in energy between 5P and 5D is about 960kjmol^-1 which is large gap and it is difficult to explain how large the energy gap is overcome.

The SP³d hybridised orbital involve orbitals of principal quantum number 5 and is much larger compared to 2P orbital of fluorine and thus effective overlap should be difficult.However,highly electronegative F-atom can cause a large contraction in the size of the d-orbitals.If this contraction is big enough,the valence bond explaination will suffice.

Structure and bonding of XeF₂ on the basis of Molecular orbital Theory

The molecular orbital explaination involving three–centre bonds is more acceptable.Assume that bonding involves the 5P_z Orbital of Xe and the 2p_z orbitals of two F-atoms.for bonding to occur,orbitals with the same symmetry must overlap.The 5P_z orbital of Xe combines with 2P_z orbitals of two F-atoms to give three molecular orbitals–one bonding,one non-bonding and one antibonding.

The four electrons of these three atomic orbitals(5P_z of Xe and 2P_zof two F) will occupy the MO of the lowest energy.The order of energy is

$$bonding MO<non-bonding MO<antibonding MO$$

Thus the two electrons occupy the bonding MO,and this pair of electrons is responsible for binding all three atoms.The remaining two electrons occupy the non-bonding MO and are concentrated mainly in the F-atoms giving some ionic character.The bonding may be classified as four-electron three-centreσ bonding.

Structure of XeF₂ on the basis of VBT

The structure of XeF₄ is square planar(distorted tetrahedral) with Xe-F distances of 1.95A°.The valence bond theory explain this by promoting two electrons from 5p orbitals to 5d orbitals as shown:

Xe-atom in ground state:

Xe-atom in excited state:

The one 5s-orbital,three 5p orbitals and two 5d-orbitals hybrised together to give six SP³d hybrid orbitals.Of these six hybrid orbitals four orbitals are singly filled and remaining two orbitals are completely filled.These four singly filled hybrid orbitals take part in overlapping with singly filled 2P_z atomic orbitalof four F-atoms.Hence six electron pairs form octahedral structure with two positions occupied by lone pairs.

(Note:The MO explaination of XeF₄ is similar to that for XeF₂.The Xe atom bonds to four F-atoms.The Xenon 5P_x orbital forms a three centre MO with 2p orbitals from two F-atoms similar to XeF₂.The 5P_z orbital forms another three-centre MO involving two more atoms.The two three centre orbitals are right angles to each other).

Structure of XeF₆

The bonding in XeF₆ has caused considerable controversy which is not completely resolved.The structure of XeF₆ may be explained in valence bond terms by promoting three electrons in Xe from 5p to 5d.

Xe-atom in excited state:

The six singly filled SP³d³ hybrid orbitals overlap with six F-atoms to form six Xe-F σ bonds.However the remaining one completely filled hybrid orbital remains as non bonding lone pair.Thus,XeF₆ should have a distorted pentagonal bipyramidal shape having one lone pair at its axial position.

The structure of XeF₆ is also assumed as a capped octahedron.A capped octahedron has a lone pair pointing through one of the faces of the octahedron.Since there are six bonds and one lone pair,a capped octahedron would give a distorted octahedral molecule.Monomeric XeF₆ in the liquid and gaseous state has been shown to contain octahedron distorted by the lone pair at the centre of one triangular face.The structure of the molecule rapidly fluctuates between structures where the lone pair occupies each of the eight triangular faces_.In various non-aqueous solvents,XeF₆ forms a tetramer Xe₄F₂₄.Solid XeF₆ is polymeric.Except at very low temperature it contains tetramers,where four square pyramidal XeF₅⁺ ions are joined to two similar ions by means of two bridging F-ions.

Structure of XeO₃

Xe-atom in ground state=

Xe-atom in excited state=

Three oxygen atoms from three π-bonds with three orbitals so the remaining four orbitals(one 5s, and three 5P) are hybridised to give SP³ hybrid orbitals.The three half filled hybrid orbitals form σ bonds with three O-atoms and the remaining one completely filled hybrid orbitals remain as lone pair.Hence the XeO₃ molecule has pyramidal shape(tetrahedral with one position occupied by lone pair).

Structure of XeO₄

Xe-atom in excited state=

The four O-atoms form four π-bonds with four orbitals.So the remaining four orbitals (one 5s and three 5p) are hybrised to give four singly filled SP³ hybrid orbitals.These four singly filled orbital overlap with four oxygen atoms to form σ bonds.Hence XeO₄ molecule has tetrahedral structure.

Structure of XeOF₄

Xe-atom in excited state=

XeOF₄ molecule has octahedral structure with one position occupied by lone pair(square pyramidal).

Structure of XeO₂F₂ molecule

Xe-atom in excited state=

Two oxygen atoms form π-bonds with two orbitals.so,the remaining five orbitals (one 5s,three 5P and one 5d) are hybrised to give SP³d² hybrid orbitals.The four singly filled hybrid orbitals form σ-bonds with two F-atoms and two O-atoms.

The remaining one completely filled hybrid orbital remains as non bonded lone pair.Hence XeO₂F₂ molecule has trigonal bipyramidal structure with one equatorial position occupied by a lone pair.

Structure of persenate ion[XeO₆]^4-

References

Poudel, Bhoj Raj Poudel and Dr.Megh Raj. A Text Book Of Inorganic Chemistry. Kathmandu: National Book Centre, 2012.

Sons, John Wiley and. J.D Lee Concise Inorganic Chemistry. 2007.