Hypervalent Molecules: 1 Examples of molecules with three centre four electron bonds And their representation within the covalent bond classification XeF2 – an example of a 3c-4e bond Xe uses its 5s and three 5p orbitals for bonding The molecule is linear The Xe 5ps orbital and two F 2ps orbitals form a 3 centre bond The inphase combination of the two F 2ps orbitals are nonbonding Occupancy of these two orbitals results in a build up of charge on the fluorines The equivalent Lewis formulation is given below F Xe+ F- F- Xe+ F Xe contributes 2 electrons to the bonding and retains its octet. The XeF bond order is ½. 3c–4e Interactions X L X X X L L Z L L L Z L X X L Z L L Z X X L L • •• • • • •• •• •• •• •• 1e 2e 1e 1e 1e 2e 2e 0e 2e 2e 2e 0e X–µ-L–X X–µ-X–L L–µ-Z–L L–µ-L–Z H H H H (X2) (LX) (L2) (LZ) number of electrons number of electrons number of electrons number of electrons contributed to the central contributed to the central contributed to the central contributed to the central atom (L): 0 atom (X): 1 atom (Z): 2 atom (L): 0 contribution to the valence contribution to the valence contribution to the valence contribution to the valence of the central atom (L): 2 of the central atom (X): 1 of the central atom (Z): 0 of the central atom (L): 20 Method of Writing Formula of Covalent Molecules that contain 3 center - 4 electron bonds H H H MLl(Ll) Xx(Xx) Zz(Zz) This may be condensed to Element M ML2lX2xZ2zHl+x+z Number l of L ligands in 2c-2e bonds Number lH of LH ligands in 3c- 4e bonds The general formula being Number x of X ligands in 2c-2e bonds Number xH of XH ligands in 3c-4e bonds MLlXxHh Number z of Z ligands in 2c-2e bonds Number zH of ZH ligands in 3c- 4e bonds H indicates a hypervalent interaction h is the number of hypervalent interactions Counting using the general formulae H is not to be confused with H for hydrogen H Non-metal fluoride examples of (X2) 3c-4e bonds F F F F F Xe F Cl F S F F P F F F F H F XeF2 H H ClFF2 SF2F2 VN = 2, EN = 8 VN = 3, EN = 8 VN = 4, EN = 8 H F F PF3F2 F F F VN = 5, EN = 8 F Xe F F Cl F F S F F F F F H XeF H ClFF4 4 H VN = 4, EN = 8 VN = 5, EN = 8 SF2F4 VN = 6, EN = 8 The 3c-4e bonds are linear being formed by a p orbital which favours linear overlap. They are longer than the 2c-2e bonds having a bond order of ½ rather than 1. Examples of (LX)H 3c-4e bonds H( H H( H SiF3F NH3) SnBr3Br AsPh3) VN = 4 EN = 8 VN = 4 EN = 8 H Examples of (L2) 3c-4e bonds H H AlH3(NH3)2 InCl3(PMe3)2 VN = 3 EN = 8 VN = 4 EN = 8 3c-4e bonds with first and second period elements as the central atom Most examples of 3c-4e bonds are from the 3rd period and below. However the - classic example from the first period is the bifluoride ion, [HF2] . Some more exotic examples with B and C are shown below.