9.5 Comparison of sulfur to oxygen

Size

[link] summarizes the comparative sizes of oxygen and sulfur.

Electronegativity

Sulfur is less electronegative than oxygen (2.4 and 3.5, respectively) and as a consequence bonds to sulfur are less polar than the corresponding bonds to oxygen. One significant result in that with a less polar S-H bond the subsequent hydrogen bonding is weaker than observed with O-H analogs. A further consequence of the lower electronegativity is that the S-O bond is polar.

Bonds formed

Sulfur forms a range of bonding types. As with oxygen the -2 oxidation state prevalent. For example, sulfur forms analogs of ethers, i.e., thioethers R-S-R. However, unlike oxygen, sulfur can form more than two covalent (non-dative) bonds, i.e., in compounds such as SF ₄ and SF ₆ .

Such hypervalent compounds were originally thought be due to the inclusion of low energy d orbitals in hybrids (e.g., sp ³ d ² for SF ₆ ); however, a better picture involves a combination of s and p ortbitals in bonding ( [link] ). Any involvement of the d orbitals is limited to the polarization of the p orbitals rather than direct hydridization. In this regard SF ₆ represents the archetypal hypervalent molecule. Finally, sulfur can form multiple bonds, e.g., Me ₂ S=O.

Catenation

Catenation is defined as the ability of a chemical element to form a long chain-like structure via a series of covalent bonds. Oxygen’s extent of catenation is limited to ozone (O ₃ ) and peroxides (e.g., R-O-O-R). In contrast, the chemistry of sulfur is rich in the formation of multiple S-S bonds.

While elemental sulfur exists as a diatomic molecule (i.e., S ₂ ) in the gas phase at high temperatures, sulfur vapor consists of a mixture of oligomers (S ₃ to S ₈ ) as a temperature dependant equilibrium. In the solid state the formation of Sn dominates, and sulfur exists as a range of polymorphs in which extended S-S bonding occurs in either rings of 6 to 20 atoms (e.g., [link] ) or chains (catenasulfur).

The higher level of catenation for sulfur is due to the greater strength of a S-S bond (226 kJ/mol) as compared to the O-O bond (142 kJ/mol). In general the homoleptic bond strength is expected to decrease going down a period of the Periodic Table. The reason for the unexpected weakness of the O-O bond is that the electronegative oxygen atoms repel each other and thus weaken the bond.