σ-Bond Prevents Short π-Bonds: A Detailed Theoretical Study on the Compounds of Main Group and Transition Metal Complexes

Abstract

The unusual shortness of the bond length in several main group and transition metal compounds is explained on the basis of their π-alone bonding. The detailed electronic structure calculation on C2, HBBH, and Fe2(CO)6 shows that each of them has two π-alone bonds (unsupported by an underlying σ-bond), whereas B2 has two-half π-bonds. The C–C bond length in C2 is 1.240 A, shorter than any C–C double (σ + π, in C2H4, C–C=1.338 A) bonded species. The B–B bond distance in B2 (1.590 A, two half-π bonds) is shorter than any B–B single σ-bonded (~1.706 A) species. The calculated Fe–Fe bond distance of 2.002 A in Fe2(CO)6 is shorter than those of some experimentally known M–M single bonded compounds in the range of 2.904–3.228 A. Here, our detailed studies on the second and third row diatomics (five, six, seven and eight valence electrons species) and transition metal complexes show that π-alone bonds left to themselves are shorter than σ-bonds; in many ways, σ-bonds prevent π-bonds from adopting their optimal shorter distances.