Hextuple bond

In this chapter we illustrated how the CASSCF/CASPT2 method can be used to explore the nature of such chemical bonds. Classic cases are the Re-Re multiple bond in Re2Cl, and the Cr-Cr bond ranging from the quadruply bonded Cr(II)-Cr(II) moiety to the formal hextuple bond between two neutral chromium atoms. The bonding between the 3d5 electrons is weak and should be considered as an intermediate between two pairs of antiferromagnetically... 

The other five pairs form the 3d bonds in the Cr-Cr complexes and they present quite a different binding power. The 3do-orbital pair creates a bond with an EBO of 0.77, whereas the 3dtt bonds exhibit an EBO of 0.81 each, and the 3d5 bonds an E BO of 0.58 each. The total EBO then adds up to 4.45. Therefore, the Cr-Cr bond is closer to a quadraple bond, rather than a hextuple or quintuple bond, because the 5 bonds are very weak and could also be considered as two pairs of antiferromagnetically coupled 3d5 atomic orbitals. The CASSCE/CASPT2 conclusions presented by Roos [25] yield a bond distance of 1.66 (1.68) A and a bond energy (Dg) of 1.65 (1.53) eV (experimental values are given within parentheses). 

---