Quintuple Bond - Bonding

Bonding

As stated above metal-metal quintuple bonds have a σ2π4δ4 configuration. This means the of the five bonds present between the metal centers one is a sigma bond, two are pi bonds, and two are delta bonds. the σ-bond is the result of mixing between the d_z2 orbital on each metal center. The first π-bond comes from mixing of the d_yz orbitals from each metal while the other π-bond comes from the d_xz orbitals on each metal mixing. Finally the δ-bonds come from mixing of the d_xy orbitals as well as mixing between the d_x2-y2 orbitals from each metal. A graphical representation of this mixing can be seen below (note: the directions of the y and z axes have been mistakenly switched; the atoms are along a line parallel to the z-axis).

Molecular orbital calculations have elucidated the relative energies of the orbitals created by these bonding interactions. As shown in the figure below, the lowest energy orbitals are the pi bonding orbitals followed by the sigma bonding orbital. The next highest are the delta bonding orbitals which represent the HOMO. Because the 10 valence electrons of the metals are used to fill these first 5 orbitals, the next highest orbital becomes the LUMO which is the delta antibonding orbital. Though the pi and delta orbitals are represented as being degenerate, they in fact are not. This is because the model shown here is a simplification and that hybridization of s, p, and d orbitals is believed to take place, causing a change in the orbital energy levels.