If we are considering species containing two neighboring d-block atoms, as in Hg and ClaReRecil' , we should also allow for the possibility of forming bonds from d orbitals. The dz2 orbital has cylindrical symmetry with respect to the internuclear (z) axis, and hence can contribute to the α orbitals that are formed from s and pz orbitals. The dyz and dzx orbitals
both look like p orbitals when viewed along the axis, and hence can contribute to the π orbitals formed from px and py. The new feature is the role of dx2-y2 and dxy, which have no counterpart in the orbitals discussed up to now. These two orbitals can overlap with matching orbitals on the other atom to give rise to doubly degenerate pairs of bonding and antibonding … orbitals (Fig. 3.19). As we shall see in Chapter 7, orbitals are important for the discussion of bonds between d-metal atoms and lead to descriptions of some species in terms of quadruple bonds, as in