Pπ-dπ bonding is also important in transition metal complexes. For example, phosphines (R 3P:) are good σ donors in complexes with transition metals, as shown below. Transition metal d-orbitals can also form σ bonds, typically with s-p hybrid orbitals of appropriate symmetry on ligands. Compounds with metal-metal δ bonds occur in the middle of the transition series. δ bonds are generally quite weak compared to σ and π bonds. For this reason, compounds containing C=C double bonds are very common, but those with Si=Si bonds are rare. π-bonded compounds of heavier elements are rare because the larger cores of the atoms prevent good π-overlap. Because pπ-pπ bonding involves sideways overlap of p-orbitals, it is most commonly observed with second-row elements (C, N, O). In each case, we can make bonding or antibonding combinations, depending on the signs of the AO wavefunctions. Some possible σ (top row), π (bottom row), and δ bonding combinations (right) of s, p, and d orbitals are sketched below.
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