The sigma- and pi-bonding modes of a tetraanionic porphyrinogen ligand in zirconium(IV) complexes: a theoretical investigation

The bonding modes of the porphyrinogen ligand L4–(H4L = 5,10,15,20,22,24-hexahydroporphyrin) with the metal ion Zr4+ are analysed in detail for the limiting co-ordinations η5,σ,η5,σ and σ,σ,σ,σ. It is concluded that in both co-ordination modes σ bonding is by far the most dominant, mainly due to strong charge donation from the pyrroli nitrogen lone pairs into the empty 4dx2-y2 and 4dxz, with additional effects from donation into the 4dz2 and 5s orbitals. The π bond, resulting from donation from occupied pyrrolyl π orbitals into the metal 4dxy and 4dyz orbitals is significant for η5,σ,η5,σ- but rather weak for σ,σ,σ,σ-co-ordination due to the diminished donation into the 4dxy orbital. The total orbital interaction contribution (the covalent component) is about one third of the ionic component of the bond, the latter being identified as the sum of the Pauli repulsion and the attractive electrostatic interaction between L4– and Zr4+. The ionic contribution is about the same in the two configurations, but the covalent component of the bond decreases by ca. 3 eV for σ:σ:σ:σ co-ordination due mostly to a weaker π-bond interaction. The η5,σ,η5,σ complex is only 2.1 eV more stable, indicating that the interconversion between the two co-ordination modes is a relatively easy process. The most stable co-ordination mode is preserved after interaction of the substrate with the Lewis bases tetrahydrofuran or H–.