THE SIGMA-BONDING 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 eta5,sigma,eta5,sigma and sigma,sigma,sigma,sigma. It is concluded that in both co-ordination modes sigma bonding is by far the most dominant, mainly due to strong charge donation from the pyrrolic nitrogen lone pairs into the empty 4d(x2-y2) and 4d(xz), with additional effects from donation into the 4d(z2) and 5s orbitals. The pi bond, resulting from donation from occupied pyrrolyl pi orbitals into the metal 4d(xy) and 4d(yz) orbitals is significant for eta5,sigma,eta5,sigma- but rather weak for sigma,sigma,sigma,sigma-coordination due to the diminished donation into the 4d(xy) 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 sigma:sigma:sigma:sigma co-ordination due mostly to a weaker pi-bond interaction. The eta5,sigma,eta5,sigma 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-.