Coordination Chemistry Solutions: #7

7.* (1996 F 13) Below are structural formulas for two iron porphyrins. The pyridine complex, X, is paramagnetic whereas the carbonyl derivative is diamagnetic. The pyridine complex strongly binds a second pyridine forming a diamagnetic bis-pyridine complex; however, the carbonyl complex, Y, only weakly binds a second carbonyl.

1. Backbonding: The carbonyl has antibonding pi orbitals that are optimally situated for backbonding. The Fe d_xz orbital (or d_yz, depending on how you draw it) can donate electron density into C-O p*, thereby stabilizing itself and increasing DO to make the complex go low-spin. Although this weakens the C-O p bond, it strengthens the Fe-CO bond and makes less Fe d_xz electron density available for backbonding to the second CO. Hence the second CO is less tightly bound than the first. The pyridine, on the other hand, has orbitals less optimally situated for backbonding. Therefore it is high spin and the binding of the second pyridine, since it does not disrupt any stabilizing interactions, is strong.
2. We are told in the problem that adding the second pyridine causes the complex to be diamagnetic, which means that all 6 of the Fe d electrons go into bonding t_2g orbitals, causing the complex to become more stable.