MOSSBAUER STUDIES ON FERROCENE COMPLEXES .18. A COMPARATIVE-STUDY OF AZAFERROCENES AND PHOSPHAFERROCENES AND THEIR DERIVATIVES BY FE-57 MOSSBAUER AND C-13 NMR-SPECTROSCOPY

The C-13 NMR chemical shifts (delta) for azaferrocenes, phosphaferrocenes and their precursor ligands have been used to determine coordination shifts, DELTAdelta (= delta(complex) - delta(ligand)) for the complexes. For azaferrocenes, DELTAdelta values were similar to that of ferrocene, but the phosphaferrocenes showed much larger coordination shifts. These observations are discussed in terms of the difference in bonding in these complexes. The Fe-57 Mossbauer data are reported for a number of azaferrocene derivatives and compared with those for the phosphaferrocene analogues in terms of orbital contributions to the electric field gradient that governs the quadrupole splitting (QS). An excellent correlation is found between QS and the orbital population parameter (2p2 - p1). where p2 and p1 are electronic populations of the e2 and e1 iron-based orbitals, respectively. Protonation and quaternisation of azaferrocene causes a lowering of QS, in contrast to the slight increase observed for monophosphaferrocene in CF3CO2H solution- The results confirm that protonation at the N atom takes place for azaferrocene. The W(CO)5 adduct of azaferrocene shows little change in QS from that for azaferrocene, which is interpreted as evidence for back bonding by the tungsten.