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, phosphaferroce nes and their precursor ligands have been used to determine coordinati on shifts, DELTAdelta (= delta(complex) - delta(ligand)) for the compl exes. For azaferrocenes, DELTAdelta values were similar to that of fer rocene, but the phosphaferrocenes showed much larger coordination shif ts. These observations are discussed in terms of the difference in bon ding in these complexes. The Fe-57 Mossbauer data are reported for a n umber of azaferrocene derivatives and compared with those for the phos phaferrocene analogues in terms of orbital contributions to the electr ic field gradient that governs the quadrupole splitting (QS). An excel lent correlation is found between QS and the orbital population parame ter (2p2 - p1). where p2 and p1 are electronic populations of the e2 a nd e1 iron-based orbitals, respectively. Protonation and quaternisatio n of azaferrocene causes a lowering of QS, in contrast to the slight i ncrease observed for monophosphaferrocene in CF3CO2H solution- The res ults confirm that protonation at the N atom takes place for azaferroce ne. The W(CO)5 adduct of azaferrocene shows little change in QS from t hat for azaferrocene, which is interpreted as evidence for back bondin g by the tungsten.