Weak coordination of neutral S- and O-donor proximal ligands to a ferrous porphyrin nitrosyl. Characterization of 6-coordinate complexes at low T.

The interaction of the S- and O-donor ligands tetrahydrothiophen (THT) and tetrahydrofuran (THF) with the ferrous nitrosyl complex Fe(TTP)(NO) (TTP(2-) is meso-tetra-p-tolyl-porphyrinatodianion) was studied at various temperatures both in solid state and solution using electronic and infrared absorption spectroscopy. Upon addition of these ligands to a cryostat containing sublimed layers of Fe(TTP)(NO), no complex formation was detected at room temperature. However, upon lowering the temperature, spectral changes were observed that are consistent with ligand binding in axial position trans to the NO (the proximal site) and formation of the six-coordinate adducts. Analogous behavior was observed in solution. In both media, the six-coordinate adducts are stable only at low temperature and dissociate to the 5-coordinate nitrosyl complexes upon warming. The NO stretching frequencies of the six-coordinate thioether and ether complexes were recorded and binding constants for the weak bonding of proximal THF and THT ligands were determined from the spectral changes. These parameters are compared with those obtained for the N-donor ligand pyrrolidine.