Porphyrins ligand field effects

Yb(5d)(H20)3]Cl, and [Yb(5d)(CoP] (fig. 12) where CoP is the cyclopentadienyl-tris(dieth-ylphosphito)cobaltate(I) anion (see fig. 13) for which the luminescence intensity increases in the proportions 1 22 36 62 271, upon excitation at 512 nm (Meng et al., 2000). The effect of coordinated water molecules on the metal-centered fluorescence intensity is clearly seen in the more than four-fold enhancement obtained by replacing the three water molecules in [Yb(5d)(H20)3 ]C1 by CoP the 2F5/2 lifetime of the latter complex (40 ps) is also much longer than lifetimes reported for other Ybm porphyrinates. In addition, the cobaltate anion rigidities the molecule, which results in a much finer structure of the ligand-field split electronic levels. 

Many iron-aryl or iron-alkyl a-bonded porphyrins have been described as low spin complexes on the basis of NMR data . However, alkyl and aryl a-bonded derivatives may exist as a mixture of high and low spin state Fe(III) complexes or, alternatively, (P)Fe(R) can exist as a purely high spin state Fe(III) complex. In general, the spin state of a-bonded iron(III) porphyrins will vary as a function of the porphyrin ring basicity and/or the ligand field strength of the a-bonded ligand. The spin state of (P)Fe(R), in turn, seems to directly or indirectly effect the electrochemistry of the complex. 

Unlike the case of optical or MCD spectroscopy, the presence of a nearby free radical (porphyrin or amino acid) has only a small effect on the Mossbauer spectra of ferryl iron (by contrast the nature of the axial ligand appears to have a greater effect on the Mossbauer spectra). Thus in the absence of a magnetic field there is little difference between the Mossbauer spectra of horseradish peroxidase compounds I and II [134,181,183]. Due to... 

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