  We study the current discrepancy between the model-predicted and measured
concentrations of Fe++ and Fe3+ in ionized nebulae. We calculate a set of
photoionization models, updated with the atomic data relevant to the problem,
and compare their results with those derived for the available nebulae where
both [Fe III] and [Fe IV] lines have been measured. Our new model results are
closer to the measured values than the results of previous calculations, but a
discrepancy remains. This discrepancy translates into an uncertainty in the
derived Fe abundances of a factor up to ~4. We explore the possible causes of
this discrepancy and find that errors in the Fe atomic data may be the most
likely explanation. The discrepancy can be fully accounted for by any of the
following changes: (1) an increase by a factor of ~10 in the recombination rate
(radiative plus dielectronic, or charge transfer) for Fe3+, (2) an increase by
a factor of 2-3 in the effective collision strengths for Fe++, or (3) a
decrease by a factor of 2-3 in the effective collision strengths for Fe3+. We
derive the Fe abundances implied by these three explanations and use the
results to constrain the degree of depletion of Fe in our sample nebulae. The
Galactic H II regions and planetary nebulae are found to have high depletion
factors, with less than 5% of their Fe atoms in the gas phase. The
extragalactic H II regions (LMC 30 Doradus, SMC N88A, and SBS 0335-052) have
somewhat lower depletions. The metal-deficient blue compact galaxy SBS 0335-052
could have from 13% to 40% of Fe in the gas phase. The depletions derived for
the different objects define a trend of increasing depletion at higher
metallicities.
