It depends on whether the compound is diamagntic or parmagnetic. Compounds such as ferrocene are diamagetic Fe(0) and therefore the NMR spectra can be measured. Fe(III) compounds are paramagnetic so the NMR signals are usually very broad. Fe(II) compounds are usually paramagnetic but in some situations can be diamagnetic. Magnetic suscepibilities of paramagnetic compounds can be determined using an NMR spectrometer using Evan's Method (D. Evans, J. Chem. Soc., 1959, p2003-2005).