Why does the proton NMR of ferrocenecarboxaldehyde appear to be four singlets?
I like to address some interesting NMR features of ferrocene derivatives. Try Hard's question about $\mathrm{^{13}C-NMR}$ of ferrocenecarboxaldehyde showing one signal for the substituted cyclopentyl ring prompt me to find some solid evidence. However, it seems like most of ferrocene derivatives give similar results regardless of how strong magnetic field you used. Therefore, Aldrich NMR spectra may not be the best spectra with high definition (e.g., they use $\lt \pu{300 MHz}$ machine for $\mathrm{^{1}H-NMR}$). 

Following $\mathrm{^{13}C-NMR}$ values from very recent paper, which published NMR data of few ferrocene derivatives using a $\pu{400 MHz}$ (but $\pu{100 MHz}$ for $\mathrm{^{13}C}$) machine (Ref.1):



The values in left hand side is for $\mathrm{C_p}$ ring (substituted) while single value at the bottom is for all 5 $\ce{C}$s of unsubstituted  $\mathrm{C_{p'}}$ ring. If you look closely, you'd see monosubstituted rings have so close values that they may overlap in spectra taken in low resolution machines.

Reference:


N. D'Antona, R. Morrone, G. Gambera, S. Pedotti, “Enantiorecognition of planar “metallocenic” chirality by a nitrile hydratase/amidase bienzymatic system,” Org. Biomol. Chem. 2016, 14, 4393-4399 (DOI: 10.1039/C6OB00689B).