Microwave spectra of seven isotopomers of tetracarbonylethyleneiron were recorded using a Pulse-Beam Fourier Transform Microwave Spectrometer. Rotational transitions for a "c" dipole moment;with J' <-- J from 2 <-- 1 to 6 <-- 5 were measured in the 4-12 GHz range. Rotational constants were determined by fitting the measured microwave spectra to a Watson "A" reduced Hamiltonian with centrifugal distortion parameters. The measured rotational constants of the main isotopomer are A = 1031.1081(4) MHz, B = 859.8055(4) MHz, and C = 808.5675(3) MHz. Data were also obtained for three C-13-substituted species and two O-18-substituted species in natural abundance. Additional spectra were measured for an isotopically enriched sample of perdeuterated tetracarbonylethyleneiron. The moments of inertia of the seven isotopomers were used in a Kraitchman analysis and in two different least-squares fitting analyses to determine the molecular structure of the compound. The ethylene ligand exhibits significant structural changes upon complexation to iron, primarily an increase in C-C bond length with movement of the hydrogen atoms away from the metal center. The C-C and C-H bond lengths were found to be r(o) = 1.419(7) and = 1.072(4) Angstrom, respectively. The C-C-H angle and the Fe-C-C-H dihedral angle were found to be angle(C-C-H)(o) = 120.6(5)(o) and angle(Fe-C-C-H)(o) = 103.6(9)(o), respectively. The plane of the hydrogen atoms is displaced:0.217(2) Angstrom above the ethylene carbon atoms, along the c axis. Extensive DFT calculations were carried out prior to the experimental research. The calculated structure proved extremely valuable in obtaining accurate predictions for the spectra, and provided structural parameters in excellent overall agreement with measured parameters.