Molecular structure of tetracarbonyldihydroiron: Microwave measurements and density functional theory calculations

Microwave spectra of seven isotopomers of tetracarbonyldihydroiron were measured in the 4-16 GHz range using a Flygare-Balle type microwave spectrometer. Measured transitions were fit using a rigid rotor Hamiltonian with five independent distortion constants. Structural parameters from a least-squares fit to the rotational constants are r(Fe-H) = 1.576(64) Å, r(Fe-C1) = 1.815(54) Å, r(Fe-C3 = 1.818(65) Å, r(C1-O1) = 1.123(80) Å, r(C3-O3) = 1.141(74) Å, <(H-Fe-H) 88.0(2.8)°, <(C1-Fe-C2) = 154.2(4.2)°, <(C3-Fe-C4) = 99.4(4.3)°, <(Fe-C1-O1) = 172.5(5.6)°, and <(Fe-C3-O3) 177.8(6.8)°. All of the carbonyl groups are bent slightly toward the hydrogen atoms. The least-squares-determined structural parameters are in excellent agreement with the substitution coordinates determined from the Kraitchman equations, the structural parameters calculated using density functional theory, and the previously published electron diffraction data. The C(2v) molecular symmetry is consistent with the results ofthe microwave data and with theoretical calculations. All of the analyses show that the H atoms are separated by about 2.2 Å, and this indicates that the complex is clearly a 'classical dihydride' rather than an η²-'dihydrogen' complex. Structural parameters obtained from a density functional theory calculation agreed with measured values to within 2%. The density functional theory analysis of the anharmonicity in the Fe-H symmetric stretching potential is shown to support the observed deuterium isotope effects observed for the hydrogen atom coordinates. The anharmonicity effects are larger for the Fe-H stretching coordinate than for the <H-Fe-H interbond angle. The r₀(Fe-D) bond lengths were observed to be 0.05(4) Å shorter than the r₀(Fe-H) bond lengths.