MOLECULAR-STRUCTURE OF TETRACARBONYLDIHYDROIRON - MICROWAVE MEASUREMENTS AND DENSITY-FUNCTIONAL THEORY CALCULATIONS

Microwave spectra of seven isotopomers of tetracarbonyldihydroiron wer e measured in the 4-16 GHz range using a Flygare-Balle type microwave spectrometer. Measured transitions were fit using a rigid rotor Hamilt onian with five independent distortion constants. Structural parameter s from a least-squares fit to the rotational constants are r(Fe-H) = 1 .576(64) Angstrom, r(Fe-Cl) = 1.815(54) Angstrom, r(Fe-C3) = 1.818(65) Angstrom, r(C1-O1) = 1.123(80) Angstrom, r(C3-O3) = 1.141(74) Angstro m, angle(H-Fe-H) = 88.0(2.8)degrees, angle(C1-Fe-C2) = 154.2(4.2)degre es, angle(C3-Fe-C4) = 99.4(4.3)degrees, angle(Fe-C1-O1) = 172.5(5.6)de grees, and angle(Fe-C3-O3) = 177.8(6.8)degrees. All of the carbonyl gr oups are bent slightly toward the hydrogen atoms. The least-squares-de termined structural parameters are in excellent agreement with the sub stitution coordinates determined from the Kraitchman equations, the st ructural parameters calculated using density functional theory, and th e previously published electron diffraction data. The C-2 upsilon mole cular symmetry is consistent with the results of the microwave data an d with theoretical calculations. All of the analyses show that the H a toms are separated by about 2.2 Angstrom, and this indicates that the complex is clearly a ''classical dihydride'' rather than an eta(2)-''d ihydrogen'' complex. Structural parameters obtained from a density fun ctional theory calculation agreed with measured values to within 2%. T he density functional theory analysis of the anharmonicity in the Fe-H symmetric stretching potential is shown to support the observed deute rium isotope effects observed for the hydrogen atom coordinates. The a nharmonicity effects are larger for the Fe-H stretching coordinate tha n for the angle H-Fe-H interbond angle. The r(0)(Fe-D) bond lengths we re observed to be 0.05(4) Angstrom shorter than the r(0)(Fe-H) bond le ngths.