From the viewpoint of the fixation of atmospheric nitrogen, there is a long-term general interest in the identification and characterization of compounds that result from the reaction of transition metals with dinitrogen. In addition, the investigation of the interaction of N 2 with transition-metal compounds has intrinsic significance because of the isoelectronic, isolobal nature of dinitrogen and carbon monoxide. There is a voluminous amount of literature [1] on transition-metal carbonyl compounds, from which it is concluded that the nature of the generally strong bonding interaction of CO with the transition-metal center in such compounds is well-understood.Recently, a series of simple linear transition-metal carbonyl complexes of the type OCÀMÀX (M = Cu, [2] Ag, [3] or Au [4] ; X = F, Cl, or Br) were thoroughly investigated in the gas phase by means of their rotational spectra to give details of their geometry and electronic structure. Information about the electric-charge redistribution at the metal center is contained in the nuclear quadrupole coupling constants for M = Cu or Au. The interaction between CO and M was also found to be strong in these transient species. On the other hand, the hydrogen-bonded species OC···H À X, which may be envisaged, formally at least, as generated from OC À M À X by replacement of M by H, have been investigated extensively by rotational spectroscopy. [5][6][7] Such complexes are weakly bound (here depicted with ···) with intermolecular binding energies of only a few kJ mol À1 . The dinitrogen complexes N 2 ···HÀX have linear geometries similar to those of OC···H À X, but with slightly weaker hydrogen bonds. [8][9][10] The isostructural nature of OC···H À X and N 2 ···H À X can be understood by assuming that the electrophilic region of HX seeks the most nucleophilic region of CO or N 2 (namely, the axis of a nonbonding

Carbonyl gold iodide, OCAuI, is generated in the gas phase through synchronising the laser ablation of a gold sample with the pulsed, supersonic expansion of gas containing carbon monoxide and an iodine precursor. Fourier Transform microwave (FT-MW) spectroscopy has been used to detect and assign lines in the pure rotational spectrum of the molecule. The frequencies of lines in the spectra of three isotopomers are presented and used to determine rotational, centrifugal distortion and hyperfine coupling constants through iterative least-squares fitting. Rotational constants, B 0 , of the three isotopomers allow a determination of the r 0 molecular geometry. The structure is linear. The bond lengths are consistent with previous studies of OCMX complexes (M=Cu, Ag, Au; X=F, Cl, Br, I). The frequency of the lowest energy vibrational stretching mode is estimated from the centrifugal distortion constant of the 16 O 12 CAuI isotopomer. The nuclear quadrupole coupling constants of both iodine, I aa χ , and gold, Au aa χ , have been determined. The ionic character of the Au-I bond is calculated using I aa χ. Nuclear shielding parameters for OCAuI and other OCAuX species are determined from the measured nuclear spin-rotation constants, M bb C and X bb C .

Carbonyl gold iodide, OCAuI, is generated in the gas phase through synchronising the laser ablation of a gold sample with the pulsed, supersonic expansion of gas containing carbon monoxide and an iodine precursor. Fourier Transform microwave (FT-MW) spectroscopy has been used to detect and assign lines in the pure rotational spectrum of the molecule. The frequencies of lines in the spectra of three isotopomers are presented and used to determine rotational, centrifugal distortion and hyperfine coupling constants through iterative least-squares fitting. Rotational constants, B 0 , of the three isotopomers allow a determination of the r 0 molecular geometry. The structure is linear. The bond lengths are consistent with previous studies of OCMX complexes (M=Cu, Ag, Au; X=F, Cl, Br, I). The frequency of the lowest energy vibrational stretching mode is estimated from the centrifugal distortion constant of the 16 O 12 CAuI isotopomer. The nuclear quadrupole coupling constants of both iodine, I aa χ , and gold, Au aa χ , have been determined. The ionic character of the Au-I bond is calculated using I aa χ. Nuclear shielding parameters for OCAuI and other OCAuX species are determined from the measured nuclear spin-rotation constants, M bb C and X bb C .

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