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A complex of ethyne and chlorine monofluoride has been identified and characterised in a supersonically expanded mixture of the two gases by means of its ground-state rotational spectrum. A fast-mixing nozzle was used in combination with a pulsed-nozzle, Fourier-transform microwave spectrometer to preclude chemical reaction of the components and to observe the spectrum of each of the isotopomers C2H2. e3'C1F, C2H2. * 37ClF and C2D2. * e3'ClF. Analysis of the spectrum led to the rotational constants A , , B, and C,, the centrifugal distortion constants A , , A,, and 6,, the C1-nuclear quadrupole coupling constants x,,(Cl) and zbb(C1)xoa(CI), and the spin-rotation coupling constant $[kfbb(C1) + M,(Cl)] in each case. Arguments based on nuclear-spin statistical weight effects and a detailed interpretation of the spectroscpic constants revealed that the detected pre-reactive intermediate has a planar T-shaped geometry, with C2H2 acting as the bar of the T, and ClF as the stem but oriented so that the C1 atom was closer to the ethyne n-bond than F. The distance between the centre (*) of the n-bond and the C1 nucleus is r(*. * C l ) = 2.783 (8) A. The small intermolecular stretching force constant k, = 10.0 N m-' and the minor changes in the C1 nuclear quadrupole coupling constants of ClF on formation of the complex demonstrate that the complex is weakly bound and that the electric charge distribution of ClF suffers only a small perturbation. It is concluded that C2H2..-C1F is best described formally as a complex of the Mulliken bn . a a weak outer type.

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