A prototype transition-metal olefin complex C2H4···AgCl synthesised by laser ablation and characterised by rotational spectroscopy and ab initio methods.

C(2)H(4)···Ag-Cl has been synthesised in the gas phase in a pulsed-jet, Fourier-transform microwave spectrometer by the reaction of laser-ablated metallic silver with carbon tetrachloride to give AgCl, which subsequently reacts with ethene to give the complex. The ground-state rotational spectra of six isotopologues (C(2)H(4)···(107)Ag(35)Cl, C(2)H(4)···(109)Ag(35)Cl, C(2)H(4)···(107)Ag(37)Cl, C(2)H(4)···(109)Ag(37)Cl, (13)C(2)H(4)···(107)Ag(35)Cl, and (13)C(2)H(4)···(109)Ag(35)Cl) were recorded and analysed to give rotational constants A(0), B(0), and C(0), centrifugal distortion constants Δ(J) and Δ(JK), and Cl nuclear quadrupole coupling constants χ(aa)(Cl) and χ(bb)(Cl)-χ(cc)(Cl). These spectroscopic constants were interpreted in terms of a geometry for C(2)H(4)···Ag-Cl of C(2V) symmetry in which the AgCl molecule lies along the C(2) axis of ethene that is perpendicular to the C(2)H(4) plane. The Ag atom forms a bond to the midpoint () of the ethene π bond. A partial r(s)-geometry and a r(0)-geometry were determined, with the values r(···Ag) = 2.1719(9) Å, r(C-C) = 1.3518(4) Å, and r(Ag-Cl) = 2.2724(8) Å obtained in the latter case. The C-C bond lengthens on formation of the complex. Detailed ab initio calculations carried out at the CCSD(T)/cc-pVQZ level of theory give results in good agreement with experiment and also reveal that the ethene molecule undergoes a small angular distortion. The distortion is such that the four H atoms move in a direction away from Ag but remain coplanar. The two C atoms are no longer contained in this plane, however. The electric charge redistribution when C(2)H(4)···Ag-Cl is formed and the strength of the π···Ag bond are discussed.