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

C2H4 center dot center dot center dot 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 (C2H4 center dot center dot center dot(AgCl)-Ag-107-Cl-35, C2H4 center dot center dot center dot(AgCl)-Ag-109-Cl-35, C2H4 center dot center dot center dot(AgCl)-Ag-107-Cl-37, C2H4 center dot center dot center dot(AgCl)-Ag-109-Cl-37, (C2H4)-C-13 center dot center dot center dot(AgCl)-Ag-107-Cl-35, and (C2H4)-C-13 center dot center dot center dot(AgCl)-Ag-109-Cl-35) were recorded and analysed to give rotational constants A(0), B-0, and C-0, centrifugal distortion constants Delta(J) and Delta(JK), and Cl nuclear quadrupole coupling constants chi(aa)(Cl) and chi(bb)(Cl)-chi(cc)(Cl). These spectroscopic constants were interpreted in terms of a geometry for C2H4 center dot center dot center dot Ag-Cl of C-2V symmetry in which the AgCl molecule lies along the C-2 axis of ethene that is perpendicular to the C2H4 plane. The Ag atom forms a bond to the midpoint (*) of the ethene pi bond. A partial r(s)-geometry and a r(0)-geometry were determined, with the values r(*center dot center dot center dot Ag) = 2.1719(9) angstrom, r(C-C) = 1.3518(4) angstrom, and r(Ag-Cl) = 2.2724(8) angstrom 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 C2H4 center dot center dot center dot Ag-Cl is formed and the strength of the pi center dot center dot center dot Ag bond are discussed. (C) 2011 American Institute of Physics. [doi:10.1063/1.3604821]