Calculations on indium and thallium cyclopentadienyls. Metal-metal interactions and possible new species

The possibility of an attractive interaction between two closed-shell molec ules containing In(I) or Tl(I) was studied using relativistic pseudopotenti als and correlated ab initio methods ranging from MP2 to CCSD(T). The resul ts for the simple test system (TlH)(2) qualitatively agree with earlier one s by Schwerdtfeger (Inorg. Chem., 1991, 30, 1660). The calculated dimerizat ion energy is -20 kJ mol(-1). The results for the more realistic [M(eta(5)- Cp)](2) model (M=In, Tl; Cp=cyclopentadiene, C5H5) yield a weaker attractio n of about - 12 and - 16 kJ mol(-1), respectively. The M-M secondary bond l engths are somewhat longer and the M-M-Cp angles more acute than in the dim ers found in solids. This is attributed to the crystal effects. The calcula ted structures for [M(eta(5)-Cp)(2)](-) anions agree with experimental ones for the known M=Tl case. Predicted structures are given for the [M(eta(5)- Cp-2)](+) cations and for a hypothetical M(eta(5)-Cp)(3).