The strength of the σ-, π- and δ-bonds in Re2Cl82-

The geometry of Re2Cl82- has been optimized for the eclipsed (D-4h) equilibrium conformation and for the staggered (D-4d) conformation at BP86/TZ2P. The nature of the Re-Re bond which has a formal bond order four has been studied with an energy decomposition analysis (EDA). The EDA investigation indicates that the contribution of the b(2)(delta(xy)) orbitals to the Re-Re bond in the (1)A(1g)(delta(2)delta*(0)) ground state is negligibly small. The vertical excitation of one and two electrons from the bonding delta orbital into the anti-bonding delta* orbitals yielding the singly and doubly excited states (1)A(1g)(delta(1)delta*(1)) and (1)A(1g)(delta(0)delta*(2)) gives a destabilization of 17.5 and 36.1 kcal/mol, respectively, which is nearly the same as the total excitation energies. The preference for the D-4h geometry with eclipsing Re-Cl bonds is explained in terms of hyperconjugation rather than delta bonding. This is supported by the calculation of the triply bonded Re2Cl8 which also has an eclipsed energy minimum structure. The calculations also suggest that the Re-Re triple bond in Re2Cl8 is stronger than the Re-Re quadruple bond in Re2Cl82-. A negligible contribution of the delta orbital to the metal-metal bond strength is also calculated for Os2Cl8 which is isoelectronic with Re2Cl82-.