The strength of the σ-, π- And δ-bonds in Re ₂Cl₈^2-

The geometry of Re₂Cl ₈ ^2- 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 (δ _xy ) orbitals to the Re-Re bond in the ¹A_1g (δ²δ ^*0) ground state is negligibly small. The vertical excitation of one and two electrons from the bonding δ orbital into the antibonding δ*orbitals yielding the singly and doubly excited states ¹A_1g (δ¹δ^*1) and ¹A_1g (δ⁰δ^*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 δ 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 Re ₂Cl₈ is stronger than the Re-Re quadruple bond in Re ₂Cl ₈ ^2- . A negligible contribution of the δ orbital to the metal-metal bond strength is also calculated for Os ₂Cl₈ which is isoelectronic with Re₂Cl ₈ ^2- .