Methods and Effective Bond Orders

Most of the description of metal-metal multiple bonds presented in this paper are based on calculations performed with the complete active space self-consistent-fleld (CASSCF) method [8] followed by second order perturbation theory, CASPT2 [9], The CASSCF method guarantees a qualitatively correct representation of the electronic structure, and the metal-metal multiple bonds. The subsequent 

The Chemical Bond Chemical Bonding Across the Periodic Table, First Edition. 

I 9 Modeling Metal-Metal Multiple Bonds with Multir rence Quantum Chemical Methods 

G SPT2 treatment ensures quantitatively correct energy differences among different electronic states. 

The choice of the active space is the challenging aspect of these calculations. An appropriate active space should include the metal-metal bonding and antibonding orbitals and also the necessary hgand orbitals that interact with the metal d- and s-type orbitals. This active space comprises at least all the nd (n= 3, 4 or 5) and ( +l)s orbitals forming the M-M multiple bond, namely, 12 orbitals. From a G SSCF wavefunction it is possible to compute the effective bond order (EBO) [10, 11] which quantifies the formation of a chemical bond. For a single bond the EBO is defined as  

---