Structure and bonding of low-valent (Fischer-type) and high-valent (Schrock-type) transition metal carbene complexes

Quantum mechanical ab initio calculations are reported at the MP2 level of theory with effective core potentials for the heavy atoms of the low-valent carbene complexes [(CO)₅WCH₂] (1), [(CO)₅WCF₂] (2), [(CO)₅WCHF] (3), and [(CO)₅WCH(OH)] (4), and for the high-valent carbene complexes [F₄W(CH₂)] (5), [F₄W(CF₂)] (6), [Cl₄W(CH₂)] (7), [Br₄W(CH₂)] (8), [I₄W(CH₂)] (9), [(OH)₄W(CH₂)] (10), [F₅W(CH₂)](11), and [F₅W(CF₂)]_-(12). Metal-carbene bond energies are predicted at CCSD(T) with MP2 optimized geometries. The bonding situation is analyzed with the help of Bader's topological theory of atoms in molecules, Weinhold's NBO-partitioning scheme and the CDA method for donor-acceptor interactions. The analysis of the calculated data shows that the chemical and physical properties of the two types of compounds can: be understood when the electronic configuration at the metal is considered. The Taylor and Hall model is supported by the CDA results for the neutral compounds. This model suggests that the metal-carbene bonds in Fischer-type complexes are due to donor-acceptor interactions between the metal fragment and singlet carbenes, while Schrock-type complexes have normal covalent bonds between open-shell metal fragments and triplet carbenes. Donor-acceptor bonds are found for 1-4 and normal covalent bonds are found for 5-10. The high-valent negatively charged complexes 11 and 12, however, have donor-acceptor bonds.