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

Quantum mechanical ab initio calculations are reported for 13 low-valent (Fischer-type) and 13 high-valent (Schrock-type) tungsten carbyne complexes. The geometries have been optimized at the HF and MP2 levels of theory with relativistic effective core potentials for the heavy atoms with valence basis sets of DZP quality. Tungsten-carbyne bond dissociation energies are predicted at CCSD(T) with MP2 optimized geometries. The electronic structure of the complexes and the metal-ligand bonding have been analyzed with the help of the NBO method, the topological analysis of the electron-density distribution and the CDA method. The L(n)W-CR bonds of the Fischer and Schrock carbyne complexes are much stronger than those of related carbene complexes. The strength of the L(n)W-CR bond is strongly influenced by the nature of R. Substituents with p(π) lone-pair electrons yield lower bond dissociation energies. This can be explained by a bonding model that uses the ₁Σ⁺ ground state of CR⁺ as reference state for the ligand of the Fischer complexes and the ₄Σ^- excited state of CR as reference state for the ligand of Schrock complexes.