Complexes of Transition Metals in High and Low Oxidation States with Side-On-Bonded π-Ligands

The equilibrium geometries of the transition metal compounds WCl₄L and W(CO)₅L (L = HCCH, C₂H₄, CO₂, CS₂, CH₂O) have been calculated at the HF and MP2 levels of theory by using a relativistic effective core potential for tungsten and valence shell basis sets of DZ+P quality. The W-L bond dissociation energies are predicted at CCSD(T). The calculations show that the inclusion of correlation energy is essential for the accurate description of transition metal donor—acceptor complexes. The theoretically determined geometries at the MP2 level are in very good agreement with experimental results. The calculated (CO)₅W—CO₂ bond strength (D₀ = 10.2 kcal mol^-1) is in accord with the experimental estimate (D₀ = 8.2 ± 1.0 kcal mol^-1). The calculations predict that the metal—ligand bond strength has the order for W(CO)₅L of L = C₂H₄ > HCCH > CH₂O > CS₂ > CO₂. For the WCl₄L complexes, the order is L = HCCH > CH₂O > C₂H₄ > CS₂ > CO₂. The different sequences of W-L bond strengths are explained by the nature of the metal-ligand interactions in the two sets of compounds. The W-L bonds of WCl₄L are covalent bonds, while the W(CO)₅L complexes have donor-acceptor bonds. Ethylene is a better donor than acetylene and formaldehyde and, therefore, forms a stronger bond in the W(CO)₅L complexes. The WCl₄L compounds with L = HCCH, C₂H₄, CH₂O should be considered as metallacycles. Since the carbon atoms of the metallacyclopropenes are approximately sp² hybridized, while the carbon atoms of the metallacyclopropanes are sp³ hybridized, the former W-L bonds are stronger than the latter. In general, the W-L bond dissociation energies are clearly smaller for the WCl₄L complexes than for W(CO)₅L, although the metal-ligand bonds are much shorter in the former compounds. This puzzling result can easily be explained by the high excitation energy that is necessary to promote the ligand into the triplet state prior to formation of the strong covalent bond. The classification of the WCl₄L compounds as covalently bonded molecules and the W(CO)₅L complexes as donor—acceptor species is supported by the examination of the electronic structure using topological analysis of the electron density distribution, covalent bond orders, and charge decomposition analysis (CDA). The CDA method is found to be particularly useful for the analysis of the metal-ligand interactions. For the W(CO)₅L complexes, reasonable values are calculated for the L → W donation, the W → L back-donation, and the W ↔ L repulsive polarization by using closed-shell fragments W(CO)₅ and L. The CDA results for WCl₄Lusing closed-shell fragments WCl₄ and L are physically unreasonable, which indicates that the Dewar-Chatt-Duncanson model for these compounds is not appropriate.