| Abstract
| - The equilibrium geometries and bond dissociation energies of 16VE and 18VE complexes ofruthenium and iron with a naked carbon ligand are reported using density functional theory at the BP86/TZ2P level. Bond energies were also calculated at CCSD(T) using TZ2P quality basis sets. The calculationsof [Cl2(PMe3)2Ru(C)] (1Ru), [Cl2(PMe3)2Fe(C)] (1Fe), [(CO)2(PMe3)2Ru(C)] (2Ru), [(CO)2(PMe3)2Fe(C)] (2Fe),[(CO)4Ru(C)] (3Ru), and [(CO)4Fe(C)] (3Fe) show that 1Ru has a very strong Ru−C bond which is strongerthan the Fe−C bond in 1Fe. The metal−carbon bonds in the 18VE complexes 2Ru−3Fe are weaker thanthose in the 16VE species. Calculations of the related carbonyl complexes [(PMe3)2Cl2Ru(CO)] (4Ru),[(PMe3)2Cl2Fe(CO)] (4Fe), [(PMe3)2Ru(CO)3] (5Ru), [(PMe3)2Fe(CO)3] (5Fe), [Ru(CO)5] (6Ru), and [Fe(CO)5] (6Fe) show that the metal−CO bonds are much weaker than the metal−C bonds. The 18VE ironcomplexes have a larger BDE than the 18VE ruthenium complexes, while the opposite trend is calculatedfor the 16VE compounds. Charge and energy decomposition analyses (EDA) have been carried out forthe calculated compounds. The Ru−C and Fe−C bonds in 1Ru and 1Fe are best described in terms oftwo electron-sharing bonds with σ and π symmetry and one donor−acceptor π bond. The bonding situationin the 18 VE complexes 2Ru−3Fe is better described in terms of closed shell donor−acceptor interactionsin accordance with the Dewar−Chatt−Duncanson model. The bonding analysis clearly shows that the16VE carbon complexes 1Ru and 1Fe are much more strongly stabilized by metal−C σ interactions thanthe 18VE complexes which is probably the reason why the substituted homologue of 1Ru could becomeisolated. The EDA calculations show that the nature of the TM−C and TM−CO binding interactionsresembles each other. The absolute values for the energy terms which contribute to ΔEint are much largerfor the carbon complexes than for the carbonyl complexes, but the relative strengths of the energy termsare not very different from each other. The π bonding contribution to the orbital interactions in the carboncomplexes is always stronger than σ bonding. There is no particular bonding component which is responsiblefor the reversal of the relative bond dissociation energies of the Ru and Fe complexes when one goesfrom the 16VE complexes to the 18VE species. That the 18 VE compounds have longer and weaker TM−Cand TM−CO bonds than the respective 16 VE compounds holds for all complexes. This is because theLUMO in the 16 VE species is a σ-antibonding orbital which becomes occupied in the 18 VE species.
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