Density Functional Studies On The Structures And Thermal Stabilities Of The Derivatives Composed Of Transition Metal And Steric Ligands

Dispersion effects widely exist between molecules and between different functional groups in molecules,which play an important role in the stability of molecules,especially for molecules with large geometry or ligand volume.In this paper,the B3PW91-D3 and?B97x D methods density functional methods are used to systematically study dispersion effect on the thermal stabilits of transition metal derivatives（1-norbonyl）₄Fe and（cyclohexyl）₄Fe with high valent M⁴⁺transition metal atom and large steric ligands;The geometric structures,thermal properties,and the bonding characters of the（1-norbonyl）Co（CO）_n（n=4,3,2,1）and（1-norbonyl）₂Co₂（CO）_n（n=7,6,5）compounds are also investigated.The specific research results are as follows:（1）Studies on the compounds（1-norbonyl）₄Fe and（cyclohexyl）₄Fe show that the high stability of the experimentally known homoleptic 1-norbornyl derivative（1-norbonyl）₄Fe of iron in the unusual+4 oxidation state is a consequence of the high reaction barriers of the singlet or triplet potential surfaces constrained by the global dispersion attraction and the large steric demands of the norbornyl groups.The much more limited stability of the corresponding cyclohexyl derivative（cyclohexyl）₄Fe may result from the conical intersection between the singlet potential surface and the quintet spin potential surface arising from the weaker dispersion attraction and the reduced steric effect of the cyclohexyl groups relative to the1-norbornyl groups.In contrast,the high stability of the likewise experimentally known（cyclohexyl）₄M（M=Ru or Os）structures results from the larger ligand field splitting（Δ）of thed-orbital energies for the second and third row transition metals ruthenium and osmium relative to that of the first row transition metal iron.The cyclohexyl derivative（cyclohexyl）₄Fe is predicted to be reactive towards carbon monoxide to insert CO into up to two Fe–C bonds.However,in the（1-norbonyl）₄Fe derivatives,the strong dispersion effect and large steric hindrance between the 1-norbonyl ligands on the cryostat substrate make it difficult for carbon monoxide to approach the iron atom center in（1-norbonyl）₄Fe,and similar insertion reactions occur with carbon monoxide.（2）Stuides on the structures of（1-norbonyl）Co（CO）_n（n=4,3,2,1）and（1-norbonyl）₂Co₂（CO）_n（n=7,6,5）show that low-energy structures of the unsaturated systems（1-norbonyl）Co（CO）_n（n=3,2）and（1-norbonyl）₂Co₂（CO）_n（n=6,5）have agostic hydrogen atoms from a CH₂group adjacent to the Co–C bond.Such agostic hydrogen atoms form a C–H–Co bridge with a bonding Co–H distance less than～2?.In such structure unsaturation is relieved by donation of an additional two electrons from the C–H bond of this norbornyl CH₂ group.In addition,structures in which carbonyl migration from cobalt to carbon has occurred to form acyl derivatives with 1-norbonyl CO ligands are among the lowest energy structures.The resulting acyl carbonyl groups of the 1-norbonyl CO ligands serve as spacers between the bulky 1-norbornyl ligand and the cobalt carbonyl moiety.Furthermore,such neutral 1-norbonyl CO acyl ligands can either be one-electron donors to a cobalt atom,bonding solely through the carbonyl carbon,or three-electron donorη²-μ-1-norbonyl CO groups bridging a central Co₂unit through both the acyl carbon and oxygen atoms.The strengths of the agostic C–H–Co interactions have been characterized by their reduced density gradient（RDG）values.