Computational study of the energetics of³Fe(CO)₄,¹Fe(CO)₄and¹Fe(CO)₄(L), L = Xe, CH₄, H₂and CO

Abstract

Large basis CCSD(T) calculations are used to calculate the energetics of 3Fe(CO)4, 1Fe(CO)4 and 1Fe(CO)4(L), L = Xe, CH4, H2 and CO. . The relative energy of the excited singlet state of Fe(CO)4 with respect to the ground triplet state is not known experimentally, and various lower levels of theory predict very different results. Upon extrapolating to the infinite basis set limit, and including corrections for core-core and core-valence correlation, scalar relativity, and multi-reference character of the wavefunction, the best CCSD(T) estimate for the spin-state splitting in iron tetracarbonyl is 2 kcal mol(-1). Calculation of the dissociation energy of 1Fe(CO)4(L) into singlet fragments, taken together with known experimental behaviour of triplet Fe(CO)4, provides independent evidence for the fact that the spin-state splitting is smaller than 3 kcal mol(-1). These calculations highlight some of the challenges involved in benchmark calculations on transition metal containing systems.

Computational insights into the mechanism of iron carbonyl-catalyzed ethylene hydrosilylation or dehydrogenative silylation

“…Our gas phase calculation show Fe(CO) 5 has a D 3h ground state, which is in agreement with previous literatures [49,50]. The most stable Fe(CO) 4 (1) has a triplet ground state in C 2v symmetry and the corresponding singlet state is calculated to be slightly less stable, being consistent with previous observations and computations [51][52][53]. The other conformations for Fe(CO) 4 are much higher in energy.…”

“…The other conformations for Fe(CO) 4 are much higher in energy. Our calculated singlet-triplet splitting energy of 1.79 kcal/ mol is close to the CCSD(T) value of 2 kcal/mol [53] and the B3PW91 value of 3.5 kcal/mol [54], although the experimental singlet-triplet splitting is unknown. The computed CO bond dissociation enthalpy (DH 298 ) leading to the most stable C 2v triplet state is 42.16 kcal/mol, in excellent agreement with the experimental value of 42 ± 2 kcal/mol [55], and other theoretical values [56][57][58].…”

A new local density functional for main-group thermochemistry, transition metal bonding, thermochemical kinetics, and noncovalent interactions

“…52,60,[133][134][135][136][137][138][139][140][141][142][143][144][145][146] Thus M06-L is the only functional in the top three for all these databases.…”

“…The addition of this data set is prompted by the increasing attention being paid by many workers to the relative energies of spin states of transition metal systems because of the importance of spin states for structures, properties, and chemical reactivities of organometallic complexes and for functional nanotechnology. [52][53][54][55][56][57][58][59][60][61][62] Holthausen made a systematic study 63 of the ability of density functionals to predict 3d-4s excitation energies in transition metal cations and found that some functionals, even though performing well for main group atomization energies, show large errors for transition metal atomic excitation energies. Because we seek a functional that is accurate for both main-group and transition metal chemistry, our small database has both types of atoms.…”

Spin-state splittings, highest-occupied-molecular-orbital and lowest-unoccupied-molecular-orbital energies, and chemical hardness

“…4 The more efficient Hartree-Fock or density-functional theory ͑DFT͒ methods are unreliable and tend to give systematic errors. HartreeFock theory, which accounts for electron exchange exactly, but neglects correlation between opposite-spin electrons, always favors high-spin configurations.…”