An Experimental Determination of the Cr−DMB (DMB = 3,3-Dimethyl-1-butene) Bond Energy in Cr(CO)₅(DMB):  Effects of Alkyl Substitution on Chromium−Olefin Bond Energies in Cr(CO)₅(olefin) Complexes

Abstract

The chromium−olefin complex Cr(CO)5(DMB) (DMB = 3,3-dimethyl-1-butene) has been studied in the gas phase using transient infrared spectroscopy. This complex forms by addition of DMB to photogenerated Cr(CO)5 with a rate constant, k L = (7.0 ± 1.5) × 10-11 cm3 molecule-1 s-1. The bond enthalpy for the DMB−Cr(CO)5 bond has been determined from the kinetics for the decay of Cr(CO)5(DMB) to be 20.1 ± 1.7 kcal/mol at 298 K. An energy decomposition analysis has been performed for a series of Cr(CO)5(olefin) complexes (olefin = DMB, ethylene, propene, 1-butene, 1-hexene, cis-2-butene, trans-2-butene, isobutene, and tetramethylethylene (TME)) using density functional theory. These calculations provide insights into trends in the chromium−olefin bond energy. The results reveal that the trend in bond energies in these complexes correlates with the number and the nature of the alkyl groups around the double bond, and that the dominant factor in this trend is the deformation energy of the olefin and Cr(CO)5, where the deformation energy is the energy required to deform the olefin ligand and the unsaturated metal centered moiety from their isolated ground-state geometries to the geometry they adopt in the bound complex.

A Density Functional Theory Study of η2 Acyl Bonding in Fe and Mn Carbonyl Complexes

“…This result is not surprising because the greater steric interactions that would be expected as the (CH 3 ) 3 C moiety approaches the unsaturated Fe(CO) 3 CH 3 species would be expected to lead to a longer bond than when the smaller C 2 H 5 moiety approaches Fe(CO) 3 CH 3 . A similar effect was seen in the bonding of substituted olefins to Cr(CO) 5 . With everything else being equal, a decrease in the degree of orbital overlap would be expected to lead to a weaker metal−alkyl bond.…”

“…Our DFT calculations demonstrate that the CH 3 −Fe(CO) 3 CH 3 BDE of ∼41 kcal/mol is greater than the C 2 H 5 −Fe(CO) 3 CH 3 BDE of ∼34 kcal/mol, which is significantly greater than the (CH 3 ) 3 C−Fe(CO) 3 CH 3 BDE of ∼18 kcal/mol. This trend is counter to what is expected in terms of the degree of electron density available on the alkyl group, which is expected to increase with increasing chain length . A well known approach to calculating molecular orbital interaction energies takes into account overlap integrals as well as the energies of the interacting oribtals.…”

“…Though the trend is the same in both cases, there are considerable differences in the magnitudes of the energies calculated using eq 6 and the BDEs calculated by DFT (41.08, 33.91, and 18.21 kcal/mol respectively). DFT calculations of bond energies take into account orbital interaction energies as well as factors such as the deformation energy that lead to a bond energy that can be significantly less than the orbital interaction energy . Thus, DFT calculations of BDEs would be expected to be more accurate than the energies calculated using eq 6.…”

Electron-Withdrawing Effects on Metal−Olefin Bond Strengths in Ni(PH3)2(CO)(C2XnH4-n), X = F, Cl; n = 0−4: A DFT Study

“…Comparisons to Other Systems. Recent studies have focused on the analysis of bond energy trends in different organometallic olefin complexes. − ,− Among these studies, those by Weitz and co-workers, , Nunzi et al, and Massera and Frenking 47 have recognized the importance of reorganizational and steric effects in the metal−olefin bond strength. In their study of the series M(PH 3 ) 2 (C 2 X 4 ) (M = Ni, Pd, Pt and X = H, F, CN) Nunzi et al found that for M = Ni the Ni−C 2 X 4 bond strength follows the trend C 2 (CN) 4 > C 2 F 4 ≈ C 2 H 4 .…”

Transition Metal-Olefin Bonding Interactions: A Density Functional Theory Study [M(CO) x (Æ? 2 - C 2 H 3 - C 6 H 4 - Y)]

“…The interaction energy (ΔE int ) can then be further delineated as the sum of attractive ( DFT calculations and experimental data. 1,22,38 In this thesis, metal-olefin equilibrium geometries, bond formation energies (ΔE), enthalpies (ΔH), and free energies (∆G) for a select series of transition (M = Ni, Fe, Cr,…”