Computational Studies of Metal−Ligand Bond Enthalpies across the Transition Metal Series

Abstract

Relative to the p-block of the periodic table, data for transition metal-ligand bond dissociation enthalpies are less comprehensive. Recent developments in computational methods make systematic assessment of trends in metal-ligand bond enthalpies across the transition series a relatively rapid and accurate exercise. We report a systematic study of metal-ligand bond enthalpies for saturated transition metal complexes that encompasses the entire d-block of the periodic table and a wide assortment of ligands. The saturated complexes have the form MH n -L such that closed-shell molecules are formed with the maximum number of two-center, two-electron (2c/2e) bonds under the constraint that the metal electron count does not exceed 12. Bond enthalpies for MH n -L molecules with higher electron counts (14 and 16 electrons) are assessed for some group 10 and 11 metals. The primary methods are density functional theory (DFT) using the hybrid B3LYP density functional and CCSD(T) ab initio computations. Bond enthalpies are reported as the first bond dissociation enthalpies for neutral and cationic complexes of the type MH n -R (R ) H, CH 3 , C 2 H 5 , CH(CH 3 ) 2 , C(CH 3 ) 3 , CH 2 F, C 2 H, C 2 H 3 , NH 2 , OH, F, and BH 2 ) for all transition elements. Electronic structure analysis of the complexes features natural bond orbital (NBO) analysis of bond polarity.

Hydrogen‐Atom Transfer in Reactions of Organic Radicals with [CoII(por)]. (por=Porphyrinato) and in Subsequent Addition of [Co(H)(por)] to Olefins

“…(8) and Table 1] represents one of the lower BDE values known for transition-metal-hydrogen bonds. [38] The computed Co À H BDE of 51 kcal mol À1 is consistent with the inability to obtain NMR observable concentrations…”

The Effects of Ancillary Ligands on Metal–Carbon Bond Strengths as Determined by C–H Activation

“…First, from the competition experiments, all of the hydrocarbons are activated with similar barriers -that is, the ΔΔG { only spans 1.8 kcal/mol, which corresponds to a 22:1 ratio at 25 C. This is because in the rate-determining step, the substrate is coordinating to the [Tp 0 Rh(CNR)] fragment via its C-H bond, and all of the hydrocarbons have similar binding affinities. For aromatic substrates, the arene can bind through its π-system, and this is why benzene and mesitylene are the Reproduced with permission of the ACS from Jones and Wick [9] 74 W.D.…”

“…One caveat that we encountered was that many of these substituted derivatives proved to be very stable. Loss of alkane from the n-pentyl hydride complex has a half-life of about an hour at 25 C. Methane loss from 3 has a half-life of about 5 h. Loss of benzene from 2, however, is extremely slow (months), and therefore, the rate of benzene reductive elimination at 25 C was determined by extrapolation from the rate at higher temperatures. The Eyring plot of ln(k/T) vs. 1/T gave activation parameters for reductive elimination of benzene ΔH { ¼ 37.8 (1.1) kcal/mol and ΔS { ¼ 23 (3) e.u., which can be used to calculate the rate at other temperatures.…”

Catalysis Involving the H• Transfer Reactions of First‐Row Transition Metals

“…Second -order kinetics have been 4 VH 57.9 dppe(CO) 4 VH 57.5 dppp(CO) 4 VH 56.0 dppb(CO) 4 VH 54.9 established (the rate constants are shown in Table 1.2 ) for the transfer of H • from a variety of hydrides to the tris( p -tert -butylphenyl)methyl radical [24 -26] . The bulky t -Bu substituents keep it entirely monomeric in solution [27,28] 15.7 a) Ref [120] . b) Ref [13] .…”

“…On the other hand V -H bonds are particularly weak. Calculations at the B3LYP level of theory on the hypothetical VH 5 give it the weakest M -H bond (43 kcal mol − 1 ) among neutral " valency -saturated " MH n (i.e., among complexes where M forms the maximum number of M -H bonds) [15] . Experimentally Table 1.1 [13] shows very weak V -H bonds for (P -P)(CO) 4 VH (P -P = Ph 2 P(CH 2 ) n PPh 2 , with n = 1 (dppm), n = 2 (dppe), n = 3 (dppp), and n = 4 (dppb) [16,17] .…”