Revisiting exo–endo isomerization of transition metal half-sandwich η3–allyl complexes

Rotation of the η3‐Propargyl Ligand in [(C6Me5H)M(CO)2(η3‐C3H3)]+ (M=Cr, Mo, W) Complexes

“…The nuclei movements corresponding to the single imaginary frequency were visualized and analysed for all found transition states. For the allyl complexes, the transition states are similar to those obtained earlier [8] . For the propargyl molybdenum complexes, the transition states were additionally verified using the Nudged Elastic Band [34] ( TSr1 : Mo , TSr2 : Mo , and TS : Mo ) and Intrinsic Reaction Coordinate [35] ( TSc : Mo ) methods.…”

“…In accordance with the abovementioned literature data (Scheme 1a, b), it was initially assumed that this process involves η 3 ‐η 1 hapticity slippage of the propargyl ligand followed by a rotation around the σ(Mo−CH 2 ) bond [17] . However, later theoretical investigations showed that this η 3 ‐η 1 ‐η 3 isomerization pattern is unfeasible for similar allyl d 4 compounds [8,9] . For this reason, here we report the first systematic study on the dynamic behavior of [(C 6 Me 5 H)M(CO) 2 (η 3 ‐C 3 H 3 )] + ( comp : M , M=Cr, Mo, W) complexes.…”

“…4) The feasibility of the η 3 ‐η 1 ‐η 3 mechanism is governed by the strength of the metal‐allyl/propargyl bonding because it proceeds with the partial decoordination of the corresponding ligand. The binding energies for all considered complexes except [CpFe(CO)(C 3 H 5 )] hinder this process [8] . However, the activation energy is lower for η 3 ‐propargyl complexes (Figure 3, left) in accordance with their lower binding energies (Figure 3, right).…”

“…Geometry optimizations were carried out without symmetry constraints using B3LYP density functional [24] and tight convergence criteria as implemented in the ORCA 5.0.3 program package [18] . This functional was successfully used for studying the dynamic behaviour of allyl complexes of the same type [8,9] . The zeroth‐order regular approximation (ZORA) Hamiltonian [25] was used together with the relativistically recontracted Karlsruhe basis set ZORA‐def2‐TZVPP [26] for light atoms and SARC‐ZORA‐TZVPP [27,28] for a 4d or 5d metal, if any.…”

“…In contrast to analogous allyl complexes, the dynamic behaviour of η 3 ‐propargyl derivatives is not well understood. The exo‐endo isomerism in neutral and cationic allyl complexes was systematically investigated using experimental and theoretical approaches across the entire transition metals block [8–12] . It was found that two mechanisms (η 3 ‐η 3 ‐η 3 and η 3 ‐η 1 ‐η 3 paths) are responsible for the exo‐endo interconversion, and the feasibility of both mechanisms is strongly dependent on the type of complex [8] …”