There has been discussion as to whether or not π-loading is a real phenomenon, how significant it may be, and if so, is its influence overstated? Π-loading is defined by Wigley as the weakening of transition metal-ligand π-bonds through competition for the M dπ – E pπ molecular orbitals in systems with more than one multiply bonded ligands. In the years since Wigley’s hypothesis, a variety of organometallic catalytic mechanisms have been rationalized through the lens of π-loading. Moulder and Cundari’s earlier findings of M=E π-bond strengths as diagnostic for the C-H bond activation mechanism is suggestive that further π-loading to reduce M=N π-bond strengths may be of catalytic benefit. This strategy may be particularly interesting for complexes like nitridos, which are often unreactive in the absence of high dn count, electron-rich metals. Yet, π-loading has never been rigorously tested. This computational investigation into the π-loading phenomena uses small early transition metal models to isolate the impact of competition among metal dp – ligand pp molecular orbitals while minimizing steric and other effects that may lead to ambiguity. To assess how the supporting and actor ligand environment can impact metal-element bonds, as well as their σ- and π-bond enthalpy components, a series of models using molybdenum and tungsten is employed initially with hopes to expand to a larger data set. The use of such simple models also allows for coupled cluster calculations to further interrogate the nature of the bonding, and assess the accuracy and precision of DFT methods when, as is routinely the case, there is a dearth of corroborating experimental thermodynamic data.

Is π-loading real: And, if so, could it be used in catalyst design?

ACS Spring Meeting 2021

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