Past exam of the chemistry B course of the University of Cambridge / 2023 / Paper 2 / B8 / b / ii / Solution

A useful strongly bound limit is Zr(IV), $d^0$, and Ta(V), $d^0$, with an aryne dianion in each complex. Charge balance uses two Cp${}^{-}$ ligands for zirconium, and Cp*${}^{-}$ plus two methyl ligands for tantalum:Equivalently, an unreduced neutral-aryne description starts from Zr(II) and Ta(III), both $d^2$, and transfers that pair strongly into the aryne acceptor. These are alternative bookkeeping limits for the same covalent interaction; saying that a formal $d^0$ assignment has no available electrons must not erase the strong metal–carbon bonds in the reduced-ligand description.

Both ligands have therefore lost much of the extra in-plane C–C bonding of free benzyne and approach the same reduced, approximately double-bond-length C1–C2 framework. Their identical $1.364\text{A˚}$ C–C distances are consistent with similar aryne reduction in both complexes. The distance is intermediate in a delocalized ring, so it should not be converted into an exact integer bond order.

The M–C distances depend additionally on metal size, effective nuclear attraction and orbital overlap. The high formal charge of Ta(V), the size effects associated with the lanthanide contraction, and effective $5d$ covalent bonding favour shorter Ta–C contacts than Zr–C contacts. This is consistent with $2.075$ versus $2.250\text{A˚}$. Different coordination envelopes and ancillary ligands also contribute, so this pair is not a controlled measurement of a radius or a metal–carbon bond energy. Similar internal ligand bond order can coexist with different metal–ligand distances.