Structure and bonding in trimetallic arrays containing a Cr–Cr quadruple bond: A challenge to density functional theory

• The electronic structure of the quadruply bonded Cr2 unit is challenging.
• Optimised bond lengths are strongly dependent on the chosen functional.
• The presence of paramagnetic metals in a Cr2M array perturbs the Cr2 unit.
• The paramagnetic moment seeds the localisation of the electrons in the Cr–Cr bond.
• Judicious choice of initial spin polarisation can block the bond cleavage.

Structural trends within the series of trimetallics Cr2M(dpa)4Cl2, M = Cr, Mn, Fe, Ni and Zn offer an important test bed for the theoretical treatment of the quadruple Cr–Cr bond. Cr–Cr bond lengths are dramatically functional dependent, reflecting the different treatment of exchange and correlation in each case. The presence of a paramagnetic heterometal in a Cr2M(dpa)4Cl2 array has the effect of elongating the Cr–Cr bond by polarising the spin density in the Cr–Cr σ bond, effectively seeding the localisation of the remaining π and δ components.

Unpaired spin density on a heterometal can perturb the Cr–Cr quadruple bond in a linear trimetallic MCr2(dpa)4Cl2 array. The ability of density functional theory to capture these subtle effects in a series where M is Cr, Mn, Fe, Ni and Zn is critically assessed.Figure optionsDownload as PowerPoint slide