Ligand Field Theory for Linear ML₂Complexes

Abstract

Point‐charge crystal field theory (CFT) for linear ML2 systems guarantees dz2>dxz/dyz>dxy/dx2‐y2 (i. e. dσ>dπ>dδ). This is not what is found for CuCl2 and other linear, divalent MX2 complexes where dπ>dσ. This failure of CFT has also been attributed to its successor, ligand field theory (LFT). However, taking LFT to be parameterised CFT, any d orbital sequence is possible. The real test is whether the LFT parameters make chemical sense. A new cellular ligand field (CLF) analysis of CuCl2, including the equatorial toroidal void, which affects all the d orbitals, confirms eπ(Cl)>eσ(Cl). To account for this, the ‘distributed donation’ model is proposed linking σ and π bonding such that enhanced π donation is at the expense of reduced σ bonding and vice versa. This is the π‐donor analogue of the Dewar‐Chatt‐Duncanson model for π‐acceptor ligands. Ab initio LFT (AI LFT) calculations support a progressive π‐donation increase/σ donation decrease from Cu(II) to Sc(II) with eσ finally becoming negative. The model further predicts that adding equatorial ligands will force the π density back into the σ bond which is also confirmed by AI LFT calculations for trans‐[CuCl2(NH3)4] where eσ(Cl)>eπ(Cl) even though the CuCl2 unit is unchanged. The distributed donation model for linear σ‐only [MII(Lσ)2]n systems (Lσ=NH3, H), where increasing π donation is not possible, implies that the M−L bonding in linear systems should be inherently weak.

Quantifying the influence of 3d–4s mixing on linearly coordinated metal-ions by L2,3-edge XAS and XMCD

“…1) is of general importance to fundamental understandings of electronic structure. Both ligand-field theories 9–11 and ab initio calculations 12,13 predict d–s orbital mixing for metal-ions spanning the periodic table from transition metals, to lanthanides 14,15 and actinides. 12,16 Electron paramagnetic resonance (EPR) can be applied for Kramers ions to quantify d–s orbital mixing via the hyperfine effect.…”