Understanding and Predicting Distorted T- versus Y-Geometries for Neutral Chromous Complexes Supported by a Sterically Encumbering β-Diketiminate Ligand

A series of three-coordinate Cr(II) complexes sharing the common molecular fragment “(nacnac)Cr” (nacnac− = [ArNC(tBu)]2CH, Ar = 2,6-iPr2C6H3) were prepared via salt metathesis with the dimer [(nacnac)Cr(μ-Cl)]2. Single-crystal X-ray diffraction studies revealed that the complexes (nacnac)Cr(L) (L = CH2tBu, CH3, CH2CH3, SiH{2,4,6-Me3C6H2}2, O{2,6-iPr2C6H3}, N{CH3}2) represent a rare class of mononuclear, neutral chromium complexes with a three-coordinate high-spin chromous metal center. Depending on the nature of the third ligand, L−, these complexes can adopt either distorted T-shaped or Y-shaped coordination geometries. Density functional theory calculations and molecular orbital analyses in combination with a detailed molecular fragment energy decomposition were used to establish an intuitive concept of the key electronic structure patterns that determine the coordination geometry of preference. The frontier orbitals of the (nacnac)Cr(II) fragment direct π-donating ligands to adopt Y-shaped geometry, whereas ligands that are primarily σ-donors prefer T-shaped coordination. The relationship between electronics at the metal center and coordination geometry was extended to include the putative neutral three-coordinate high-spin complexes of Sc(II) and Mn(II), which are predicted to both adopt Y-shaped geometry.