Figure 4.

Computational works predict a certain flexibility of the Cr(II) sites at the silica surface. The figure shows several proposed structures for different Cr(II) sites and related carbonyl species as obtained from DFT calculations by (a) Scott and co-workers¹²⁰ and (b) Damin and co-workers¹⁰⁷. Species 1–3 are embedded in a 6CR, while species 4 belongs to an 8CR (in blue). Additional weaker oxygen ligands are shown in red. For species 4, the Cr(II) is stabilized by an extra oxygen ligand belonging to its own 8CR. Relevant bond distances and angles are reported, as well as the computed Δν(CO) values. (a) Adapted with permission from ref (120). Copyright 2012 Elsevier. (b) Adapted with permission from ref (107). Copyright 2015 Elsevier.