Fig. 18 Computational models for grafted “(SiO) 2 Cr” sites, differing in total Cr coordination
number, are shown without (1d and 4d) and with (3d and 5d) coordinated CO. The numbers over
the arrows denote binding energies in kcal mol
À1
, relative to the CO-free Cr(II) clusters (2d and
4d) and free ligands (CO or siloxane). Color key: Cr (yellow), O (red), Si (blue), C (green), H
(white). Inner layer: ball-stick; outer layer: wireframe
Scheme 11 Proposed structures for chromate sites (1e, 2e) embedded in six-membered
chromasiloxane rings (blue) on highly dehydroxylated amorphous silica; the corresponding Cr
(II) sites, Cr(II)/S948-500 (3e) and Cr(II)/S948-800 (4e) formed upon reduction, (showing the
distances obtained by EXAFS curve fitting); and their corresponding carbonyl complexes (5e, 6e).
Additional siloxane and carbonyl ligands are shown in red. Bond length is in A ˚
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number, are shown without (1d and 4d) and with (3d and 5d) coordinated CO. The numbers over
the arrows denote binding energies in kcal mol
À1
, relative to the CO-free Cr(II) clusters (2d and
4d) and free ligands (CO or siloxane). Color key: Cr (yellow), O (red), Si (blue), C (green), H
(white). Inner layer: ball-stick; outer layer: wireframe
Scheme 11 Proposed structures for chromate sites (1e, 2e) embedded in six-membered
chromasiloxane rings (blue) on highly dehydroxylated amorphous silica; the corresponding Cr
(II) sites, Cr(II)/S948-500 (3e) and Cr(II)/S948-800 (4e) formed upon reduction, (showing the
distances obtained by EXAFS curve fitting); and their corresponding carbonyl complexes (5e, 6e).
Additional siloxane and carbonyl ligands are shown in red. Bond length is in A ˚
168
R. Cheng et al.