## 3 Results and Discussion  
### IR Spectra and Chemical Images of Materials  
The average solid-state IR spectra of CF, CF-1, and CF-12 are shown in Figure 2. Isolated CF's show characteristic absorption peaks of silicon dioxide with the asymmetric \(\
u\)(Si-O-Si) mode being predominant at 1100 cm\({}^{-1}\). In addition to the characteristic silica signature, after chemisorption of ReCORM-2 and B\({}_{12}\)-ReCORM-2, the IR spectra of CF-1 (red spectrum in Figure 2) and CF-12 (blue spectrum in Figure 2) show respectively weak absorptions at 1970 and 1785 cm\({}^{-1}\), and 2000 and 1860 cm\({}^{-1}\). These are assigned respectively to the symmetric and asymmetric Re-C\(\equiv\)O \(\
u\)(CO) stretching modes of chemisorbed ReCORMs. It should be noted that following chemisorption of ReCORM-2, CF-1 also showed weak Si-O-Si overtone modes around 2000 and 1870 cm\({}^{-1}\) assigned to hydrated silicon dioxide [72] and two peaks centered at 1460 and 1400 cm\({}^{-1}\). These latter peaks are assigned to a combination of \(\delta\)(CH\({}_{3}\)) and \(\delta\)(CH\({}_{2}\)) vibrations either due to residual organic solvents or the to the Et\({}_{4}\)N\({}^{+}\) counter ion of _cis_-[Re(CO)\({}_{2}\)Br\({}_{4}\)]\({}^{2-}\). In the case of CF-12 the Si-O-Si overtones are hidden under the Re-C\(\equiv\)O \(\
u\)(CO) stretching modes.  
Figure 3 shows optical and MCT detector generated chemical images of single CF-1 and CF-12 valves, which were isolated following chemisorption of _Coscindiscus_ frustules treated with solutions of the CO releasing molecules ReCORM-2 and B\({}_{12}\)-ReCORM-2 [63]. The chemical maps of the selected vibrations show the following features. In both cases, the asymmetric \(\
u\)(Si-O-Si) band at 1100 cm\({}^{-1}\) shows the highest intensity in the central region of the diatom valve. The intensity of the \(\
u\)(Si-O-Si) mode slopes to lower values radiating away from the centre to the outer rim of the diatom, known as the girdle band. In the 3D surface representations of the chemical maps (Figure 3E) the roughness of the valve surface, composed of pores of ca. 0.5 \(\upmu\)m in diameter, can be somewhat appreciated. The symmetric and asymmetric Re-C\(\equiv\)O \(\
u\)(CO) stretching modes of chemisorbed ReCORM-2 and B\({}_{12}\)-ReCORM-2, show the highest intensity in the girdle band. This is most evident in the 3D surface representations (Figure 3F) of the symmetric \(\
u\)(CO) vibrations. Depending on the CORM used, the broad symmetric \(\
u\)(CO) mode is found centered at 2010 and 1996 cm\({}^{-1}\) for CF-12 and CF-1, respectively. The shift is fully consistent with the assignment. In B\({}_{12}\)-ReCORM-2, two of the coordinated Br\({}^{-}\) ions are substituted by  
Figure 2: Solid-state IR spectra (2200–600 cm\({}^{-1}\) range) of untreated _Coscindiscus_ frustules (CF, black), ReCORM-2 (CF-1, red trace) and B\({}_{12}\)-ReCORM-2 (CF-12, blue trace) treated _Coscindiscus_ frustules. Insert shows magnified 2100–1700 cm\({}^{-1}\) region. Asterisks (*) in the insert indicate hydrated silicon dioxide Si-O-Si overtone modes; \(\
u_{s}\)CO and \(\
u_{\mathrm{as}}\)CO indicate respectively the symmetric and asymmetric stretching Re-CO bands of the molecules.  
the cyanide group of vitamin B\({}_{12}\) and a solvent molecule [27]. On the basis of both theoretical [73; 74] and previous experimental evidence [61; 75], we demonstrated that by substituting \(\pi\)-basic ligands (such as Br\({}^{*}\)) with \(\sigma\)-donor or \(\pi\)-acidic ligands (such as CH\({}_{3}\)CN) the symmetric \(\
u\)(CO) stretching frequency of [M(CO)\({}_{\mathrm{x}}\)L\({}_{6\cdot\mathrm{x}}\)]\({}^{\mathrm{m}}\) complexes shifts to higher wavenumbers. The apparent discrepancy in the position of the symmetric Re-C\(\equiv\)O \(\
u\)(CO) stretching in CF-1 and CF-12, with respect to the solid-state IR (Figure 2), is due to the fact that MCT images were recorded with frustules immersed in a film of paratone oil on round CaF\({}_{2}\) windows.