Figure 5.6 illustrates the infrared spectra of the Co(ll), Cr(ll) and Ni(ll) hex

Question

Figure 5.6 illustrates the infrared spectra of the Co(ll), Cr(ll) and Ni(ll) hexamine complexes [9). The spectra indicate that the type of metal in the complex produces important differences in the infrared bands of each complex. Significant differences in the avenumber values of each com-plex are noted for the metal-NH3 rocking (900-600 cm-1) and NH3 bending (1400-1100 cm-1) bands. Bands due to N-H stretching and N-H bending are also observed in these spectra in the 3700-2500 and 1750-1500 cm-1 regions, respectively. 100 80 60 40 20 Cr Ni 4000 3000 2000 1800 1600 1400 1200 1000 800 650 Wavenumber (om-) Figure 5.6 Infrared spectra of metal hexamine complexes. From Nakamoto, K., Infrared and Raman Spectra of Inorganic and Coordination Compounds, Part B, Applications in Coordination, Organometallic and Bioinorganic Chemistry, 5th Edn. Copyright © (1991 n Wilev&Sons.Inc.;). This material is used by permission of John Wiley &Sons;, Inc. Given that the stability order of the hexamine compounds, the spectra of which are shown in Figure5 Ni(lI), what do the infrared spectra indicate about the bonding in these compounds?

Explanation / Answer

In the given IR spectra shows a trend wavenumber shifts for the three hexamine complexes, the N-H band shifts to lower wavenumbers from Co to Cr to Ni. THis shows that N-H bond order or bond strength decreases as the metal-N bond order increases in the stability order Co > Cr > Ni.

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