Pentacoordinate nitrosyls

Coordinated NO is found in two basic geometries in transition metal complexes, linear and bent, exemplified by the molecules 17.57 and 17.58 as well as structures intermediate between the two. We shall concentrate on five-coordinate examples 

FIGURE 17.10. (a) bncrgclic behavior of tlie stjuare planar ML4 J IcvcLs on pyramidalization. (b) Interaction of these levels with those ol NO to ive a stjuuTe pyramidal MU4NO species. 

FIGURE 17.11. I nerj clic behavior of Ihc metal d and nit rosy 1 -n levels on bending the M-N-0 unit. Adapted from Ret. 

Cotton and R. A. Walton. Multiple Bonds Between Metal Atoms, Wiley, New York (1982). 

Energetic behavior of the metal d and nitrosyl jt levels on bending the M-N-O unit. Adapted from Reference [54], the extended Hiickel calculation refers to an iridium nitrosyl species. 

Plot of the M-N-O bond angle versus the symmetry measure for a trigonal bipyramid, S(TBP) in molecules with a MNO electron configuration. The plot has been adapted from Reference [59], 

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A similar analysis has been carried out for pentacoordinate nitrosyls (37) and precise rules were derived for the preferred conformations, on orbital interaction grounds alone. 

Hoffmann R, Chen MML et al (1974) Pentacoordinate nitrosyls. Inorg Chem 13 2666-2675... 

Figure 4. Resonance Raman spectra of human nitrosylhemoglobin in the T and R structures. The R structure shows a single band at 1634 cmcharacteristic of hexacoordinated nitrosyl heme. The T structure shows two bands of equal intensity, one at 1634 cmand the other at 1644 cm x, characteristic of hexa- and pentacoordinated hemes, respectively (40).

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