Isocyanide and nitroso species

In contrast to the dioxygen, carbon-monoxide, and nitric-oxide ligands, the isocyanide and nitroso functions bear an organic tail. Moreover, nitroso ligands are isoelectronic with dioxygen.

Thus, in principle, not only may the steric bulk of the ligand be varied, in order to probe the dimensions 35 of the dioxygen-binding pocket, * but also the (jdonor/ 7T-acceptor properties of the ligands may be varied by appropriate substituents on the aryl ring.
 

Isocyanide groups may bind to metals in a variety of ways. For 1: I adducts (Figure 4.19), the isocyanide group is approximately linear, although some flexibility seems to exist in a bis(t-butylisocyanide)iron(Il)tetraphenylporphyrinato complex. 135 For zerovalent metals with much electron density available for donation

into ligand 77"* orbitals, the isocyanide ligand has been observed to bend at the N atom. 136 One prediction exists that an isocyanide ligand binds in this manner to hemoglobin. 

For 1: 1 adducts of nitroso ligands, side-on, 138 0-, and N-ligated modes are possible (Figure 4.19). No O-nitroso complexes have been definitively characterized by diffraction methods. For hemoglobin the N-nitroso mode is likely, since this is the mode found for the nitrosoalkane in Fe(TPP)(amine)(RNO).139 To date isocyanide ligands have not achieved their potential as probes of the geometry of the ligand-binding pocket in hemoglobin, partly because we lack structural data on the preferred geometry of attachment of these ligands in a sterically uncongested environment.