Nitrosyl derivatives of metals

As a monodentate ligand in metal complexes NO behaves in one of two ways (a) as a 12-electron unit (NO ) when it forms a single bond to M with M-N—0 around 125°, and (b) more usually, as a 10-electron unit (NO, when M—N is a multiple bond and M-N-0 is approximately linear. Examples of NO behaving as a bridging ligand are given later. 

Some of the compounds in Table 18.5 are also of interest in connection with the stereochemistry of the transition-metal atom, for example, the tetragonal pyramidal structures of [Ir(CO)I(P03)2NO] and the Fe and Co compounds M(N0)[S2CN(CH3)2] 2, (c), and Mo(CsH5)3(NO), (d). The Fe molecule of type (c) 

In the molecule (e) NO behaves both as a monodentate ligand of type (b) (angle Cr—N—0, 179°) and also as a bridging ligand. The Cr—Cr distance indicates a metal-metal bond. An even more complex behaviour is exhibited in (CsHs)3Mn3(NO)4. Here NO is functioning in two ways, as a doubly and as a 

The crystal structure of one of the black Roussin salts, CsFe4S3(NO)7. HjO, has been determined. The nucleus of the ion consists of a tetrahedron of Fe atoms with S atoms above the centres of three of the faces (Fig. 18.4(a)). One iron atom 

The molecule of the red ester, Fe2S2(NO)4(C2Hs)2, has the structure shown in Fig. 18.4(b), in which Fe forms tetrahedral bonds to two S and two NO. The Fe—Fe distance (2-72 A) is very similar to that in the black salt, and there must be sufficient interaction between these atoms to account for the diamagnetism of this compound. For nitrosyl carbonyls see p. 772. 

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