Nitrosyl complexes square pyramidal

Coordination about the osmium in 47 is best regarded as distorted trigonal bipyramidal with axial triphenylphosphine ligands. The distortion is toward a square pyramidal geometry with an apical nitrosyl ligand. Coordination about Os in the six-coordinate phenylcarbene complex is octahedral, as expected. 

Fig. 7 also shows that base binding in the trans position weakens the metal to nitrosyl backbonding in that the Fe-NO bond length increases by 0.3 A and the Fe-N-O bond angle decreases 9°. In concert with this, the displacement of the Fe from the porphyrin plane decreases from 0.21 A in the square pyramidal structure to 0.07 A in the octahedral complex. The equilibria of NO binding to hemes is summarized in Fig. 8 [64], which indicates that NO binds to Fe" better... 

The product is square pyramidal with a bent nitrosyl ligand (Z. Ir—N—O - 124 ) at ihe apical position (Fig. 15.15a).44 Other complexes with a bent M—NO have been found, including the remarkable example [Rubent nitrosyl groups45 (Fig. 15.15b). 

Nitrosyl and S02 ligands in axial positions in trigonal-bipyramidal and basal sites in a square-pyramidal MB 8 complex prefer to be linearly (or q1-planar) coordinated. 

In the solid, the square pyramidal ion has bent apical and linear basal NO groups. The linear-bent transition probably requires little energy and may well be involved in reactions of dinitrosyl intermediates in reactions such as that noted previously, leading to a hyponitrite complex, or in reactions of NO and CO, catalyzed by nitrosyl complexes, to give N20 and C02. 

A. This type of argument was first proposed to account for the tendency of nitrosyls to bend toward the acceptor ligand in square pyramidal (d ) ML4NO complexes ... 

From presently known structural data, the orientation of the SO2 group in most SP complexes can be rationalized by considering the effect of basal donors and acceptors on the ML4(S02) molecular orbitals. The factors involved in determining the rotational orientation of the bent axial nitrosyl in MNO square-pyramidal complexes have been considered extensively by several authors For convenience, we reproduce two rules, which may be of some utility in rationalizing orientational features of the SO2 complexes discussed here (see especially Ref. 48 for underlying assumptions) (1) (a) In... 

Although the bis(dithiocarbamate) complexes of Fe(II) are relatively unstable to air oxidation, early studies (12, 15) produced stable adducts of NO and CO. Both 5-coordinate [Fe(NO)(I dtc)2] and 6-coordinate [Fe(NO)2(R2dtc)2] complexes are known. There has been considerable interest in the mode of attachment of the NO molecule, as there are six possibilities (see Scheme 4). (A) and (B) represent valence-bond structures of the linear Fe-NO bond. Structure (C) involves a symmetric Fe-NO TT-bond. Structure D illustrates the bent mode of attachment, in which nitrosyl is coordinated to the metal through the nitrogen atom, but the Fe-NO bond-angle differs greatly from 180°. Structures (E) and (F) are valence-bond formalisms of an unsymmetrical, metal-NO 7r-bond. The structure of [Fe(NO)(R2dtc)2] (R = Me or Et) has been shown (230,231) to be square pyramidal, with four sulfur atoms in... 

M = Rh, 1626 cm M = Ir, 1545 cm ) led to the incorrect conclusion that different solid-state structures are adopted by the two compounds (60). However, X-ray powder data showed the rhodium material to be isomorphous with the structurally characterized iridium complex, which in the solid state has a square-pyramidal geometry and a bent apical nitrosyl ligand (61). 

Well-characterized complexes of the MNO 8 class are scarce (Table 7.2).62 Five-coordinate species with bound NO and square pyramidal geometries are dominant,10 18 probably related to the great trans-effect of NO-. Most common are the cobalt nitrosyl porphyrins.18 By reacting with monodentate ligands (NCS-, Cl-, Br-, etc.), six-coordinate complexes may be obtained. Reaction 7.28 describes the preparation of the first compound of this type studied by X-ray methods.13s... 

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