Bent-bond formalism

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... 

Solution. A ring structure is not likely, because of the strain of the bent bonds. The linear structure N—N=Q completes the octet for each atom, but we reject it because of the double negative charge on the end nitrogen atom. The two other structures that complete the octet for each atom are N=N—O and N=N=Ot, each of which has formal charges on two atoms as shown. These two structures look equally good, and we conclude that the molecule can best be described as the resonance hybrid with the two structures contributing about equally. 

For example, leinicyanoel%lene has a formal C=C douMe bond (l33.9[an) in the free ligand but m Ihe complex [Pl C2(CN)4l(PPh3)2) the C-C dislance (152pm) is that of a single bund and Ihe CN o[Pg.931]

Group 2 complexes are formally electron deficient and conformationally floppy only small energies (often only 1-2 kcal mol-1) are required to alter their geometries by large amounts (e.g., bond angles by 20° or more). In such cases, the inclusion of electron-correlation effects becomes critical to an accurate description of the molecules structures. Both HF/MP2 and density functional theory (DFT) methods have been applied to organoalkaline earth compounds. DFT approaches, which implicitly incorporate electron correlation in a computationally efficient form, are generally the more widely used. Molecular orbital calculations that successfully reproduce bent... 

Method (i) is a route commonly utilized in monometal nitrosyl complexes. The nitrosyl ligand may function as (formally) a three-electrop donor (NO+) with a linear bonding mode, or as (formally) a one-electron donor (NO ) with a bent (—120°) M-N-0 arrangement. Conversion of the M-NO system to a M-NO system has two effects. First, it increases the metal oxidation state by two second, it generates a vacant coordination site. The dinitrosyl cluster Os3(CO)8(NO)2, which has... 

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