Seven-electron ligands

Cycloheptatriene complexes, in which the olefin is acting as a 6-electron ligand, may undergo hydride abstraction to give ir-cycloheptatrienyl metal cations, e.g. 

The reverse type of reaction can also occur, when ir-cycloheptatrienyl metal cations are treated with anions such as Y = CN , OMe, CH(COOEt)2 

In some cases, the expulsion of a hydrogen atom from cycloheptatriene occurs spontaneously, e.g. 

The structure of Tr-cyclopentadienyl-Tr-cycloheptatrienyl vanadium, 7.30, has been determined by X-ray diffraction. The C7H7 ring is planar and the C—C bonds distances are equal within experimental error. 

An example of a ring replacement reaction involving mixed sandwich complexes is, 

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Seven-electron ligands, mixed sandwich complexes, related azulene derivatives and cyclo-octatetraene complexes... 

SEVEN-ELECTRON LIGANDS as shown in the carbonyl displacement reactions,... 

However, its isoelectronic unsymmetrical counterpart, 7r-cyclopen-tadienyl-7r-cycloheptatrienylchromium(0) can be reduced to a stable anion radical (41). The orbital occupied by the unpaired electron has dominant ligand tt character. It appears to consist of a combination of the antibonding e2 MO s of the cyclopentadienyl (Cp) and cyclohepta-trienyl (Tr) 7r-systems with about one-third Cp and two-thirds Tr, respectively (41). Reduction of 7r-cyclopentadienyl-7r-methylcyclohepta-trienylchromium leads to an unstable radical anion that decomposes by loss of the seven-membered ligand (41). 

The following compounds of the p-block elements contain seven electron pairs in the valence shell, but the six unidentate ligands are octahedrally arranged, with only small distortions 264... 

Fig. IS. Triply bridging ligands as seven-electron donors.

T. A. Stephenson Metal compounds containing six-electron and seven-electron organic ligands. 

Loss of two CO ligands from Os.- ( - / -Ph)( -C4H6N)(CO)io results in a rearrangement of the phenyl bridge to the seven-electron donating ju-rj ri -mode. 

Given that phenyl ligands can have the various modes of coordination A to C (Fig. 3) using just the one carbon atom, one can imagine that seven-electron donation could be achieved by in conjunction with different types of coordina-... 

According to the Enemark-Feltham approach, the factor that determines the bent vs. linear NO ligands in octahedral complexes is the sum of electrons of ji-symmetry. Complexes with Jt-electrons in excess of 6 tend to have bent NO ligands. Thus, [Co(ethylenediamine)2(NO)Cl], with seven electrons of Jt-symmetry (six in t2g orbitals and one on NO), adopts a bent NO ligand, whereas [Fe(CN)5(NO)] , with six electrons of ji-synunetry, adopts a linear nitrosyl [40]. 

Complexes Containing Seven and Eight Electron % Ligands... 

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