Tetracyanoethylene complexes, bonding

Intramolecular rearrangement in five-co-ordinate cobalt(i) and rhodium(i) complexes of the type [ML4((CN)2C—C(CN)2 ]+, where L represents various cyano-substituted organic ligands, are shown to be due to Berry pseudorotation, to rotation around the tetracyanoethylene double bond, and to catalysis by anions due to formation of ion pairs. 

The dipole moment varies according to the solvent it is ca 5.14 x 10 ° Cm (ca 1.55 D) when pure and ca 6.0 x 10 ° Cm (ca 1.8 D) in a nonpolar solvent, such as benzene or cyclohexane (14,15). In solvents to which it can hydrogen bond, the dipole moment may be much higher. The dipole is directed toward the ring from a positive nitrogen atom, whereas the saturated nonaromatic analogue pyrroHdine [123-75-1] has a dipole moment of 5.24 X 10 ° C-m (1.57 D) and is oppositely directed. Pyrrole and its alkyl derivatives are TT-electron rich and form colored charge-transfer complexes with acceptor molecules, eg, iodine and tetracyanoethylene (16). 

In another publication, Holsboer et al. [280] have investigated bond properties in the five-coordinated Ir(I) complexes (PPh3)3lr(CO)H, (PPh3)3lr (CO)CN, and the tetracyanoethylene, fumaronitrile, and acrylonitrile adducts of (PPh3)2lr(CO)Cl by X-ray photoelectron and fr Mossbauer spectroscopy. 

The existence of a Pd° >Mo" dative bond elongates the quadruple Mo2 bond to some extent. This complex has two Pd° centers that can react with a variety of olefinic compounds, shown schematically in Figure 111 such reactions give Mo2Pd2(pyphos)4(L)2 [L = acrylonitrile, fumar-onitrile, tetracyanoethylene] in moderate yields. Unlike the previous compounds which have a strictly trans arrangement of the P-donor atoms, the last compound has a cis arrangement and thus the four metals are not linear.959... 

Fullerenes, among which the representative and most abundant is the 4 symmetrical Cgg with 30 double bonds and 60 single bonds, are known to behave as electron-deficient polyenes rather than aromatic compounds [7]. The energy level of the triply degenerate LUMO of Cgg is almost as low as those of p-benzoquinone or tetracyanoethylene. Thus, a wide variety of reactions have been reported for Cgg such as nucleophilic addition, [4-1-2] cycloaddition, 1,3-dipolar addition, radical and carbene additions, metal complexation, and so on [7]. Fullerene Cgg also undergoes supramolecular complexation with various host molecules having electron-donating ability and an adequate cavity size [8]. 

The complexes of tetraalkylstannanes SnR4 with tetracyanoethylene (TCNE)85 are more stable than those with oxygen. The Sn—C bond serves as a a -donor in this case. The Kq values of such complexes are also defined predominantly by the degree of shielding of the donor centre. With change of R these constants diminish in the series Me >> Et > i- Pr > i-Bu > f-Bu. The steric hindrance to the complexation of Me4- PbEt with TCNE is smaller in comparison with the isostructural tin compounds as expected on the basis of the larger Pb interatomic Pb-C distance85. 

Particularly relevant to the present crmtext is the fact that the olefinic double bond is considered as a soft base in Pearson s theory, while many Lewis acids used in cationic polymerisation (BF3, BCI3, AICI3, etc.) are classed as hard acids. Obviously, n-acceptors like chloranil or tetracyanoethylene are considered as soft acids. Thus, the interactions between Lewis acids and olefins must be considered as very weak in the context of the HSAB theory. This prediction is well substantiated by the tenuous character of the complexes observed in experimental studies (see Chap. IV). On the other hand, carbenium ions are usually placed at the borderline between hard and soft acids and are definitely softer than the Lewis acids mentioned above. Consequently, their interactions with olefins must be rather strong, which suggests that that propagation in cationic polymerisations promoted by Lewis acids should be faster than initiation. 

The Si-Si bond provided the first example of a a-bond donor in the formation of charge-transfer complexes with TCNE (tetracyanoethylene). Conversely, the Si-Si bond can act as an electron acceptor to form disilanyl radical anions. 

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