Donor-carbon Atom

In a preliminary report on the structure of the nickel(n) complex (40) the five-membered chelate ring is said to be almost fiat and coplanar with the nickel atom and the isocyanide ligands. The Ni-CNBu distances of 1.83(2) and 1.77(2) A are significantly shorter than the Ni-C(chelate) distance of 2.00(2) A. The Ni-O distance is 1.81(1) A.  

Nickel is found in the rare +1 oxidation state in Rb4[Ni2(CN)J. There are two crystallographically independent [Nij(CN)6] anions which contain planar Ni(CN)a units linked together by a strong Ni-Ni bond, so that the co-ordination at each nickel atom is approximately square-planar. The mean angle between the nickel co-ordination planes is 86°. The two Ni-Ni distances are 2.32 and 2.29 A, not significantly different from twice the Pauling covalent radius of nickel (2.30 A). The mean Ni-C distance is 1.91 A. Two crystal structures containing the tetracyanonickelate anion have been described. -  

Complexes of Olefins, Acetylenes, and Related Molecules. A number of compounds have been described which contain nickel atoms in trigonal-planar co-ordination, one or more of the co-ordination sites being occupied by the multiple bond of a mono-olefin or similar species. A review of such compounds, containing structural data, has appeared.  

A preliminary report has appeared on the structure of the unusual complex Ni[(CF3)2C=0](PPh3)2 (41), in which the carbonyl group is w-bonded to 

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Introduction of an additional methyl group on the donor atom of TMM moiety gives a low 33% yield of the perhydroindans (49, X=H2) and (50, X=H2) with substantial production of the diene by-products [24]. However, it is still remarkable that the reaction works at all since the corresponding intermolecular cycloaddition failed. Incorporation of a carbonyl moiety adjacent to the donor carbon atom doubles the yield of the cycloadducts to 66% (Scheme 2.15). This so-called acyl effect works by making the donor carbon of the TMM unit "softer," thus facilitating the initial step of the conjugate addition, as well as inhibiting base-induced side reactions [22]. 

The donor carbon atoms exert a very strong trans influence, causing an elongation of the Pt—Pt bond. 

Thus intramolecular hydride transfers occur quite readily if the conformation is suitable for the efficient interaction of acceptor, usually carbonyl, and hydride donor carbon atom. In the transformation of (59) to (62 Scheme 5) intramolecular hydride transfer occurs from an amine (59) rather than an alco-hol." This reaction is probably mechanistically akin to the chemistry discussed in Section I.3.3.5.3. 

The former reaction discriminates the enantiofaces of the donor, and the asymmetric center is formed on the donor carbon atom. The latter reaction proceeds via enantioface discrimination of the Michael acceptor generating a chiral carbon center on the acceptor. Although both reactions are known, their mecha-... 

There is also a group of reactions in which hydride is transferred from carbon. The carbon-hydrogen bond has little intrinsic polarity or tendency to break in the way required for hydride transfer. These reactions usually proceed via cyclic transition states in which new C-H bonds are formed simultaneously with the cleavage. Hydride transfer is also facilitated by high charge density on the donor carbon atom. The Cannizzaro reaction, the base-catalyzed disproportionation of aldehydes, is one example of a hydride-transfer reaction. A general mechanism is outlined below. The hydride transfer is believed to occur from a species bearing two... 

Ligands with One Donor-carbon Atom. The complexes [PtCln(py)(CsH5-NCHCH2CH3)], where n = 2 in the square-planar platinum(ii) compound (62) and n = 4 in the octahedral platinum(iv) compound (63), are both... 

This structure indicates that carbon monoxide should have donor properties, the carbon atom having a lone pair of electrons. Carbon... 

If an open-chain organic molecule contains an electron acceptor and an electron donor site, two carbon atoms may be combined intramolecularly. This corresponds to the synthesis of a monocyclic compound. 

The reactions described so far can be considered as alkylation, alkenylation, or alkynylation reactions. In principle all polar reactions in syntheses, which produce monofunctional carbon compounds, proceed in the same way a carbanion reacts with an electropositive carbon atom, and the activating groups (e.g. metals, boron, phosphorus) of the carbanion are lost in the work-up procedures. We now turn to reactions, in which the hetero atoms of both the acceptor and donor synthons are kept in a difunctional reaction produa. 

Another group of macrocyclic ligands that have been extensively studied are the cycHc polyethers, such as dibenzo-[18]-crown-6 (5), in which the donor atoms are ether oxygen functions separated by two or three carbon atoms. The name crown ethers has been proposed (2) for this class of compounds because of the resemblance of their molecular models to a crown. Sandwich stmctures are also known in which the metal atom is coordinated with the oxygen atoms of two crown molecules. 

Beeause the anion acts as an electron donor, we can find clues to its reactivity preferences by examining the shape of its HOMO. The HOMO is delocalized over several sites, but the largest contribution to the HOMO clearly comes from the terminal carbon atom. Therefore, we expect electron movement and bond formation to occur at this carbon, and lead to the product shown on the left. 

Ti -Cyclopentadienyl(triphenylphosphine)cobalt reacts with phosphites and forms complexes of 1-alkoxyphosphole oxides 251 (R = Me, Et, Ph) through a step involving (ri -cyclopentadienyl)(phosphite)cobalt (80JA4363). (ri -Cp)Co(PF3)2 reacts with hexafluorobut-2-yne and 252 is formed, which hydrolyzes into 253 (X = OH) [73JCS(CC)583 75JCS(D)197]. The five-member ring has the envelope conformation, in which the carbon atoms are coplanar, and the phosphorus atom deviates from this plane in the direction opposite to the cobalt atom. The heterocycle is a four-electron donor relative to the metal center. 

Phospholes and analogs offer a wide variety of coordination modes and reactivity patterns, from the ti E) (E = P, As, Sb, Bi) through ri -dienic to ri -donor function, including numerous and different mixed coordination modes. Electrophilic substitution at the carbon atoms and nucleophilic properties of the phosphorus atom are well documented. In the ri -coordinated species, group V heteroles nearly acquire planarity and features of the ir-delocalized moieties (heterocymantrenes and -ferrocenes). 

As well as phosphorus ligands, heterocyclic carbenes ligands 10 have proven to be interesting donor ligands for stabilization of transition metal complexes (especially palladium) in ionic liquids. The imidazolium cation is usually presumed to be a simple inert component of the solvent system. However, the proton on the carbon atom at position 2 in the imidazolium is acidic and this carbon atom can be depro-tonated by, for example, basic ligands of the metal complex, to form carbenes (Scheme 5.3-2). 

If the reaction just described is conducted in the presence of a suitable hydrogen atom donor such as tri-n-butyltin hydride or tert-butyl hydrosulfide, reductive decarboxylation occurs via a radical chain mechanism to give an alkane (see 125—>128, Scheme 24). Carboxylic acids can thus be decarboxylated through the intermediacy of their corresponding thiohydroxamate esters in two easily executed steps. In this reducjtive process, one carbon atom, the carbonyl carbon, is smoothly excised... 

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