As a n Donor

In the complex xenon functions as a n-donor toward Au2+. This is reflected in the calculated charge distribution within the cation, where the main part of the positive charge resides on the xenon atoms. Relativity plays a large role in stabilizing this and other predicted Au—Xe compounds about half of the Au—Xe bonding energy comes from relativistic effects.1993... 

Two colorimetric methods have been developed which were based on the charge transfer complexation reaction of pantoprazole sodium (acting as a n-donor) with either a 7t-acceptor (such as 2, 3-dichloro-5,6-dicyano-l,4-benzoquinone, or DDQ) or with a o-acceptor (such as iodine). 

Other organic ligands can also be bound in more than one way. For example, the allyl group can be bound in a a bond to one carbon atom (h1) or as a n donor encompassing all three of the carbon atoms (h3). These bonding modes of the allyl ligand are shown as follows ... 

The two <-butyl groups initiate via the + M-effect (n donor) a destabilization of the it orbital with respect to the a orbital. Probably the ethinyl group acts as a n donor, which is shown in the reaction given by Eq. (14). 

The TTF molecule (Chart 1) is a u-delocalized system with readily accessible redox states. TTF. TTF, and TTF ". The reduced form ftmctions as a n-donor in FF interactions, but the oxidized forms have n-acceptor character. Incorporation of one TTF moiety into a donor cycle also containing 1.5-dioxynaphthalene, and catenation with a Z /5 -4,4 -bipyridinium n-acceptor cycle, leads to a switchable [2]catenane (Fig. 10). In the reduced TTF form (Fig. 10b), the TTF molecule, as donor, is sandwiched between the two 4,4 -bipyridinium acceptors. When oxidized (chemically or electrochemically). so that the TTF has a positive or 2+ charge (Fig. 10a), electrostatics cause rotational translation of one cycle relative to the other to displace the positive TTF to the outer section of the catenane. This switching is reversible. 

First examine the geometry of methyl radical. Is it planar or puckered Examine the geometries of 2-methy 1-2-propyl radical, trifluoromethyl radical, trichloromethyl radical and tricyanomethyl radical. Classify each of the substituents (methyl, fluoro, chloro and cyano) as a n-electron donor or as a Tt-electron acceptor (relative to hydrogen). Does replacement of the hydrogens by 7t-donor groups make the radical center more or less puckered Does replacement by Jt-acceptor groups make the radical center more or less puckered Justify your observations. 

Takemoto et al. applied the Pd-catalyzed coupling reaction to N-phlhaloyl dehydroalanine 102 and benzaldehyde (Scheme 27) [41]. Instead of hydrosilanes, M-BuiiSnH was capable of serving as a hydrogen donor to promote C - C bond formation giving 103. 

Chelated species are less common. The substituted thiourea 7V-ethoxycarbonyl-/V-[3-(/>-anisyl) pyrazol-5-yl]thiourea (eatu) acts as a monoanionic 0,S-chelate to cobalt(II), forming Co(eatu)2.506 Whereas ethylenethiourea (etu) has been observed to act as a S-donor monodentate in Co (etu)2X2(H20)2 (X = C1, Br), it binds as an S,N-chelate in Co(etu)2(OOCCH3)2.507 Both violet octahedral and blue tetrahedral cobalt(II) compounds Co(pptu)2Cl2 and Cp(pptu)Cl2 respectively were prepared with bidentate A -2-(5-pico 1 y 1)- A"-pheny 11hiourea (pptu) chelated.508 Refluxing Co powder with thiourea in 4-methylpyridine provides a route to isothiocyanates, which form by decomposition of thiourea to ammonium thiocyanate509 with one product, Co(pic)4(NCS)2, characterized by a crystal structure. 

Another important N-donor group is the amide group. Contrary to the basic amino groups, the more acidic amide functions tend to be deprotonated in the complex and therefore operate as a monoanionic donor. Alkaline conditions promote the deprotonation and subsequent complex formation. The amide group is a very useful component of mixed donor sets, such as N2S2 or N3S, as discussed in the next chapter. Whether a pure amide coordination may occur in M(V) complexes has not yet been proved. Tetrapeptides do form Tc(V) complexes [68], apparently without involvement of the carboxyl group. The N-donor atom provided by Schiflf bases plays only a role in mixed donor sets and will be discussed below. 

Buono and co-workers (215) suggest that the increased activity of mixed P,N-ligands compared to bis(oxazohnes) is due to the ability of phosphorus to behave as a o donor and n acceptor. With this in mind, Buono and co-workers designed chiral quinoline-phosphine 302 as a ligand for copper-catalyzed Diels-Alder reactions. Acryloylimide and cyclopentadiene afford high selectivity of the cycloadduct in the presence of a catalytic amount of 302 Cu(OTf)2, Scheme 20. Buono invokes a square-planar metal center to account for the observed selectivity. 

There is some uncertainty whether this complex should be described as [Vm(bipy- )3] or as [V°(bipy)3], In fact, given that 2,2 -bipyridine can act either as a cr-donor or a n-acceptor, the metal-ligand bond in these complexes is constituted by a cr-bond between the lone pair of electrons of the nitrogen atom and an unoccupied s-orbital of the metal. Such electron donation, increasing the electron density on the metal, can in turn favour a back-bonding from the d-orbitals of the metal and the unoccupied rt -orbitals of the aromatic pyridine ring. In short, if the metal ion is in a high oxidation state pyridine will act as a a donor, whereas if the metal is in a low oxidation state pyridine will act as a n acceptor. 

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