Donor bonds

The covalently bonded oxygen atom still has two lone pairs of electrons and can act as an electron pair donor. It rarely donates both pairs (to achieve 4-coordination) and usually only one donor bond is formed. A water molecule, for example, can donate to a proton, forming H30, and diethyl ether can donate to an acceptor such as boron trifluoride ... 

Since equatorial attack is roughly antiperiplanar to two C-C bonds of the cyclic ketone, an extended hypothesis of antiperiplanar attack was proposed39. Since the incipient bond is intrinsically electron deficient, the attack of a nucleophile occurs anti to the best electron-donor bond, with the electron-donor order C—S > C —H > C —C > C—N > C—O. The transition state-stabilizing donor- acceptor interactions are assumed to be more important for the stereochemical outcome of nucleophilic addition reactions than the torsional and steric effects suggested by Felkin. 

The P-N bond in phosphinous amides is essentially a single bond, so the lone pairs on N and P are available for electrophiUc reagents and for donor bonding towards metal atoms. Proton addition to the N atom of HjPNHj has been calculated to loosen the P-N bond, whereas protonation at P renders this bond stronger than in the parent molecule [26]. NH-Phosphinous amides are practically not associated by intermolecular hydrogen bonds [27]. 

With the aid of a normal coordinate analysis involving different isotopomers a linear structure of the Pd-Si-0 molecule is deduced. The results of ab initio MP2 calculations (Tab. 4) confirm the experimentally obtained IR spectra and their interpretation. The Pd-C bond in PdCO is similar to the Pd-Si bond in PdSiO which means, that the donor bond is strengthened by x acceptor components. This conclusion is in line with the high value of the Pd-Si force constant (exp. f(PdSi) = 2.69, f(SiO) = 8.92 mdyn/A) as well as with the energy of PdSiO (Pd + SiO —> PdSiO + 182 kJ/mol for comparison Pd + CO —> PdCO + 162 kJ/mol, MP2 level of theory). 

Additional bonds are thus donor bonds, and to accept electron pairs from neutral and anionic ligands, zinc uses the two remaining 4p orbitals to form sp2 and sp3 hybrids. In the absence of steric effects, discrete, homoleptic, anionic tri- and tetraorganozinc compounds (zincates) have almost always ideal trigonal-planar and tetrahedral geometries, respectively. 

As shown in Figure 3, the Lewis-basic pyridyl groups form donor bonds with the Lewis-acidic zinc to yield an -symmetric polycyclic compound. The pyramidalized zinc atoms are three coordinate, being surrounded by two relatively long Zn-C bonds (2.032(7) A) and one short Zn-N donor bond (2.031(5) A). 

A ferrocenyl-based aldimine chelated dimethylzinc with its imino and oxygen atom to produce the tetrahedral dimethylzinc complex 44. Both zinc-carbon bonds are equidistant (1.974(2) A) and the zinc-oxygen donor bond (2.381(2) A) is substantially longer than the zinc-nitrogen counterpart (2.213(2) A) (Scheme 37).91... 

The complex has crystallographic ///-symmetry, the mirror plane bisecting the unique benzyl group, the nitrogen atom to which it is attached, and the ethylzinc moiety. The pseudo-tetrahedral zinc atom has a short (1.930(4) A) zinc-ethyl bond, but comparatively long (2.230(2) A) nitrogen-zinc donor bonds. 

The zinc atom is coordinated by three phenyl groups, and displays mean Zn-C bond lengths of 2.049(2) A, and a relatively long Zn-N donor bond (2.340(3) A). 

Fig. 4. Experimental reactivation energies for each donor element versus hydrogen-donor species bond strength. The formation of hydrogen-donor bonds is supported by the good correlation between these two parameters (Pearton et al., 1986).

MR donor bonds, is o-bond metathesis. Alternatively, as the Lewis-acid strength of M increases, the tendency toward agostic or bridging interactions can finally result in H—H bond scission and formal migration of hydride to the metal atom,... 

The monosubstituted adduct offers the ready synthesis of a whole range of monosubstituted adducts (see Scheme 6) it is often possible to isolate in these reactions intermediates that are not readily obtained by alternative methods. Thus, in the reaction with halogen acids to yield the bridged hydrido complexes HOs3(CO)10X, it is possible to identify the intermediate HOs3(CO)uX complex in which the halogen functions as a one-electron donor bonding to only one metal center (158). 

By noting that the best conformation will be the one which allows the interaction of the best donor bond (C—Br) with the best acceptor bond (C—Br), the second best donor bond (C—Cl) with the second best acceptor bond (C—Cl), and the third best donor bond (C—F) with the third best acceptor bond (C—F), we predict the best conformation to be I for the meso form. The best conformation for the racemic form will be K, since it is the only one which allows for the interaction of the best donor and best acceptor bonds. We further predict that L will be more stable than J. Experimentally, I and L are found to be the most stable conformations of CClFBrCClFBr371>. 

This occurs because the donor bond, C-H, and the best acceptor bond C—X, are in an anti geometrical relationship in the cis conformation. 

The incoming ligand (or metal ion in the previous section) simulates H+ by preventing reclosing of the Ni-donor bonds as these are successively broken (see also Fig. 8.7)... 

The metal-donor bonds are predominantly ionic and become more labile for calcium, strontium, and barium compared to beryllium and magnesium. The solubility and stability of the complexes decrease from calcium to barium. The 1 1 adducts of NHCs with BH3 or BF3 (28 and 29) are thermally stable and can be sublimed without decomposition. This is in sharp contrast to the properties of conventional carbenes, which rely on a pronounced metal-to-ligand back donation and are, thus, not suited to forming adducts with electron-poor fragments such as... 

By its nature, the acceptor bond, like the donor bond, may be purely ionic or purely homopolar or, in the general case, a mixed one. As we shall see below, this depends on how the electron or the hole captured by the particle and participating in the bond is distributed between the adsorbed particle and the adsorption center. In other words, this depends on the type of localization of the electron or the hole, which in turn, is determined by the nature of the adsorbate and the adsorbent. 

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