Basicity of electron pairs

Molecular hydrogen has neither a lone pair nor a tt bond, yet it also binds as an intact molecule to metals in such complexes as [W(t -H2)(CO)3L2]. The only available electron pair is the H—H a bond, and this becomes the donor ( 3 in Fig. 1,9b). Back donation in this case ( 4 in Fig. 1.96) is accepted by the H2 (x orbital. The metal binds side-on to H2 to maximize a-d overlap. Related o-bond complexes are formed with C—H, Si—H, B—H, and M—H bonds. In general, the basicity of electron pairs decreases in the following order lone pairs > w-bonding pairs > o-bonding pairs, because being part of a bond stabilizes electrons. The usual order of binding ability is therefore as follows lone pair donor > ir-bond donor > o-bond donor. 

The N-basicity of the commonly used amines (pyrrolidine > piperidine > morpholine) drops by 2-3 orders of magnitude as a consequence of electron pair delocalization in the corresponding enamines. This effect is most pronounced in morpholino enamines (see table below). Furthermore there is a tendency of the five-membered ring to form an energetically favorable exocyclic double bond. This causes a much higher reactivity of pyrroUdino enamines as compared to the piperidino analogues towards electrophiles (G.A. Cook, 1969). 

In—W bond. Use of Ph3Al leads to a complex in which the oxygen atom of a carbonyl ligand is the site of electron pair basicity in a WC=OAl link. Solutions of [n-Bu4N][Ph3GaCpW(CO)3] in CH2CI2 contain, in addition to free [CpW(CO)3], two isomeric complexes a metal-metal-bonded species and a C- and O-bonded adduct of the type found in the Ph3Al case. 

A molecule is composed of positively charged nuclei surrounded by electrons. The stability of a molecule is due to a balance among the mutual repulsions of nuclear pairs, attractions of nuclear-electron pairs, and repulsions of electron pairs as modified by the interactions of their spins. Both the nuclei and the electrons are in constant motion relative to the center of mass of the molecule. However, the nuclear masses are much greater than the electronic mass and, as a result, the nuclei move much more slowly than the electrons. Thus, the basic molecular structure is a stable framework of nuclei undergoing rotational and vibrational motions surrounded by a cloud of electrons described by the electronic probability density. 

The idea of Stm. III.2 is intuitively basic in characterizing a hypothetical covalent bonding state. Moreover, the idea of Stm. III.2 is commonly coupled to the MO language via Ay = Ay which, together with Hyps. III.l, III.2, III.4, and III.5 (specialized to 0y = 1), constitutes an altenative specification of electron-pair covalency . 

Certain acid dyes can have their Fastness properties improved by combining Ihe dye with a metal atom (chelation) The most common inelal is chromium, although cobalt is sometimes used, and this can be introduced in a number of ways. The basic mechanism is donation of electron pairs by groups in the dye l ligands) to a metal ion. 

A more interesting result of the unimportance of the nucleophile in the rate is that very poor nucleophiles indeed may react in the absence of anything better. In Chapter 8 we established that nitriles are only weakly basic because the lone pair nitriles are only weakly basic of electrons on the nitrogen atom is in a low-energy V ... 

In the imidazole ring of histidine, the electron pair of one nitrogen is also part of the aromatic n system and is unavailable for bonding this nitrogen is not basic. The electron pair on the other nitrogen, however, is in an sp orbital available for bonding, and is about as basic as pyridine. 

The condensation of one ester with another under basic conditions is generally thought to proceed by an ionic mechanism involving anionic species and the transfer of electron pairs. Substantial evidence supports this view, but recent work does indicate the possibility of a single electron transfer (SET) process in some cases. 

A number of attempts have been made to account for the electronic structures of clusters by theoretical descriptions of their bonding. The valence-bond approach, based on localized two-center bonds formed by directed hybrid orbitals have been used by a number of workers. Sutton and Dahl 357) have rationalized the structures of a number of clusters in terms of bent metal-metal bonds. This conceptis contrary, however, to the basic notion of the valence-bond approach, which is that orbitals should be directed to give maximum overlap. Moreover, the situation can arise in clusters where there is not an integral number of electron pairs per bond. 

Lewis basically threw out the two previous ideas and instead looked at the reactions based on the transfer of electron pairs. He postulated that acids accept electron pairs, and bases donate electron pairs. 

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