Lone pair orbitals donor ability

Comparison of Eq. (1) derived from the r(C-OR) versus pKz (ROH) plot for the anri-periplanar P-trimethylsilyl esters 98-102 with Eq. (3) derived for the 2-substituted tetrahydropyran derivatives 117 reveals a similar response of the C-OR bond distance to the electron demand of the OR substituent. This is consistent with the similar energies (and hence donor abilities) of a C-Si bond and an oxygen lone-pair orbital (Tables 1 and 2). Thus the above structural data suggest that the oc Si-Oj- o interaction in 98-102 is similar in magnitude to the n0-Oc o present in 117. Also consistent with the present interpretation is the plot of C-OR distance versus pKd (ROH) for structures derived from the gauche P-trimethylsilyl alcohol 118,97... 

Contrary to previous arguments, the significantly enhanced donor ability of DMPP is likely due to the repulsive interaction between the methyl groups which places more s-character in the intracyclic P-C bonds leaving the lone pair orbital with an increased amount of p-character. Data supporting these conclusions are found in Table 1. 

In a review written by Genet and co-workers on electron-deficient phosphines the authors have briefly discussed the relationship between the a-donor ability of a phosphine group and the magnitude of Vpse in the phosphineselenides. As was established by Allen and Taylor some time ago, an increase in this coupling indicates an increase in the ct character of the phosphorus lone-pair orbital (i.e., a less basic phosphine). 

The reactivity of carbenes is strongly influenced by the electronic properties of their substituents. If an atom with a lone pair (e.g. O, N, or S) is directly bound to the carbene carbon atom, the electronic deficit at the carbene will be compensated to some extent by electron delocalization, resulting in stabilization of the reactive species. If both substituents are capable of donating electrons into the empty p orbital of the carbene, isolable carbenes, as e.g. diaminocarbenes (Section 2.1.6), can result. The second way in which carbenes can be stabilized consists in complexation. The shape of the molecular orbitals of carbenes enable them to act towards transition metals as a-donors and 71-acceptors. The chemical properties of the resulting complexes will also depend on the electronic properties of the metallic fragment to which the carbene is bound. Particularly relevant for the reactivity of carbene complexes are the ability of the metal to accept a-electrons from the carbene, and its capacity for back-donation into the empty p orbital of the carbene. 

The strongest response (largest slope) of the C-OR bond distance to the electron demand of the OR substituent occurs for 117, which has an oxygen lone pair anri-periplanar to the C-OR bond. The cyclohexyl esters 116 in which the C-C bond is anri-periplanar to the C-OR bond show a much weaker response of the C-OR distance to the electron demand of the OR substituent. Thus the slopes of the lines [r(C-OR) vs. pA- (ROH)] appear to provide qualitative measures of the donor abilities of orbitals that are anri-periplanar to the C-OR bond. 

The adducts of Lewis bases known so far often show re-dissociation in solution or under low pressure. If the acceptor ability of the tetrylene is enhanced, stable compounds with shorter (stronger) C — E bonds are obtained, however. One possibility of enhancing the acceptor ability of the tetrylene is by the introduction of electropositive substituents such as silyl groups. As shown by theoretical calculations, such substituents will simultaneously raise the energy of the HOMO (lone-pair on E) and lower the energy of the LUMO (empty p-orbital on E) and therefore lead to both an enhanced donor and an enhanced... 

Photoelectron spectroscopy reveals that the highest occupied molecular orbital (HOMO) in NHC is the lone pair in the sp hybrid orbital of the carbene carbon atom (see Figure 1.18). However, a molecular orbital with jt symmetry centred around the N-C-N heteroallyl system is of almost equal energy. This gives the carbene the ability to engage in n-face donor interactions with the transition metal fragment. For unsaturated NHC these n-face donor interactions are more favourable than for saturated ones [104]. 

Comprehensive reviews on the coordination chemistry of thiolates and hindered thiolates have appeared. " One of the major problems in preparing complexes is oxidation of the thiolate to the corresponding disulfide. This can be more readily controlled with hindered thiolates, which can stabilize highly reactive, coordinatively unsaturated complexes. An important feature of thiolates is their ability to level charge within metal complexes. Thus, both jr-acceptor (d-orbital on S, or S-C cr -orbital) and jr-donor (lone pair on S) properties... 

All these ligands (I-III) are pyramidal at phosphorus, but with varying degrees of sp hybridization. Due to their reduced phosphorus inversion barrier phospholes are more planar than typical phosphines and their phosphorus lone pair possesses less s-character than does the phosphorus in either I or II. Thus from a frontier orbital point of view since the lone pair is likely the HOMO for all these ligands, we would anticipate that I and II might be poorer donors than III. Likewise, III is considerably less bulky than either I or II. In sum then, if cyclic conjugation is not large in III, its donor ability should approximate those of I and II. 

The formation of a strong pi bond is responsible for the stabilization afforded by the pi-donor. However, pi bonds between orbitals from different shells necessarily have poor overlap and are weak. A sulfur atom is a poor pi-donor because there is poor overlap of the sulfur 3/ orbital with the carbon 2p orbital. The two worst lone pair donors are poorer donors than alkyl groups because their high electronegativity is not compensated by their weak pi donation ability. For example, a chlorine atom is very electronegative and is a poor resonance pi-donor because of poor 2p-3p overlap. 

Bonding of the CO molecule to a transition metal atom, either in a carbonyl complex or possibly on a metal surface, can be visualized as first proceeding by the donation of the lone pair on the carbon atom into vacant d orbitals of the metal atom. The donor ability (Lewis basicity) of CO in this manner is known to be extremely small, and stabilization of the metal-carbon bond is believed to be obtained by back donation of electrons from filled d orbitals on the metal into vacant antibonding tt orbitals on the CO molecule. It is thought that the two mechanisms. 

As we have already discussed in Section 1.2, there are several cases which may obtain in comparing two orbital interactions. Since all of them cannot be incorporated in any single simple framework, we have chosen to develop a model which leads to correct predictions whenever A5 (A,B) < 0 and AHij (A,B) > 0 or A6 ejj (A, B) < 0 and AHy (A, B) 2 0 (see Section 1.2). Accordingly, interactions, which are matrix element controlled should be anticipated and treated separately. The basic features of the model are the following a) Lone pair and bonds are classified according to their intrinsic donor and acceptor abilities. Tables 36 and 37 summarize the relative intrinsic donor and acceptor strengths of various lone pairs and bonds. 

Directionality of H-bonding Due to the high donor ability of non-bonding orbitals, most H-bonds involve a suitable lone pair as an electron donor (often alternatively, and confusingly, referred to as H-bond acceptor ) in nY o 5 jj hyperconjugative interactions. Such interactions involve two stereoelectronic components. First, the symmetry of the electron acceptor defines the 180° X-H...Y angle of attack (note the ste-... 

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