Bonds with Atoms Containing Unshared Electrons

Multiple Bonds with Atoms Containing Unshared Electrons The electronic spectra of molecules with such multiple bonds should be expected to be more complicated than those of simple multiple bonds or saturated compounds containing atoms with unshared electrons (auxochromes). One finds low intensity bands at long wavelengths due to n n  

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The water molecule is composed of two hydrogen atoms covalently bonded to an oxygen atom with tetrahedral (sp3) electron orbital hybridization. As a result, two lobes of the oxygen sp3 orbital contain pairs of unshared electrons, giving rise to a dipole in the molecule as a whole. The presence of an electric dipole in the water molecule allows it to solvate charged ions because the water dipoles can orient to form energetically favorable electrostatic interactions with charged ions. 

A parameter, which is of special importance for the description of M(OR)n in addition to the coordination polyhedron of the central atom and the M-O bond lengths, is the M-O-C bond angle. Bradley [198] was the first to emphasize the role of unshared electron pairs at the oxygen atoms that can be involved into interaction with the vacant t/-orbitals of the transition metal. Thus in the molecules containing maingroup metals such as Bi, Sn, Ge, Te, and Al. the values of these angles usually do not exceed 120 to 130°, while in those of... 

Non-tetrahedral structures of organic derivatives of the silicon subgroup elements are often caused by inter- or intra-molecular coordination interaction X M. This takes place in compounds where there is a nucleophilic substituent at the central M atom. An electronegative X atom, which has at least one unshared electron pair (X = N, P, O, S, halogen) and is directly bonded to M, can be such a substituent. Compounds of this kind tend to be involved in inter-molecular coordination. There can be also a heteroatom X as part of the organic substituent at M in this case an intra-molecular coordination usually occurs . Such compounds, which contain five- or six-membered coordination rings, include, for example, draconoides (4) , their analogues (5) , metalloatranes (6) and others. The stability of a coordination bond X —> M increases with the atomic number of M Si < Ge < Sn < Pb. 

It is expected that hyperconjugation would be enhanced in certain systems containing heteroatoms. If one atom with an unshared electron pair is a particularly good electron donor and another a good cr acceptor, the u contribution should be enhanced. This is represented by a charged, no-bond resonance structure. 

Propylein, the other alkaloid of this group, appears to be the sole alkaloid of the beetle Propylaea quatuordecimpmctata Propylein is an amorphous, unstable, laevorotatory base of molecular formula C13H21N which affords precoccinellin on hydrogenation. Since the alkaloid contains one olefinic proton that is in an enamine system (n.m.r., i.r.), three dehydroprecoccinellin structures are possible for propyleia Two of these can be excluded, since propylein exhibits only end absorption in the u.v. It thus appears that in propylein the 7r-electron system of the double bond is so oriented that it cannot interact with the unshared electrons on the nitrogen atom. This leaves only one possible structure (32) for propylein [cf. conformation (33)]. ... 

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