Bond, covalent three-centre

A common interpretation of the interaction of chalcogens with nucleophiles considers donation of electron density from a lone pair on the donor atom into the o- (E-X) orbital (Figure 15.1). As the degree of covalency increases, a hypervalent three-centre four-electron bond is formed. Real systems fall somewhere between secondary interactions and hypervalent (three centre - four electron) bonds. The two extremes can be distinguished by the correlation of X-E and E D distances.In the hypervalent case both bond distances decrease simultaneously, whereas in the secondary bond the distances are anticorrelated. This concept has been applied in a study of selenoquinones 15.17 (R = Ph, Me) with short Se 0 contacts,for... 

It has a three centre, two electron bond. In this the formal covalency of carbon is five rather than three. 

Hydrogen, having one valence orbital and one electron, may be expected to form just one a bond. In fact, the covalent bonding in many hydrides is much more complicated than this, with H atoms often being bonded to two other atoms, forming a three-centre two-electron bond. Such electron-deficient species are discussed more fully in Section 7.4. 

Beryllium and Aluminium borohydrides These are covalent and volatile. In these the [BH4]- groups acts as a ligand. One or more H atoms in a [BH4]- act as a bridge and bond to the metal, forming a three centre bond with two electrons. The structures are as... 

The difference is that after addition to the a-bond, no covalent bond remains between the two atoms of the latter the splitting of the three-centre bond results in... 

Bond and Valence Indices. - Atomic charges, bond indices (two-centre and three-centre) and valences have been calculated for a number of closed-shell molecules using HF and KS orbitals. It was observed that, compared to the HF orbitals, the KS orbitals predicted a slightly enhanced covalency. Beside AIM the authors employed MuUiken, Lowdin and Natural Population Analysis and noted that the AIM scheme led to a more polar charge distribution than NPA. 

Pantano et al used Car-Parrinello MD to compute the influence of thermal fluctuations on the proton hopping properties of a model substrate for enzymatic reactions, benzoylacetone. A plot showing the projection of ELF on the plane defined by the O-H-0 moiety unambiguously demonstrates the presence of electron pair density between the hydrogen and the two H-bond donors, suggesting the presence of a three-centre covalent bond. The character of the bond is o, as evidenced by analysing the Kohn-Sham one-electron wavefimction, in full agreement with AIM. 

Green JC, Green MLH, Parkin G (2012) The occurrence and representation of three-centre two-electron bonds in covalent inorganic compounds. J Chem Soc Chem Commun 48 11481-11503... 

The three-centre-two-electron bond is not as strong as the conventional covalent bond. The shape of the bond is that of a banana and hence called a banana bond. In diborane two such bonds exists. 

Attempts to classify carbides according to structure or bond type meet the same difficulties as were encountered with hydrides (p. 64) and borides (p. 145) and for the same reasons. The general trends in properties of the three groups of compounds are, however, broadly similar, being most polar (ionic) for the electropositive metals, most covalent (molecular) for the electronegative non-metals and somewhat complex (interstitial) for the elements in the centre of the d block. There are also several elements with poorly characterized, unstable, or non-existent carbides, namely the later transition elements (Groups 11 and 12), the platinum metals, and the post transition-metal elements in Group 13. 

The first intravascular sensor for simultaneous and continuous monitoring of the pH, pC>2, and pCC>2 was developed by CDI-3M Health Care (Tustin CA)14 based on a system designed and tested by Gehrich et al.15. Three optical fibres (core diameter = 125 pm) are encapsulated in a polymer enclosure, along with a thermocouple embedded for temperature monitoring (Figure 3). pH measurement is carried out by means of a fluorophore, hydroxypyrene trisulfonic acid (HTPS), covalently bonded to a matrix of cellulose, attached to the fibre tip. Both the acidic ( eXc=410 nm) and alkaline ( exc=460 nm) excitation bands of the fluorophore are used, since their emission bands are centred on the same wavelength (/-cm 520 nm). The ratio of the fluorescence intensity for the two excitations is measured, to render the sensor relatively insensitive to fluctuations of optical intensity. 

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