Multiple Bonds and Bond Order

The order of a bond may be defined as the number of electron pairs that constitute the bond. Thus the bond orders of single, double, and triple bonds are respectively 1, 2, and 3. As the number of electron pairs forming the bond increases, the attraction of the bonding electrons for the two atomic cores increases, so the bond strength increases and the bond length decreases. 

A well-known example of the effect of bond order on bond length is provided by the bonds in ethane, ethene, and ethyne, which have the lengths of 154, 134, and 120 pm, respectively. Covalent radii for doubly and triply bonded atoms can be obtained from double and triple bond lengths in the same way as for single bonds. Some values are given in Table 2.2. 

In many molecules the bonds between two given atoms have lengths that are intermediate between those of single and double bonds or between double and triple bonds. A familiar example is benzene for which the Lewis structure is 

So each bond has a bond order of 1.5, which is consistent with the observed bond length. These two resonance structures are often called Kekule structures because they were first proposed in 1865 by Kekule, who imagined that the molecule converted very rapidly from one form to the other. This, however, is not the case the molecule never has either of the Kekule structures but only a single structure, which is intermediate between these two hypothetical structures and is approximately represented as follows  

There are many other molecules in which some of the electrons are less localized than is implied by a single Lewis structure and can therefore be represented by two or more resonance structures. For example, the three bonds in the carbonate ion all have the same length of 131 pm, which is intermediate between that of the C—O single bond in methanol (143 pm) and that of the C=0 double bond in methanal (acetaldehyde) (121 pm). So the carbonate ion can be conveniently represented by the following three resonance structures  

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SOME OTHER ASPECTS OF COVALENT BONDING Multiple Bonds and Bond Order... 

The first diruthenium paddlewheel complex, RUjfii-OjCMej Cl, was prepared in 1966 by Stephenson and Wilkinson [3]. The paddlewheel geometry was validated in 1969 when Cotton and coworkers reported the X-ray crystal structure of an analogous complex, Ru2(p-OjC"Pr) Cl [4j. In the crystal, RUjlii-OjC Prl Cl assembles in an infinite zig-zag chain, where each chloride atom bridges two diruthenium units. More significantly, a short Ru-Ru bond distance of 2.281(4) A was revealed. The FSR of this bond is 0.92, which is consistent with a multiple bond order. 

Observed interatomic distances for diatomic transition-element interactions are estimates of the fraction, d = 0.783 of nearest-neighbor approaches in the metals [5] and may be considerably in error in the present context, especially for the second transition series. Apart from first-order La2 and Ce2, with = 245 30kJmol homonuclear diatomics have weak interactions with an average Dx = 70 40kJmor in agreement with our estimates. Multiple bond orders, in general, are characterized by stepwise reduction of the first-order golden exponent, such that... 

Bond lengths and infrared spectra support the multiple-bond character of the M—CO bonds. Coordination of a CO molecule to a metal center can change the C—O bond order. According to the description of ( - and TT-bonding given herein, increased ( -bonding between a metal and CO results in a... 

A single shared pair of electrons is called a single bond. Two electron pairs shared between two atoms constitute a double bond, and three shared electron pairs constitute a triple bond. A double bond, such as C 0, is written C=0 in a Lewis structure. Similarly, a triple bond, such as C C, is written G C. Double and triple bonds are collectively called multiple bonds. The bond order is the number of bonds that link a specific pair of atoms. The bond order in H, is 1 in the group C=0, it is 2 and, for O C in a molecule such as ethyne, C2H2, the bond order is 3. 

Kier and HaU extended the definition of the 5 connectivity index in order to incorporate heteroatoms and multiple bonds in the definition of the connectivity index % [13-15]. They noticed that the 5 connectivity (atom degree) may be expressed as ... 

That way, the Distributed Electrostatic Moments based on the ELF Partition (DE-MEP) allows computing of local moments located at non-atomic centres such as lone pairs, a bonds and n systems. Local dipole contributions have been shown to be useful to rationalize inductive polarization effects and typical hydrogen bond interactions. Moreover, bond quadrupole polarization moments being related to a n character enable to discuss bond multiplicities, and to sort families of molecules according to their bond order. 

Bond paths are observed between bonded atoms in a molecule and only between these atoms. They are usually consistent with the bonds as defined by the Lewis structure and by experiment. There are, however, differences. There is only a single bond path between atoms that are multiply bonded in a Lewis structure because the electron density is always a maximum along the internuclear axis even in a Lewis multiple bond. The value of pb does, however, increase with increasing Lewis bond order, as is shown by the values for ethane (0.249 au), ethene (0.356 au), and ethyne (0.427 au), which indicate, as expected, an increasing amount of electron density in the bonding region. 

In order to elucidate the causes of the increased stability of the hydrolyzed cluster ions compared with the unhydrolyzed ions, further studies were made of the behaviour of [Te2X8]3 (where X = Cl,Br, or I) in solutions of hydrogen halides [43,52,80,87]. The studies were performed mainly in relation to the most stable and most readily synthesized [Tc2C18]3- ion (Fig. la) kinetic methods with optical recording were employed. The identity of the reaction products was in most cases confirmed by their isolation in the solid phase. The studies showed that the stability of the [Tc2X8]3 ions (where X = Cl, Br, or I) in aqueous solutions is determined by the sum of competing processes acid hydrolysis complex formation with subsequent disproportionation and dissociation of the M-M bonds, and oxidative addition of atmospheric oxygen to the Tc-Tc multiple bond. 

A single S-O bond has a length of approximately 150 pm, but as a result of the multiple bonding between sulfur and oxygen, the observed bond length in S02 where the bond order is 1.5 is 143 pm. 

Numbering of Compounds. If the rules for aliphatic chains and ring systems leave a choice, the starting point and direction of numbering of a compound are chosen so as to give lowest-numbered locants to these structural factors, if present, considered successively in the order listed below until a decision is reached. Characteristic groups take precedence over multiple bonds. 

Most of the monomeric compounds are highly reactive liquids and are rapidly hydrolysed by atmospheric moisture. In the monomeric iminoboranes the N-B bond order is greater than unity and these compounds represent an allene-type system with cumulated multiple bonds as indicated by structure (I) 3S). 

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