Double-bond radii

In the course of the work it was found that the value assumed five years ago for the carbon double-bond covalent radius (obtained by linear interpolation between the single-bond and the triple-bond radius) is 0.02 A. too large in consequence of this we have been led to revise the double-bond radii of other atoms also. 

The revision leads to a difference of 0.06 A. between the interatomic distance in the normal oxygen molecule and the sum of the double-bond radii. This may be attributed to the presence of an unusual structure, consisting of a single bond plus two three-electron bonds. We assign this structure both to the normal 2 state, with ro = 1.204 A., and to the excited 2 state, with ro = 1.223 A., the two differing in the relative spin orientations of the odd electrons in the two three-electron bonds. We expect for the double-bonded state the separation n 1.14 A. 

Table 3.1 Van der Waals and covalent single and double bond radii. ...

A second point which must be made is that bond length depends upon bond number triple bonds are shorter than double bonds, which in their turn are shorter than single bonds. For this reason separate radii are required for these three types of bond, and it will be seen from the values given in table 4.03 that triple-bond radii are about 0-17 A less and double-bond radii about o io A less than the corresponding singlebond radii. We must, however, also remember that a bond will not necessarily have an integral bond number and that this number may be fractional if the bond involves resonance. Thus in benzene the C-C... 

The polar nature of the silicon-carbon bond is reflected in the fact that d(Si=C) in 8 is significantly shorter than the sum of the double-bond radii of the participant atoms (1.74 A)19. The charge distribution in the Si=C bond leads to the ability of silene 8 to form surprisingly stable adducts with weak Lewis bases, in which the unsaturated silicon atom is coordinated. An X-ray structure is available for the adduct 9 with THF67. As may... 

Element Single-bond radius, pm Double-bond radius, pm Triple-bond radius, pm... 

For the carbon-carbon double-bond distance in ethylene Barteli and Bonham12 have reported the value 1.334 0.003 A from an electron-diffraction study and have stated that this value is compatible with the spectroscopic values of the moments of inertia.13 This value corresponds to t lie double-bond radius 0.667 A for carbon, as given in Table 7-2. 

Double bond radius Triple bond radius... 

Covalent Radii for Atoms (in pm) Single-Bond Radius Double-Bond Radius... 

Because carbon stands at the head of its group, we expect it to differ from the other members of the group. In fact, the differences between the element at the head of the group and the other elements are more pronounced in Group 14/IV than anywhere else in the periodic table. Some of the differences between carbon and silicon stem from the smaller atomic radius of carbon, which explains the wide occurrence of C=C and G=Q double bonds relative to the rarity of Si=Si and Si=0 double bonds. Silicon atoms are too large for the side-by-side overlap of p-orbitals necessary for -it-bonds to form between them. Carbon dioxide, which consists of discrete 0=C=0 molecules, is a gas that we exhale. Silicon dioxide (silica), which consists of networks of —O—Si- O - groups, is a mineral that we stand on. 

It has been found that a carbon-oxygen double bond decreases the single-bond radius of the carbon atom involved Pauling and Brockway, paper to be submitted to This Journal. 

Revised Values of Double-Bond Covalent Radii.—This investigation has led to the value 1.34 A. for the carbon-carbon double-bond distance, 0.04 A. less than the value provided by the table of covalent radii.111 4 Five years ago, when this table was extended to multiple bonds, there were few reliable experimental data on which the selected values for double-bond and triple-bond radii could be based. The single-bond radii were obtained -from the study of a large number of interatomic distances found experimentally by crystal-structure and spectroscopic methods. The spectroscopic value of the triple-bond radius of nitrogen (in N2) was found to bear the ratio 0.79 to the single-bond radius, and this ratio was as-... 

Thus, the radius of the atom carrying the free valence has a substantial influence on the activation barrier to the addition reaction the greater the radius of this atom, the higher the activation energy. Apparently, this effect is due to the repulsion in the transition state, which is due to the interaction between the electron shells of the attacked double bond and the atom that attacks this bond. 

Before 1964 no stable compound with a localized or delocalized carbon-phosphorus double bond was known. Indeed, it was generally assumed that the atomic radius of phosphorus, being larger than that of carbon or nitrogen, would not provide sufficient 2pn—3pir overlap for such a ir system to be stable Our first communication, written jointly with Peter Hoffmann which described the synthesis of a stable phosphamethin-cyanine 1 with a delocalized P—C double bond was therefore received with skepticism However, after Allmann confirmed the structure by X-ray analysis the existence of a new type of phosphorus bond in a cationic delocalized tt system was unambigously established... 

The pure single-bond distances (second column) are calculated from the boron radius 0.80 A and the halogen radii given in Section 9-4 (with 0.72 A for fluorine), with the electronegativity correction (Sec. 7-2). The corrections for double-bond character are made in the usual way (Sec. 7-5). The fifth and sixth columns give the observed bond lengths for BF BCU, BBr and the gas molecules BF, BC1, and BBr, respectively. 

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