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The existence of a very long C–C bond length of up to 290 pm is claimed in a dimer of two tetracyanoethylene dianions, although this concerns a 2-electron-4-center bond. [4] [5] This type of bonding has also been observed in neutral phenalenyl dimers. The bond lengths of these so-called "pancake bonds" [6] are up to 305 pm.
The strength of a bond can be estimated by comparing the atomic radii of the atoms that form the bond to the length of bond itself. For example, the atomic radius of boron is estimated at 85 pm, [10] while the length of the B–B bond in B 2 Cl 4 is 175 pm. [11] Dividing the length of this bond by the sum of each boron atom's radius gives a ratio of
The term bond-dissociation energy is similar to the related notion of bond-dissociation enthalpy (or bond enthalpy), which is sometimes used interchangeably.However, some authors make the distinction that the bond-dissociation energy (D 0) refers to the enthalpy change at 0 K, while the term bond-dissociation enthalpy is used for the enthalpy change at 298 K (unambiguously denoted DH° 298).
Starting with Pauling in 1947 [12] a correlation between cation–anion bond length and bond strength was noted. It was shown later [13] that if bond lengths were included in the calculation of bond strength, its accuracy was improved, and this revised method of calculation was termed the bond valence. These new insights were developed by later ...
For example, in the case of the F 2 molecule, the F−F bond is formed by the overlap of p z orbitals of the two F atoms, each containing an unpaired electron. Since the nature of the overlapping orbitals are different in H 2 and F 2 molecules, the bond strength and bond lengths differ between H 2 and F 2 molecules.
The strength of the bond to each of those atoms is equal. It is an example of a three-center four-electron bond. This type of bond is much stronger than a "normal" hydrogen bond. The effective bond order is 0.5, so its strength is comparable to a covalent bond.
The strong bonding of metals in liquid form demonstrates that the energy of a metallic bond is not highly dependent on the direction of the bond; this lack of bond directionality is a direct consequence of electron delocalization, and is best understood in contrast to the directional bonding of covalent bonds.
R.T. Sanderson has also noted the relationship between Mulliken electronegativity and atomic size, and has proposed a method of calculation based on the reciprocal of the atomic volume. [17] With a knowledge of bond lengths, Sanderson's model allows the estimation of bond energies in a wide range of compounds. [18]