Molecular Charge Distributions and Chemical Binding. III. The Isoelectronic Series N2, CO, BF, and C2, BeO, LiF

The chemical binding and properties of the two isoelectronic series N₂, CO, BF, a 14‐electron series, and C₂, BeO, LiF, a 12‐electron series, are discussed in terms of the molecular charge distributions and the forces which they exert on the nuclei. It is found that the formation of the chemical bond in the 14‐electron series results in a transfer of charge from the bonding to the nonbonded regions of both nuclei in the molecule, the amount transferred increasing with the increasing asymmetry of the nuclear potential field. The larger and more diffuse nonbonded charge increase on B in BF as compared to that on C in CO suggests that BF may be superior to CO in its ability to act as a ligand in metal complexes. The increasing asymmetry of the potential field in the 12‐electron series results in a parallel increase in the asymmetry of the molecular charge distributions. Unlike the 14‐electron series, the charge transfer in BeO and LiF is unidirectional. The $\mathrm{\Delta \rho (\epsilon ,\; \eta )}$ or density difference contour maps $\mathrm{[\rho (}\mathrm{molecule}\mathrm{)\hspace{0.17em}-\hspace{0.17em}\rho (}\mathrm{atoms}\mathrm{)]}$ show that the atomic densities of atoms which employ $p$ orbitals in their bonding exhibit a large quadrupole polarization. The $\mathrm{\Delta \rho (\epsilon ,\; \eta )}$ maps for BeO and LiF indicate that the radial excitations of the $\mathrm{p\sigma }$ charge density of O and F, predicted by Sternheimer antishielding theory, are quenched by the exchange repulsions with the residual core densities on Be and Li.