Covalency in highly polar bonds. Structure and bonding of methylalkalimetal oligomers (CH₃M)_n (M = Li-Rb; n = 1, 4)

We have carried out a theoretical investigation of the methylalkalimetal monomers CH₃M and tetramers (CH₃M)₄ with M = Li, Na, K, and Rb and, for comparison, the methyl halides CH₃X with X = F, Cl, Br, and I, using density functional theory (DFT) at BP86/TZ2P. Our purpose is to determine how the structure and thermochemistry (e.g., C-M bond lengths and strengths, oligomerization energies) of organoalkalimetal compounds depend on the metal atom and to understand the emerging trends in terms of quantitative Kohn-Sham molecular orbital (KS-MO) theory. The C-M bond becomes longer and weaker, both in the monomers and tetramers, if one descends the periodic table from Li to Rb. Quantitative bonding analysis shows that this trend is not only determined by decreasing electrostatic attraction but also, even to a larger extent, by the weakening in orbital interactions. The latter become less stabilizing along Li-Rb because the bond overlap between the singly occupied molecular orbitals (SOMOs) of CH₃^. and M ^. radicals decreases as the metal ns atomic orbital (AO) becomes larger and more diffuse. Thus, the C-M bond behaves as a typical electron-pair bond between the methyl radical and alkalimetal atom, and, in that respect, it is covalent. It is also shown that such an electron-pair bond can still be highly polar, in agreement with the large dipole moment. Interestingly, the C-M bond becomes less polar in the methylalkalimetal tetramers because metal-metal interactions stabilize the alkalimetal orbitals and, in that way, make the alkalimetal effectively less electropositive.