Abstract
The ab initio spin-coupled model, which is one of the most useful formulations of modem valence bond theory, has been used to study the general characteristics of, and the variations in, the chemical bonding in MCH2+ (M = Sc-Co) systems. The covalent metal-methylene interaction, characteristic of Schrock complexes, exhibits simultaneously a metal to ligand sigma electron donation and a ligand to metal pi electron donation. The degree of sigma donation decreases and that of pi donation increases monotonically from ScCH2+ to CoCH2+ in parallel with the decreasing dipole moment of the system and the increasing electronegativity of the M+ center. The metal-methylene interactions are found to be well described by a balance between two resonant Lewis structures: a dominant doubly bonded closed-shell M+=CH2 form and a much less important diradical-like, singly bonded form, M-+(.)-(CH2)-C-.. The importance of this last, which accounts for the triplet character in the pi (and sigma) interaction(s), grows with the number of unpaired nonbonding electrons on the metal. Such trends may be easily understood in terms of the preservation of intraatomic exchange energy and are consistent with a general decrease in the intrinsic bond strength from ScCH2+ to MnCH2+, and vice versa from MnCH2+ to CoCH2+. In addition, the sequential filling of nonbonding orbitals across the series is found to originate from a compromise between the minimization of repulsive electrostatic interactions between them and with bonding pairs, and the maximization of the intraatomic exchange energy.
| Original language | English |
|---|---|
| Pages (from-to) | 7091-7098 |
| Number of pages | 8 |
| Journal | Journal of Physical Chemistry A |
| Volume | 104 |
| Issue number | 30 |
| DOIs | |
| Publication status | Published - 3 Aug 2000 |
Keywords
- CASSCF WAVE-FUNCTIONS
- M = CO
- CARBENE COMPLEXES
- AB-INITIO
- FISCHER-TYPE
- ELECTRONIC-STRUCTURE
- REACTION-MECHANISM
- BINDING-ENERGIES
- GAS-PHASE
- CATIONS