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CO is a well-known strong pi-accepting ligand in organometallic chemistry that will labilize in the cis position when adjacent to ligands due to steric and electronic effects. The system most often studied for the cis effect is an octahedral complex M(CO) 5 X where X is the ligand that will labilize a CO ligand cis to it.
Structure for (Cp 2 Ti) 2 CO 3. [3] Structure of [Ru 2 (CO 3) 4 Cl 2] 5+. [4] Carbonate is a pseudohalide ligand. With a saturated pi-system, it has no pi-acceptor properties. With multiple electronegative elements, it is not strongly basic. The latter is consistent with the pK a ’s of carbonic acid: pK 1 = 6.77 and pK 2 = 9.93.
The greater stabilization that results from metal-to-ligand bonding is caused by the donation of negative charge away from the metal ion, towards the ligands. This allows the metal to accept the σ bonds more easily. The combination of ligand-to-metal σ-bonding and metal-to-ligand π-bonding is a synergic effect, as each enhances the other.
For compounds with doubly bridging CO ligands, denoted μ 2-CO or often just μ-CO, the bond stretching frequency ν CO is usually shifted by 100–200 cm −1 to lower energy compared to the signatures of terminal CO, which are in the region 1800 cm −1. Bands for face-capping (μ 3) CO ligands appear at even lower energies. In addition to ...
A spectrochemical series is a list of ligands ordered by ligand "strength", and a list of metal ions based on oxidation number, group and element.For a metal ion, the ligands modify the difference in energy Δ between the d orbitals, called the ligand-field splitting parameter in ligand field theory, or the crystal-field splitting parameter in crystal field theory.
In cases where the ligand has low energy LUMO, such orbitals also participate in the bonding. The metal–ligand bond can be further stabilised by a formal donation of electron density back to the ligand in a process known as back-bonding. In this case a filled, central-atom-based orbital donates density into the LUMO of the (coordinated) ligand.
σ bonding from electrons in CO's HOMO to metal center d-orbital. π backbonding from electrons in metal center d-orbital to CO's LUMO. The electrons are partially transferred from a d-orbital of the metal to anti-bonding molecular orbitals of CO (and its analogs). This electron-transfer strengthens the metal–C bond and weakens the C–O bond.
The energies of transitions correlate with the order of the electrochemical series. The metal ions that are most easily reduced correspond to the lowest energy transitions. The above trend is consistent with transfer of electrons from the ligand to the metal, thus resulting in a reduction of metal ions by the ligand.