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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.
Ligand field molecular orbital (MO) bonding regimes for Werner-type (left), covalent (middle), and inverted ligand fields. [1] At the transition-metal - main group boundary, metal cations in organometallic complexes are more electronegative than the relatively more electropositive ligand atoms which act as z-type ligands.
The p-orbitals oriented in the z-direction (p z) can overlap end-on forming a bonding (symmetrical) σ orbital and an antibonding σ* molecular orbital. In contrast to the sigma 1s MO's, the σ 2p has some non-bonding electron density at either side of the nuclei and the σ* 2p has some electron density between the nuclei.
In general, 'hard' metal ions prefer weak field ligands, whereas 'soft' metal ions prefer strong field ligands. According to the molecular orbital theory, the HOMO (Highest Occupied Molecular Orbital) of the ligand should have an energy that overlaps with the LUMO (Lowest Unoccupied Molecular Orbital) of the metal preferential.
The s character rich O σ(out) lone pair orbital (also notated n O (σ)) is an ~sp 0.7 hybrid (~40% p character, 60% s character), while the p lone pair orbital (also notated n O (π)) consists of 100% p character. Both models are of value and represent the same total electron density, with the orbitals related by a unitary transformation.
Molecular orbital theory was seen as a competitor to valence bond theory in the 1930s, before it was realized that the two methods are closely related and that when extended they become equivalent. Molecular orbital theory is used to interpret ultraviolet–visible spectroscopy (UV–VIS). Changes to the electronic structure of molecules can be ...
Compounds that obey the 18-electron rule are typically "exchange inert". Examples include [Co(NH 3) 6]Cl 3, Mo(CO) 6, and [Fe(CN) 6] 4−.In such cases, in general ligand exchange occurs via dissociative substitution mechanisms, wherein the rate of reaction is determined by the rate of dissociation of a ligand.
As depicted in the molecular orbital diagram above, the computed electronic structure contains a purely ligand-based orbital with a 2u symmetry. [1] Invoking this ligand-only orbital allows for satisfaction of the 18-electron rule in M(CO) 8 complexes, and is stabilized by the field effect of the metal on the ligand cage. [14]