PART 9(D): LIGAND FIELD THEORY (PI DONOR & PI ACCEPTOR LIGANDS) скачать в хорошем качестве

PART 9(D): LIGAND FIELD THEORY (PI DONOR & PI ACCEPTOR LIGANDS)

In this video, I have explained the effect of pi bond on ligand field strength. An important factor that contributes to the high ligand field strength of ligands such as CO, CN-, and phosphines is π-bonding between the metal and the ligand. There are three types of pi-bonding in metal complexes: The most common situation is when a ligand such as carbon monoxide or cyanide donates its sigma (nonbonding) electrons to the metal, while accepting electron density from the metal through overlap of a metal t2g orbital and a ligand π* orbital. This situation is called "back-bonding" because the ligand donates σ-electron density to the metal and the metal donates π-electron density to the ligand. The ligand is thus acting as a σ-donor and a π-acceptor. In π-backbonding, the metal donates π electrons to the ligand π* orbital, adding electron density to an antibonding molecular orbital. This results in weakening of the C-O bond, which is experimentally observed as lengthening of the bond (relative to free CO in the gas phase) and lowering of the C-O infrared stretching frequency. d-d π bonding occurs when an element such phosphorus, which has a σ-symmetry lone pair and an empty metal 3d orbital, binds to a metal that has electrons in a t2g orbital. This is a common situation for phosphine complexes (e.g., triphenylphosphine) bound to low-valent, late transition metals. The backbonding in this case is analogous to the CO example, except that the acceptor orbital is a phosphorus 3d orbital rather than a ligand π* orbital. Here the phosphine ligand acts as a σ-donor and a π-acceptor, forming a dπ-dπ bond. The third kind of metal-ligand π-bonding occurs when a π-donor ligand - an element with both a σ-symmetry electron pair and a filled orthogonal p-orbital - bonds to a metal, as shown above at the right for an O2- ligand. This occurs in early transition metal complexes. In this example, O2- is acting as both a σ-donor and a π-donor. This interaction is typically drawn as a metal-ligand multiple bond, e.g., the V=O bond in the vanadyl cation [VO]2+. Typical π-donor ligands are oxide (O2-), nitride (N3-), imide (RN2-), alkoxide (RO-), amide (R2N-), and fluoride (F-). For late transition metals, strong π-donors form anti-bonding interactions with the filled d-levels, with consequences for spin state, redox potentials, and ligand exchange rates. π-donor ligands are low in the spectrochemical series. #PiDonarLigands #PiAcceptorLigands #TETRAHEDRONCHEMISTRYCLASSES #Pi Acid #Pi base