self test 22-4 Interpreting carbonyl stretching frequencies and phosphine comple
ID: 961988 • Letter: S
Question
self test 22-4 Interpreting carbonyl stretching frequencies and phosphine complexes which of the two isoelectronic compounds Cr(CO)_6 and [V(C0)_6] will have the higher CO stretchinq frequency? Which of the wo chromium compounds [Cr(CO)_5(Pet_3)] and [Cr(CO)_5(PPh_3)] will have the lower CO stretching frequency? Which will have the shorter M-C bond? We need to think about whether backbonding to the CO ligands is enhanced or diminished: more bockbonding results in a weaker carbon-oxygen bond. The negative charge on the V complex will result in geater pi backbonding to the CO pi^astir orbitals, compared to the Cr complex. This backbonding a weakening of the CO bond, with a corresponding decrease in stretching frequency. Thus the Cr complex has the higher CO stretching frequency, Pet_3, is more basic than PPh}and thus the PEt, complex rill have greater electron density on the metal atom than the PPh_3 complex. The greater electron density will result in greater backbonding and thus both a lower CO stretching frequency and a shorter M-C bond. Which of the two iron compounds Fe(CO)_5 and [Fe(CO)_4(PEt_3] will have the higher CO stretching frequency? Which will have the longer M-C bond? Hydrides and dihydrogen complexes Key points: The bonding of a hydrogen atom to a metal atom is a sigma interaction, whereas the bonding of a dihydrogen ligand involves pi backbonding.Explanation / Answer
PEt3 is basic. Thus it will have greater electron density on the metal atom than the complex without PEt3. The greater electron density will result in greater backbonding and thus both a lower CO stretching frequency and a shorter M-C bond.
Hence Fe(CO)5 will have higher stretching frequency and longer M-C bond.
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