There are other types of electronic effects that involve trans ligands. We will focus here on CO ligands as the most common type. The bonding of a ligand to a metal with two or more -backbonding d electrons is when there is a ligand. The weakened -backbonding trans competition of both ligands for the same orbital -backbonding d electrons reduces the amount of that can occur and, -backbonding therefore, the M-CO bond strength. weakens Complex cm1 CO Mo(CO)3[P(OMe)3]3 1977, 1888 Mo(CO)3(pyridine)3 1888, 1746 Conversely, a ligand is usually by being -backbonding strengthened to a good ligand that can’t Compare the -donating -backbond. trans CO infrared stretching frequencies for the following two complexes: The P(OMe)3 ligand has about the same ability as pyridine, but -donor is a considerably better ligand, thus completing with the -acceptor CO ligands more than pyridine. trans There is a further strengthening of M-CO when the -backbonding trans ligand has properties that can push up the energy of the filled -donation orbitals and, in turn, make them better to the CO. This can -donors d occur even when the ligand is not an especially strong donor. An example of this can be seen in the following three complexes and their stretching frequencies: CO “anomalous”
There are other types of electronic trans effects that involve ligands. We will focus here on CO ligands as the most common type. The bonding of a -backbonding ligand to a metal with two or more d electrons is weakened when there is a trans -backbonding ligand. The competition of both -backbonding ligands for the same d orbital electrons reduces the amount of -backbonding that can occur and, therefore, weakens the M-CO bond strength. Complex CO cm1 Mo(CO)3[P(OMe)3]3 1977, 1888 Mo(CO)3(pyridine)3 1888, 1746 Conversely, a -backbonding ligand is usually strengthened by being trans to a good -donating ligand that can’t -backbond. Compare the CO infrared stretching frequencies for the following two complexes: The P(OMe)3 ligand has about the same -donor ability as pyridine, but is a considerably better -acceptor ligand, thus completing with the trans CO ligands more than pyridine. There is a further strengthening of M-CO -backbonding when the trans ligand has -donation properties that can push up the energy of the filled d orbitals and, in turn, make them better -donors to the CO. This can occur even when the ligand is not an especially strong donor. An example of this can be seen in the following three complexes and their “anomalous” CO stretching frequencies:
chap11-substitution-rxns.docx