4571-HW2-2008-Answers - Organometallic Chemistry 4571...

This preview shows page 1 - 2 out of 3 pages.

Organometallic Chemistry - 4571ANSWER KEYHW # 2 Due: March 13, 2008 1. (18 pts)For each of the following pairs of metal complexes, circle the one that should have the highestaverage carbonyl IR stretching frequency. What does this tell you about the relative electron density on the metal center (electron-rich or deficient)? Briefly discuss your reasoning for each case. a) [Rh(CO)4]-or- [Fe(CO)4]2Rh and Fe have similar electronegativities and the same # of CO ligands. But Fe is in the 2 oxidation state and has an overall 2charge which makes it more electron-rich than the mono-anionic Rh complex. On page 5 of your Carbonyl chapter notes the table at the bottom lists [Fe(CO)4]2as having a rather low COof 1790 cm1, while [Co(CO)4]has a COof 1890 cm1. b) [Re(PMe3)(NCMe)(CO)4]+-or- [Mn{P(OPh)3}2(CO)4]Both complexes are cationic and have the same # of CO ligands, but the phosphite ligands in the Mn complex are poorer donors than PMe3(about the same as N=CMe) and certainly better -acceptors making the Mn center more electron-deficient. Mn is also more electronegative than Re and will hang onto it’s electrons a little more strongly have have less tendency to -backbond to the CO’s. c) CpFe(Br)(CO)2-or- Os(C-Me)(O)(CH3)3(CO) The Os complex is d0(+8 oxidation state) and can’t p-backbond effectively to the CO ligands. The Fe complex is d6and can. d) Cp*Mn(PPh3)(CO)2-or- CpIr(CO)2(Cp* = C5Me5) The Ir complex is more electron-deficient due to the less donating Cp ligand (compared to the stronger donating Cp*). The Mn complex also has an extra donating (moderately) PPh3ligand. e) [Re(dmpe)2(PMe3)(CO)]+-or- Ni(CO)3{P(OMe)3} Although the Re complex is cationic, it has two sets of strongly donating alkylated dmpe ligands (the equivalent of 4 PMe3ligands) and a PMe3ligand and only one -backbonding CO ligand. These strongly donating ligands more than compensate for the cationic charge on the Re making it more electron-rich. The Ni complex on the other hand is more electronegative and has 3 CO ligands and one poorly donating (and moderately -accepting) phosphite ligand.

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture