4571-HW2-F2008-answers - Organometallic Chemistry 4571...

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Organometallic Chemistry - 4571ANSWER KEYHW # 2 Due: October 30, 2008 (by 2 PM) 1. (15 pts)For each of the following pairs of metal complexes, circle the one that will have the highestCO stretching frequency. Briefly and clearly discuss your reasoning for each case. The metal complex that has the leastamount of electron density on the metal center (most electron deficient) will be the one with the highestIR CO stretching frequency. a) TiO2(PMe3)2(CO)2-or- Cp2Ti(CO)2Although both complexes can be counted as 18 e- systems, the first Ti complex is d0and has no delectrons to -backbond to the CO ligand. It is, therefore, the most electron deficient and will have the highest IR CO stretching frequency (and the most labile CO ligand). The second Ti complex is in the +2 oxidation state and has a rather electron-rich d2configuration. b) RhH(CO)(PPh3)2-or- IrCl(CO)[P(OMe)3]2The Ir complex is more electron-deficient due to the poorly donating Cland P(OMe)3ligands. The P(OMe)3ligands are also moderately good -backbonders. The Ir complex, therefore will have the higher CO stretching frequency. The Rh complex, on the other hand, has a strongly donating hydride and better donating PPh3 ligands. The Ir is less electronegative than Rh and as a 3rdrow metal will bind more strongly to the CO, but these are relatively minor factors compared to the more dominate ligand effects. c) PtCl2(CO)2-or- Ni(CO)2(PPh3)2The Pt complex is more electron-deficient due to the poorly donating Clligands, the +2 oxidation state, and d8electron configuration. The Ni complex is in the zero oxidation state (d10) and has moderately good donor PPh3ligands.

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