Organometallic Chemistry Name:______Ron Paul_________________ CHEM 534 Exam 1 October 9, 2008 Prof. Malachowski2 1. How could you distinguish between a square planar and a tetrahedral structure in a Ni(II) complex of which you have a pure sample, without using crystallography? (10 pts.) Ni(II) tetrahedral Ni(II) square planar Ni(II) in a tetrahedral geometry would be paramagnetic, while in the square planar it is not. 2. The three organometallic complexes shown below were reported in the most recent issue of the Journal of the American Chemical Society (2008, 130, issue 41). Choose TWO and provide the following: (i) identify each ligand as L-type or X-type; (ii) formal oxidation state of the metal; (iii) dn electron count; (iv) total electron count of the metal (v) coordination number. (10 pts.) (a) IrNHCH3NCCH3 (b) NiSSNNOOO (c) ReCONOCOCNRNNRNNR (i) X-type: Cp (L2X), Phenyl L-type: N(H)CH3, NCCH3 (ii) Ir+3 (iii) d6 (iv) 18 e-‘s (v) CN=6 (i) X-type: both N-C(O), S-CH2 L-type: S(CH2)C(O)CH3 (ii) Ni+2 (iii) d8 (iv) 16 e-‘s (v) CN=4 (i) X-type: none L-type: all six (ii) Re+ (iii) d6 (iv) 18 e-‘s (v) CN=6 _____/20 pts.3 3. Which of the two complexes below is more likely to undergo beta-elimination? Explain. (8 pts.) (a) (b) NiP h3PP h3P NiPh3PPh3P Complex (a) should undergo beta-elimination faster because it’s beta protons will be more available since the alkyl groups can freely rotate in (a), but are confined in a small ring in (b) and therefore not accessible to the Ni filled d orbitals. 4. Show two specific examples of reactions that make a metal-alkyl complex. Name the reaction type that you have shown (e.g. beta-elimination, oxidative addition, etc.). (12 pts.) Possible answers include: 1) transmetallation or ligand substitution or nucleophilic attack on M XPd RPPh3Ph3PPhPd RPPh3Ph3PX=halideSnBu3+ X-SnBu3 (2) electrophilic attack on metal FeOCCODFeCOCOOBsDDDNa+ NaOBs (3) oxidative addition IrLOCClLCH3-IIrLOCClLCH3I (4) 1,2-insertion C CRHCp2Zr(H )Cl(Schwartz's reagent)ZrClHHR _____/20 pts.4 5. The first dihydrogen  complex was reported in 1984 by G. Kubas and co-workers (JACS 1984, 106, 451) and had the structure: W(CO)3(PCy3)2(2-H2). (Cy=cyclohexyl) (a) Draw the structure of this complex if it has a meridonal orientation. (6 pts.) WCOOCPCOPH H (b) Suggest one reason why this complex may be stabilized in the sigma dihydrogen form as opposed to the classical di-hydride structure. (4 pts.) The W may be relatively electron poor with three CO’s ( acids engaged in backbonding) and this limits the e- density available to donate from W to the * bond of the H-H. Donation to * leads to formation of the classical dihydride complex. A second reason may be that the complex is coordinatively saturated (CN=6) and therefore can’t fit two hydride ligands. (c) Suggest a way that you could test your reason from part (b). (4 pts.) For reason one: replace one or more of the CO ligands with strong  donors and/or  neutral or acidic ligands, e.g. –CH3 or PMe3, to see if increasing electron density on W will cause the dihydride complex to form. For reason two: replace one or both of the P(Cy)3 ligands with PR3 ligands that have a smaller cone angle, but similar electronic donation properties, e.g. PMe3 or P(CH2CH3)3. 6. List the following in order of decreasing reactivity with trimethylamine oxide on their CO groups. (10 pts.) Mo(CO)6 Mn(CO)6+ Mo(CO)2(dppe)2 Mo(CO)52- Mo(CO)4(dppe) Least reactive Most reactive _ Mo(CO)52-_<__ Mo(CO)2(dppe)2_<__ Mo(CO)4(dppe)_<_ Mo(CO)6_<__ Mn(CO)6+_ _____/24 pts.5 7. Compare characteristics of the following two complexes. (1) (3-allyl)Co(PPh3)(CO)2 (2) (3-allyl)Co(PMe3)(CO)2 a) Select the complex with the highest CO and briefly explain your choice. (6 pts.) As a stronger  acid PPh3 in (1) will withdraw more e-density from the Co thereby reducing the back bonding to the CO ligands, increasing the strength of the CO bond and raising CO. b) Select the complex with the most sterically congested center and briefly explain your choice. (6 pts.) Complex (1) PPh3 has a much larger cone angle than PMe3 and will therefore require greater space around the Co. 8. The rate law for CO exchange in Ni(CO)4 is: rate=k[Ni(CO)4]. Ni(CO)4 + 13CO  Ni(CO)3(13CO) + CO a) Provide the total electron count of Ni(CO)4: (3 pts.) 10 e-s (for Ni) + (4 x 2 e-‘s for each CO) =18e-‘s b) Provide the dn electron count of Ni: (3 pts.) No X type ligands, therefore Ni(0) and d10. c) Provide the best mechanism for the reaction that is consistent with the rate law. (7 pts.) NiCOCOOCOC+ CONiCOOCOC13CONi13COCOOCOC dissociative substitution mechanism _____/25 pts6 9. When Cr(CO)6 is irradiated with UV light, the following reaction occurs: a) Draw the Cr d molecular orbitals before and after the irradiation to show the effect of the irradiation event. (6 pts.) Octahedral geometry for Cr(CO)6 and therefore a two-over-three arrangement of Cr’s d molecular orbitals. Light causes the promotion of one e- from d to the d orbitals. Cr(C O)6Cr(CO)5hdor egdor t2g b) Explain why this results in the loss of one CO ligand. (5 pts.) With the promotion of an electron to the anti-bonding d orbital, there is a new e-/e- repulsion between the d e- and one of the CO groups along the x, y or z axis. Furthermore, there is also one less e- for  backbonding and so the CO backbond will be compromised. _____/11