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MS M ass S pectrometry

MS M ass S pectrometry. In Organometallics. Index. MS. MS-fragmentation-1. MS-fragmentation-2. Isotope patterns. Aluminium Bromide:. AlBr + 27 Al : 100% 79 Br : 51% 81 Br : 49%. Isotope patterns. 63 Cu 35 Cl. 63 Cu: 69.1%. 35 Cl: 75.5% 37 Cl: 24.5%. 63 Cu 37 Cl. 65 Cu 35 Cl.

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MS M ass S pectrometry

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  1. MSMass Spectrometry In Organometallics Index MS MS-fragmentation-1 MS-fragmentation-2

  2. Isotope patterns Aluminium Bromide: AlBr+27Al : 100% 79Br : 51% 81Br : 49%

  3. Isotope patterns 63Cu35Cl 63Cu: 69.1% 35Cl: 75.5% 37Cl: 24.5% 63Cu37Cl 65Cu35Cl 65Cu: 30.9% 65Cu37Cl Abundance of first isotope peak M: .691 x .755 = .521 For next isotope peak: M+2: .691 x .245 = .169 } .403 .309 x .755 = .233 For next isotope peak: M+4: .309 x .245 = .076

  4. Main group organometallic Some metals in main group are mono-isotopic. (e.g. Be) These ions are difficult to distinguish from hydrocarbons For example: Be(Et)2 produce abundant ion of m/z 25 this can be C2H or BeH(CH3) only High resolution MS can solve this problem! Some metals in main group are poly-isotopic. (Ge, Sn, Hg…) Ions from these metals are relatively easy to interpret but the fact the several ions of slightly different mass can overlap can complicate the interpretation.

  5. Poly-isotopic metals Metal isotope pattern are distorted by 13C and 2H

  6. Poly-isotopic metals When more than one metal is present: pattern is affected in a predictable way

  7. Electron Impact The removal of an electron by bombardment with electrons of high energy (~ 70 eV) form energetic ions having excess of energy. This energy causes decomposition during short interval (10-5 s) Usually, the molecular ion is visible. Upon fragmentation the charge remain usually on the Metal ion The formation of multiply charged ions can occur frequently. M + ne- => Mn+ + (n+1)e Doubly charged ions are frequent in polymetallic compounds and in compounds from the second and third transition metal. Triply charged ions are less frequent (< 1%)

  8. Ions-Molecule reaction One of the greatest use of MS in organometallic compounds is the possibility to extract the molecular weight => molecular formula Some ions can occasionally be form from collision of ions with a molecule, forming ions with greater mass than the molecular ion. These process sometime might obscure the molecular ion. • Careful study is sometime needed: • Ions formed from unimolecular process are directly proportional to pressure • Ions formed from bimolecular process are proportionaltothe square of the pressure : these ions have greater mass than molecular ion e.g. Cr(CO)6+ + Cr(CO)6 --> Cr(CO)10++ 2 CO M+

  9. Fragmentation • The charge is likely to remain on Metal-containing fragment M Ln-1+ + L MLn L++ MLn-1 Less likely • Rearrangement involving hydrogen migration are frequent (M => Si, Ge, Sn, Pb…)

  10. Fragmentation • Migration of Halogens is very common M (C6F5)3+ (C6F5)2 Sn F+ + C6F4 M => Si, Ge, Sn, Pb • Migration of Alkyl to metal is also common Transfer of R is influence by electronegativity of the R group Abundance of MR+ : MI < MBr < MCl < MF

  11. Fragmentation Compounds with s bond between ligand and metal decompose by radical elimination Sn(Me)4 + e -> Sn(Me)4+.+ 2e Compounds with p bond complex loose even-electron molecules Fe(CO)5 + e -> Fe(CO)5++ 2e Electron impact remove electron from non-bonding metal orbital (non-radical) ion

  12. Rearrangement process “McLafferty” type of rearrangement involving metal instead of H This rearrangement depends on readiness of metal to become pentacoordinate (using it’s d-orbitals) Observed in: Keto-organotin: RCO (CH2)3 Sn (CH3)3

  13. Influence of operating conditions Electron deficient Alkyls and Aryls groups gives information about molecular complexity of the vapor. Ethyl Lithium: produce following ions: Li6Et5+Li5Et4+Li4Et3+Li3Et2+Li2Et+Li+ All these can be derive from the hexamer Li6Et6 In Electron deficient compounds of group II and III (Be and Al), monomeric molecules are associated by weak bridging bonds

  14. Influence of operating conditions For example: Diethylberelium: BeEt2 is dimeric in benzene solution

  15. Influence of operating conditions At 200 C : all ions derived from associated species have disappear or are very low in abundance. Since the abundance of hydrocarbon ions does not change markedly with source temperature between 50-200 C, this behavior is not due to termal decomposition of diethylberelium. The observed changes are consistent with increase thermal dissociation of the trimeric and dimeric molecules prior to ionization

  16. Metal Carbonyls Mass Spec. provides simple, quick and reliable way to determine molecular formula of such system. Since most transition metals are polyisotopic, isotope patterns can be used to determine the molecular formula (except for hydrido complexes where the loss of H can obscures the isotope pattern) Molecular peak is usually observed => highest mass is molecular ion The progressive loss of CO is a common feature of great convenience: It allows the number of CO ligands to be readily counted

  17. Metal Carbonyls (Co)

  18. Metal Carbonyls (Fe)

  19. Metal Carbonyls (Fe)

  20. Carbonyl halide compounds Terminal Bridging

  21. Carbonyl halide compounds Terminal Loss of CO and X occur with equal ease Bridging Loss of CO occur stepwise with retention of M2X2 than further X loss can occur NMR

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