The proton NMR of (C_5H_5 Fe(CO_2), as a function of temperature shows a singlet
ID: 527802 • Letter: T
Question
The proton NMR of (C_5H_5 Fe(CO_2), as a function of temperature shows a singlet at 4.4 ppm for one of the C_5H_5 groups at all temperatures. At -80 degree C, the other C_5H_5 group appears as a complex multiplet pattern of sharp peaks at 3.5 ppm and 6.2 ppm. At 30 degree C, this peak appears as a sharp singlet at 5.7 ppm. At intermediate temperatures, the multiplet pattern appears as broad peaks with chemical shifts sliding towards 5.7 ppm as the temperature increases. What is the best explanation for this series of spectra? Several isomers exist at low temperature and only one isomer exists at high temperature. The hapticity of the rings changes from eta^1 at low temperature to eta^5 at high temperature. One C_5H_5 is eta^1 at all temperatures but rapid conversion of the five positional isomers at high temperatures shows a time-average of the low temperature chemical shifts. The molecule is diamagnetic at low temperature and paramagnetic at high temperature. Which is most likely to be the first-row transition metal M in the complex (eta^1-C_6H_6M(CO_3)? Cr Mn. Fe Co What is the formal metal-metal bond order in Mn_2(CO)_3? 0 1 2 3 Which of these has the highest frequency carbonyl stretching bands? (B) Ni(CO_3) (PH_3) Ni(CO)_3(PF_3) Ni(CO)_3, (PCl_3) Ni(CO)_3(Pme_3) The metalloproteins rubredoxin and ferredoxin have iron and sulfur at their reactive sites. What is the primary function of the iron and sulfur? electron transfer hydroxylation oxygen transport oxygen storageExplanation / Answer
49. Ans. C
50.Cr
51.1
52. Ni(CO)3(PCl)3
In PMe3 case Me will donate electron to the metal center and thus encouraging pi-back bonding from metal to carbonyl group and thusdecreasing the stretching frequency. In PCl3 due to withdrawing capacity a reverse trend will be seen.
53. Electron transfer
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