91Ó°ÊÓ

The \(^{31} \mathrm{P}\) NMR spectrum of \(\mathrm{PMe}_{3}\) consists of a binomial decet \((J \quad 2.7 \mathrm{Hz})\) (a) Account for this observation. (b) Predict the nature of the \(^{1} \mathrm{H}\) NMR spectrum of PMe \(_{3}\)

(a) Explain why the location of \(\mathrm{H}\) atoms by \(\mathrm{X}\) -ray diffraction is difficult. (b) What technique (and why) is used for the accurate determination of \(\mathrm{H}\) atom positions in a structure? Comment on any limitations of this method.

(a) The IR spectrum of naturally occurring CO shows an absorption at \(2170 \mathrm{cm}^{-1}\) assigned to the molecular vibrational mode. If the sample is enriched in \(^{13} \mathrm{C}\), what change do you expect to see in the IR spectrum? (b) The IR spectra of salts of \(\left[\mathrm{Fe}(\mathrm{CO})_{6}\right]^{2+}\) and \(\left[\mathrm{Fe}(\mathrm{CO})_{4}\right]^{2-},\) respectively show absorptions at 2204 and \(1788 \mathrm{cm}^{-1}\) assigned to carbonyl vibrational modes. What do these tell you about the strengths of the carbon- oxygen bonds in the complexes compared to that in free \(\mathrm{CO} ?\) \((\mathrm{c})\left[\mathrm{Fe}(\mathrm{CO})_{4}\right]^{2-}\) and \(\mathrm{Fe}(\mathrm{CO})_{5}\) have \(T_{\mathrm{d}}\) and \(D_{3 \mathrm{h}}\) symmetries respectively. What do you expect to observe in the room temperature \(^{13} \mathrm{C} \quad \mathrm{NMR}\) spectra of \(\mathrm{Na}_{2}\left[\mathrm{Fe}(\mathrm{CO})_{4}\right] \quad\) and \(\quad \mathrm{Fe}(\mathrm{CO})_{5} ?\) Rationalize your answer.