Chemical shift tensors as a direct probe in metal-carbon bonding

Chemical shift tensors, the three-dimensional representation of chemical shift, have been recovered for a range of organometallic complexes through the collection of solid-state (SS) nuclear magnetic resonance (NMR) spectroscopy. Quantum chemical calculations using density functional theory (DFT) have also been performed to predict the chemical shift tensors. The chemical shift interaction is encoded with electronic information and can be decomposed into two contributions: diamagnetic shielding (d dia) and paramagnetic shielding (d para). dpara dominates the observed change in shielding in large nuclei and is associated with magnetically coupled transitions between frontier molecular orbitals (FMOs).
In this thesis, chemical shift tensors have been used to analyse the FMOs of organometallic compounds with a focus on metal-carbon bonding. The fluorine effect has been explored on ruthenium alkynyl and vinylidene moieties. The metal-bound carbon (C a) undergoes a large change in isotropic chemical shift between fluorinated and non-fluorinated moieties: d C ≈ 380 vs d C ≈ 350. This has been shown to be a 236 ppm deshielding shift in the dz tensor associated with the HOMO→LUMO transition. The increased deshielding arises from the contraction of the HOMO→LUMO energy and an improved magnetic coupling. The opposite trend has been explored in C between fluorinated and non-fluorinated alkynyl moieties: d C ≈ 36 vs d C ≈ 120. A 100 ppm shielding shift in the dx and dy tensors is responsible. Inclusion of fluorine decreases the energy of the sigma-bonding orbital reducing the deshielding.
High resolution 195Pt SS NMR spectra have been recorded. 103Rh solid-state NMR spectra were too technically demanding to record. Platinum chemical shift tensors have been used as a model for exploring the FMOs of metal complexes directly at the metal. This has been expanded to study a range of rhodium complexes relevant to hydroformylation looking at the effect of pnictogen and phosphine bite angle on 103Rh tensors.