Do we really understand the bonding in transition metal organometallic complexes?

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Transition-metal organometallic complexes play a pivotal role in a wide variety of applications. They are used from megaton scales for polymer synthesis to milligram scales for drug discovery in the pharmaceutical industry. In addition, recent work has explored the use of organometallics in materials chemistry (e.g. OLED materials) and in healthcare, as novel therapeutic agents. One of the much-lauded advantages of organometallic complexes is the ability to develop structure-activity relationships based on systematic changes to the steric and electronic properties of the ligands. These can in principle be used to inform the design of new metal complexes with advantageous properties for new or existing applications.In order to quantify the effects of changing co-ligands within the coordination sphere of the metal, a number of metrics have been devised, perhaps most famously by Tolman in the 1970s. In these studies a given spectroscopic parameter (such as the stretching frequency of a carbonyl ligand) is chosen as a reporting group for these electronic changes at the metal. By recording the IR spectrum, for example, of a series of compounds it is argued that the effects on the electron density at a metal caused by a co-ligand may be evaluated. More modern approaches have focussed on a multi-parameter approach to this problem, however, a common theme is that analytical data from a co-ligand is used as reporter. Therefore, all such methods are by definition indirect measures of ligand effects and there are many examples in which the nature of ligand effects may vary depending on the reporter complex used. Therefore, in many cases our understanding of metal-ligand interactions is indirect, We propose to explore new methods to directly quantify ligand effects in organometallic chemistry which will not rely on the preparation of reporter complexes but will involve measurements directly on the metal or ligand of interest. In order to achieve this, we will employ a combination of a number of different spectroscopic and analytic methods which are rarely used in organometallic chemistry to provide information about metal-ligand interactions. By using detailed analysis of methods such as solid-state NMR spectroscopy, X-ray Absorbance Spectroscopy, and Ultra-violet photoelectron spectroscopy, coupled with complementary analysis with the latest theoretical methods (e.g. DFT, CCSD(T) etc) we will be able to directly probe the electronic structure of organometallic complexes which are of direct relevance to catalysis.The initial focus will be to study the vinylidene and alkynyl complexes previously prepared in York, but then to extend these studies to molecules of both fundamental and/or applied interest.