Iron-57 NMR and Structural Study of [Fe(η5-Cp)(SnPh3)(CO)(PR3)] (PR3 = Phosphine, Phosphite). Separation of Steric and Electronic σ and π Effects

Exploring Phosphine Electronic Effects on Molybdenum Complexes: A Combined Photoelectron Spectroscopy and Energy Decomposition Analysis Study

In organometallic chemistry, especially in the catalysis area, accessing the finest tuning of a catalytic reaction pathway requires a detailed knowledge of the steric and electronic influences of the ligands bound to the metal center. Usually, the M-L bond between a ligand and metal is depicted by the Dewar-Chatt-Duncanson model involving two opposite interactions, σ-donor and π-acceptor effects of the ligand. The experimental evaluation of these effects is essential and complementary to in-depth theoretical approaches that are able to provide a detailed description of the M-L bond. In this work, we present a study of LMo(CO)₅ complexes with L being various tertiary phosphine ligands by means of mass-selected high-resolution photoelectron spectroscopy (PES) performed with synchrotron radiation, DFT, and energy decomposition analyses (EDA) combined with the natural orbitals for chemical valence (NOCV) analysis. These methods enable a separated access of the σ-donor and π-acceptor effects of ligands by probing either the electronic configuration of the complex (PES) or the interaction of the ligand with the metal (EDA). Three series of PR₃ ligands with various electronic influences are investigated: the strong donating alkyl substituents (PMe₃, PEt₃, and PiPr₃), the intermediate PPh_xMe_(3-x) (x = 0-3) set, and the PPh_xPyrl_(3-x) set (x = 0-3 with Pyrl being the strong electron withdrawing pyrrolyl group C₄H₄N). For each complex, their adiabatic and vertical ionization energies (IEs) could be determined with a 0.03 eV precision. Experiment and theory show an excellent agreement, either for the IE determination or electronic effect analysis. The ability to interpret the spectra is shown to depend on the character of the ligand. "Innocent" ligands provide the spectra that are the most straightforward to analyze, whereas the "non-innocent" ligands (which are ionized prior to the metal center) render the analysis more difficult due to an increased number of molecular orbitals in the energy range considered. A very good linear correlation is finally found between the measured adiabatic ionization energies and the interaction energy term obtained by EDA for each of these two types of ligands, which opens interesting perspective for the prediction of ligand characters.

A Nontrigonal Tricoordinate Phosphorus Ligand Exhibiting Reversible "Nonspectator" L/X-Switching

We report here a "nonspectator" behavior for an unsupported L-function σ³ -P ligand (i.e. P{N[o-NMe-C₆ H₄ ]₂ }, 1a) in complex with the cyclopentadienyliron dicarbonyl cation (Fp⁺ ). Treatment of 1a⋅Fp⁺ with [(Me₂ N)₃ S][Me₃ SiF₂ ] results in fluoride addition to the P-center, giving the isolable crystalline fluorometallophosphorane 1a^F ⋅Fp that allows a crystallographic assessment of the variance in the Fe-P bond as a function of P-coordination number. The nonspectator reactivity of 1a⋅Fp⁺ is rationalized on the basis of electronic structure arguments and by comparison to trigonal analogue (Me₂ N)₃ P⋅Fp⁺ (i.e. 1b⋅Fp⁺ ), which is inert to fluoride addition. These observations establish a nonspectator L/X-switching in (σ³ -P)-M complexes by reversible access to higher-coordinate phosphorus ligand fragments.

Syntheses and structural aspects of six-membered palladacyclic complexes derived fromN,N′,N′′-triarylguanidines with N- or S-thiocyanate ligands

Linkage isomers8·2/3PhMe and9·MeOH were synthesized, isolated and structurally characterized by SCXRD and the role of solvent of crystallization in the formation of these linkage isomers identified.

Bond dissociation energies of carbonyl gold complexes: a new descriptor of ligand effects in gold(i) complexes?

Ligand electronic effects in gold(i) chemistry have been evaluated by means of the experimental determination of M-CO bond dissociation energies for 16 [L-Au-CO]⁺ complexes, bearing L ligands widely used in gold catalysis. Energy-resolved analyses have been made using tandem mass spectrometry with collision-induced dissociation. Coupled with DFT calculations, this approach enables the quantification of ligand effects based on the LAu-CO bond strength. A further energy decomposition analysis gives access to detailed insights into this bond's characteristics. Whereas small differences are observed between phosphine- and phosphite-containing gold complexes, carbene ligands are shown to stabilize the gold-carbonyl bond much more efficiently.

Generalization of the Tolman electronic parameter: the metal–ligand electronic parameter and the intrinsic strength of the metal–ligand bond

The MLEP is a new, generally applicable measure of the metal–ligand bond strength based on vibrational spectroscopy, replacing the TEP.

Impact of Coordination Geometry, Bite Angle, and Trans Influence on Metal-Ligand Covalency in Phenyl-Substituted Phosphine Complexes of Ni and Pd

Despite the long-standing use of phosphine and diphosphine ligands in coordination chemistry and catalysis, questions remain as to their effects on metal-ligand bonding in transition metal complexes. Here we report ligand K-edge XAS, DFT, and TDDFT studies aimed at quantifying the impact of coordination geometry, diphosphine bite angle, and phosphine trans influence on covalency in M-P and M-Cl bonds. A series of four-coordinate NiCl2 and PdCl2 complexes containing PPh3 or Ph2P(CH2)nPPh2, where n = 1 (dppm), 2 (dppe), 3 (dppp), and 4 (dppb), was analyzed. The XAS data revealed that changing the coordination geometry from tetrahedral in Ni(PPh3)2Cl2 (1) to square planar in Ni(dppe)Cl2 (2) more than doubles the intensity of pre-edge features assigned to Ni-P and Ni-Cl 1s → σ* transitions. By way of comparison, varying the diphosphine in Pd(dppm)Cl2 (4), Pd(dppp)Cl2 (6), and Pd(dppb)Cl2 (7) yielded Pd-P 1s → σ* transitions with identical intensities, but a 10% increase was observed in the P K-edge XAS spectrum of Pd(dppe)Cl2 (5). A similar observation was made when comparing Ni(dppe)Cl2 (2) to Ni(dppp)Cl2 (3), and DFT and TDDFT calculations corroborated XAS results obtained for both series. Comparison of the spectroscopic and theoretical results to the diphosphine structures revealed that changes in M-P covalency were not correlated to changes in bite angles or coordination geometry. As a final measure, P and Cl K-edge XAS data were collected on trans-Pd(PPh3)2Cl2 (8) for comparison to the cis diphosphine complex Pd(dppe)Cl2 (5). Consistent with phosphine's stronger trans influence compared to chloride, a 35% decrease in the intensity of the Pd-P 1s → σ* pre-edge feature and a complementary 34% increase in Pd-Cl 1s → σ* feature was observed for 8 (trans) compared to 5 (cis). Overall, the results reveal how coordination geometry, ligand arrangement, and diphosphine structure affect covalent metal-phosphorus and metal-chloride bonding in these late transition metal complexes.