Inductive transfer and coordination of ligands in metal—metal bonded systems

Abstract

Donor-acceptor complex formation between a range of neutral, dinuclear metal carboxylates and a range of Lewis bases has been monitored to obtain equilibrium constants and enthalpies for adduct formation. The acceptors studied are Rh 2(but) 4, Rh 2(pfb) 4, Mo 2(pfb) 4, Ru 2(but) 4Cl and Cr 2(tfa) 4(Et 2O) 2 (but = O 2C 4H 7 −, pfb = O 2C 4F 7 −, tfa = O 2CCF 3 −) and the donors include oxygen, sulphur, nitrogen and phosphorous donor atom bases. Spectrophotometric and calorimetric titrations were used to measure and apportion the experimental heats. The measured enthalpies of adduct bond formation thus correspond to stepwise complexation equilibria. Measurements with bases which varied in their covalent and electrostatic donor properties allowed definition of acid parameters to describe the dinuclear complexes by the E and C treatment. The spectral and calorimetric data point towards unusual stabilization of some of the donor-Rh dimer adduct bonds which was attributed to π-backbonding from populated Rh(II)Rh(II) π orbitals to empty donor orbitals of the proper symmetry. This provides one of the first quantitative estimates of π-backbond stabilization energies in transition metal systems. The π-backbonding effects were less pronounced for adduct formation with Ru 2(but) 4Cl (half-filled π orbitals) and not observed for Mo 2(pfb) 4 (unpopulated π orbitals). Correlation of the first and second enthalpies of adduct formation allowed determination of inductive transfer parameters to describe the inductive effects of coordination of the first base through the metal-metal bond to influence the coordination of the second base. The lowered acidity of the second metal centre upon complex formation at the first could be quantified by this treatment and the results correlated with the nature of the metal-metal bond. This correlation treatment was extended to IR data, ν co of (CO)Rh 2(pfb) 4 (B) (B = base) adducts, and EPR data, g of (TEMPO) Rh 2 (pfb) 4 (B) adducts to demonstrate the relative E and C contributions to these spectral parameters. Donor complexation was observed to have profound influences upon the electronic structures of the various complexes as followed by electronic spectral shifts. The Cr 2 (tfa) 4(B) 2 adducts provided additional means for monitoring changes in electronic structure. Magnetic susceptibilities of the Cr 2(tfa) 4(B) 2 adducts were found to increase with B donor strength suggesting a direct exchange pathway to account for observed antiferromagnetism. Solid state measurements on three of the adducts (B = Et 2O, Et 3PO 4, HMPA) gave -2 J ranging from 611 to 688 cm −1, values which are larger than those measured for the well-studied cupric carboxylates, Cu 2(O 2CR) 4 (B) 2.