Thermodynamics of Phosphine Coordination to the [PNP]Rh^I Fragment:  An Example of the Importance of Reorganization Energies in the Assessment of Metal−Ligand “Bond Strengths”

Abstract

Reaction enthalpies of the complexes [RPNP]Rh(COE) ([RPNP] ) N(SiMe 2 CH 2 PPh 2 ) 2 , N(SiMe 2 -CH 2 P i Pr 2 ) 2 ; COE ) cyclooctene) with a series of phosphine ligands and CO have been measured by solution calorimetry. The measured enthalpies span a range of ca. 40 kcal/mol. These systems favor coordination of strong π-acceptor/weak σ-donor ligands as shown by the trend in ∆H rxn : CO . Ppyrl′ 3 > Ppyrl 3 > PPhpyrl 2 > PPh 2 pyrl > PPh 3 . This trend is exactly the opposite of that observed in another square planar rhodium(I) system, trans-RhCl(CO)(PZ 3 ) 2 . With the exception of CO, the ligands investigated are isosteric, and so the observed trends are electronic in nature. Single-crystal X-ray diffraction studies on several of theses complexes ([RPNP]RhL where R, L ) Ph, PPh 3 ; Ph, Ppyrl 3 ; Ph, CO; i Pr, PPh 3 ; i Pr, Ppyrl 3 ; i Pr, CO; i Pr, COE) have been performed. Although the structural trends are readily understood in terms of the electronic (donor/acceptor) nature of each ligand array, it is not obvious that the structural data predict the trends or, in particular, the trend reversal in ∆H rxn in the two Rh(I) systems. Rather, these results illustrate the importance of reorganization energies in thermodynamic analyses of metal-ligand bonding, especially in the presence of synergistic bonding involving σ-donor, π-donor, and π-acceptor ligands, interacting through shared metal orbitals (electron pushpull). In such cases the interpretation of a metal-ligand bond dissociation enthalpy (D) as an intrinsic, universal, and transferable property of that bond (e.g., a "bond strength") is an invalid proposition.

Preparation, properties, and reactivity of carbonylrhodium(I) complexes of di(2-pyrazolylaryl)amido-pincer ligands

“…All of the RheN pz bond distances in 1e5 are comparable to those found in [EtN(CH 2 pz*) 2 ]Rh(CO)] þ (pz* ¼ 3,5-dimethylpyrazolyl, avg 2.019(3) Å) [41], {[O(CH 2 pz*) 2 ]Rh(CO)} þ (avg 2.037(4) Å) [42], or {[S(CH 2 CH 2 pz*) 2 ]Rh(CO)} þ (avg 2.044(2) Å) [43]. The remaining bond distances for the rhodium-amido (RheN1 ranging from 2.027(2) Å in 1 and 2.050(4) Å in 5) and rhodium carbonyl (RheC41 ranging from 1.812(6) Å in 5 to 1.834(2) Å in 3 and C41eO1 ranging from 1.147(2) Å in 3 to 1.170(8) Å in 5) fragments in line with other carbonylrhodium(I) complexes of amido-anchored pincer complexes [19e22], [24] [44].…”

Chloro(dimethylformamide-κO)(hydridotripyrazolylborato)(tripyrrol-1-ylphosphine-κP)ruthenium(II)

“…The most significant difference in the Ru coordinations of these two compounds is that the Ru-P bond length in (I), 2.2022 (5) Å , is shorter by 0.107 Å than that in [Ru(Tp)Cl-(PPh 3 )(DMF)], where it is 2.309 (1) Å . This difference agrees qualitatively with the finding on a few other transition metal complexes, for which pairs of PPyrl 3 -and PPh 3 -containing compounds were studied and the metal-P bonds to PPyrl 3 were systematically shorter by 0.05-0.10 Å (Moloy & Petersen, 1995;Huang et al, 1998;Trzeciak et al, 1997). The PPyrl 3 ligand in (I) exerts a notable trans influence, which causes the Ru-N4 bond to be 0.067 Å longer than the mean of the Ru-N2 and Ru-N6 bond lengths (Table 1).…”

Spin State, Structure, and Reactivity of Terminal Oxo and Dioxygen Complexes of the (PNP)Rh Moiety

“…Rather, they reflect reaction thermodynamics, which can be favorable not only with oxidants, but also simply with "good ligands." Certainly there is good evidence [18,19] that CO is a better ligand (thermodynamically) than H 2 or most other ligands, but its strength relative to O 2 has not been previously established. Moreover, we deal here with Rh, which has significantly less favorable redox reaction enthalpies than, for example, the more reducing Ir.…”