Solution thermochemical study of tertiary phosphine ligand substitution reactions in the RhCl(CO)(PR3)2 system

The enthalpies of reaction of [Rh(CO)(2)Cl](2) (1) with a series of monodentate tertiary phosphine ligands, leading to the formation of RhCl(CO)(PR(3))(2) complexes, have been measured by anaerobic solution calorimetry in CH2Cl2 at 30.0 degrees C. These reactions are rapid and quantitative. The measured reaction enthalpies span a range of 43 kcal/mol. The relative stability scale established is as follows: P(NC4H4)(3) < P(NC4H4)(2)(C6H5) < P(OPh)(3) < P(p-CF3C6H4)(3) < P(NC4H4)(C6H5)(2) < P(p-ClC6H4)(3) < AsEt(3) < P(p-FC6H4)(3) < PPh(3) < P(p-CH3C6H4)(3) < P(p-CH3OC6H4)(3) < PPh(2)Me < P(OMe)(3) < PPhMe(2) < PEt(3). The relative importance of phosphine electronic ligand parameters is closely examined in terms of the presented quantitative thermochemical information. Comparisons with enthalpy data in related organometallic systems are also presented

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'

Reaction enthalpies of the complexes [RPNP]Rh(COE) ([RPNP] = N(SiMe2CH2PPh2)(2), N(SiMe2-(CH2PPr2)-Pr-i)(2); COE = cyclooctene) with a series of phosphine ligands and CO have been measured by solution calorimetry. The measured enthalpies span range of ca. 40 kcal/mol. These systems favor coordination of strong pi-acceptor/weak sigma-donor ligands as shown by the trend in Delta H-rxn: CO much greater than Ppyrl(3)' > Ppyrl(3) > PPhpyrl(2) > PPh(2)pyrl > PPh3. 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, PPh3; Ph, Ppyrl(3); Ph, CO; Pr-i, PPh3; Pr-i, Ppyrl(3); Pr-i, CO; Pr-i, 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 Delta 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 sigma-donor, pi-donor, and pi-acceptor ligands, interacting through shared metal orbitals (electron push-pull). 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

Lewis acids accelerate reductive elimination of RCN from P2Pd(R)(CN)

The rate of reductive elimination of the complexes dpppPd(CH2TMS)(CNER3) (E = B, Al) is accelerated up to 60-fold over dpppPd(CH2TMS)(CN). Based on kinetic considerations and the isoelectronic relationship of CN- and CO, a migration-type mechanism for reductive elimination is proposed. The rate acceleration correlates directly with Lewis acid strength, the latter determined by solution calorimetric analyses of the Lewis acid adduct forming reaction Pd-CN + ER3 --> Pd-CN-ER3

Synthetic, structural, and solution calorimetric studies of Pt(CH3)(2)(PP) complexes

Reaction enthalpies of the complex (COD)PtMe2 (1; COD = eta(4)-1,5-cyclooctadiene) with an extensive series of bidentate phosphines (dpype, dppf, diop, dppe, dppb, dppp, dpmcb, depe, dmpe, dcpce) have been measured by solution calorimetry. The relative stabilities of the resulting complexes PtMe2(PP) are determined by a combination of the donor and bite angle/ steric properties of the bidentate phosphine ligand. In general, good sigma donor ligands with small bite angles result in more thermodynamically stable complexes. Additionally, the molecular structures of 1, Pt(Me)(2)(pype) (2), Pt(Me)(2)(dppf) (8), Pt(Me)(2)(diop) (4), Pt(Me)(2)(dppe) (6), Pt(Me)(2)(dpmcb) (9), and Pt(Me)(2)(Et(2)dppp) (13) have been determined by single-crystal X-ray diffraction. No correlation between the thermochemical results and the structural parameters, e.g, M-P distance (as observed in other systems), is apparent in this class of complexes

N-pyrrolyl phosphines : enhanced π-acceptor character via carboalkoxy substitution

A series of 3,4-dicarboethoxy-N-pyrrolyl phosphine ligands (R2P-NC4H2(CO2Et)(2)) is described. A number of techniques, including reaction calorimetry, infrared spectroscopy, and X-ray crystallography, demonstrate that these ligands are potent pi-acceptors and that the electronic substituent parameter chi(i) for the 3,4-(EtO2C)(2)C4H2N moiety is ca. 16, placing it in a position between fluorine and chlorine