Five-Coordination in Platinum(II) and Palladium(II) Chemistry

Abstract

In recent years a number of chelating ligands. when combined with a n-acid such as an alkene, have been found to stabilize five-coordination in palladium(I1) and platinum(I1) chemistry. The complexes have invariably a trigonal bipyramidal geometry with the bidentate ligand and the alkene in the equatorial plane. The n-acceptor capacity of the alkene and a small bite of the chelate ligand (between 70 and 85') are both required for accommodating them in the equatorial plane where the d:electron concentrations (which in the square planar d X complexes are located above and below the coordination plane) are also confined. Two monodentate ligands, in place of one chelate. do not stabilize the five-coordination since for entropic reasons the dissociation of one of them is strongly favored. A great bulk of the chelate ligand is also found to stabilize five-coordination since interligand steric interactions are smaller in the trigonal plane of a five-coordinate complex than in the square-plane of a four-coordinate species. Moreover the same steric factors that destabilize four-versus five-coordination can also destabilize fourversus three-coordination, and T-shaped ML, species appear to provide a low energy path for the interconversion between four-and five-coordinate complexes. The alkene is strongly bound to the metal. the metal-carbon bond distances are shorter, and the bending back of the alkene substituents is greater than those found in four-coordinate compounds. The activation energy for alkene rotation is also greater in five-coordinate species, and in a number of cases atropisomers are generated. The chelate dinitrogen ligand also lying in the equatorial plane is. on the other hand, loosely bound to the metal, and dissociation of one end of this ligand Comments Inorg. Chem. appears to be the preliminary step in all substitution processes involving the alkene. the axial ligands. and the bidentate ligand itself. These processes should therefore occur through a mechanism analogous to that operating in four-coordinate complexes. The dissociation of one end of the bidentate ligand also provides a low energy path to proton exchange and inversion of configuration at coordinated nitrogens. Other aspects related to the conformation of the alkene and of the chelate ligand as well as to the preparation and reactivity of this class of compounds are discussed.

Hindrance, Donor Ability of Men∩NN Chelates and Overall Stability of Pentacoordinate [PtCl2(η2‐CH2=CH2)(Men∩NN)] Complexes as Observed by η2‐Olefin 1JPt,C Modulation: An NMR Study

“…Common decomposition and formation pathways for pentacoordinate complexes [PtCl 2 (η 2 -CH 2 =CH 2 )(_ NN)]; _ NN = dinitrogen ligand; X, Y = general ligands. [17] The presented pattern is also common for other unsaturated ligands coordinated to the metal, instead of simple ethene.…”

“…In particular, the [PtXY(η 2 -alkene)(_ NN)] trigonal bipyramidal species show peculiar common features such as (i) the Pt-X and Pt-Y distances are very similar to those generally found in the analogous four-coordinate species; [17] (ii) the length of the equatorial platinum-alkene bond (in the trigonal plane) is generally shorter than that found in square-planar species, [26][27][28] with a greater activation energy for rotation about the platinum-alkene bond; [20] (iii) the equatorial Pt-N bonds are generally weaker and about 0.2 Å longer than observed in square-planar species obtained by olefin loss; [17][18][19]29,30] (iv) the optimal N-Pt-N angle between metal-bis-nitrogen ligand is lower than 80°( with a preference for the low-membered chelate rings); (v) the unsaturated ligand is able to reduce the electron density on the metal (back electron-donation from filled d orbitals of the metal to the empty π* orbital of the olefin), which favors coordination of the second N-donor of the chelate. [5,[17][18][19] Previous data and the theoretical description of pentacoordinate complexes, made by Kepert [17,31] and Lin, [32] following the fundamental studies by Hoffmann and coworkers, [33] did not rule out the possibility of thinking of this kind of pentacoordinate complex as particular squareplanar "trans" complexes (pseudo-trans in the following text), where the same coordination position is occupied by the two N-donors.…”

“…A rationale for the stability changes observed in such pentacoordinate complexes is suggested, based on the NMR spectroscopic data analysis. [17] Scheme 2. Common decomposition and formation pathways for pentacoordinate complexes [PtCl 2 (η 2 -CH 2 =CH 2 )(_ NN)]; _ NN = dinitrogen ligand; X, Y = general ligands.…”

“…); [17][18][19][21][22][23] moreover, they have also been proposed as protecting groups for the stabilization of unstable olefin derivatives. [24,25] For this reason, we considered the role of steric and electronic properties in determining the thermodynamic stability of such [PtXY(η 2 -alkyne)(_ NN)] and [PtXY(η 2 -alkene)(_ NN)] five-coordination complexes to be worth of further investigation.…”

“…[5][6][7][8][9][10][11][12][13][14][15][16] Although some very stable pentacoordinate complexes are well known, [17][18][19][20] [PtXY(η 2 -alkyne)(_ NN)] and [PtXY(η 2 -alkene)(_ NN)] complexes generally undergo characteristic decomposition patterns, including loss of alkyne or alkene, loss of an axial ligand, chelate ring opening, reductive elimination, and insertion of the unsaturated ligand into a σ-bond. [18] In this context, the main decomposition pattern generally involves the formation of reactive cationic intermediates of the type [PtX(η 2 …”