Four-versus five-co-ordination in palladium(II) and platinum(II) complexes containing 2,9-dimethyl-1,10-phenanthroline (dmphen). Crystal structures of [PtCl2(dmphen)] and [Pt(η2-C2H4)Cl2(dmphen)]

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Abstract
The four-co-ordinate complexes with 2,9-dimethyl-1,10-phenanthroline [MX2(dmphen)](M = Pt, X2= Cl21, ClBr 2, Br23, or I24; M = Pd, X2= Cl25, ClBr 6, Br27 or I28) have been prepared for the first time. The crystal structure of [PtCl2(dmphen)]1, has been determined by X-ray diffraction methods. The complex has a square-planar geometry around the metal and the steric interaction between the methyl groups of dmphen and the cis chlorine ligands causes a narrowing of the Cl–Pt–Cl angle [85.8(1)°], a displacement of the two chlorine atoms from the N–Pt–N plane [0.286(4) and 0.372(4)Å respectively], a bending of the phenanthroline (ca. 17°), and a rotation of the overall ligand plane with respect to the platinum co-ordination plane (ca. 28°). All interligand steric constraints are released in the five-co-ordinate complexes obtained from the four-co-ordinate species by direct uptake of olefins, [M(η2-olefin)X2(dmphen)](olefin = ethylene, a; propene, b; but-1-ene, c; cis-but-2-ene, d; trans-but-2-ene, e; or styrene, f). The structure of [Pt(η2-C2H4)Cl2(dmphen)]1a, has been determined by X-ray diffraction methods. The complex has a trigonal-bipyramidal geometry around the Pt atom; the phenanthroline ligand and the olefinic carbons occupy the equatorial plane while the two chlorine atoms are in axial positions. Bond distances and angles are similar to those found in other five-co-ordinate complexes of platinum(II). The rate of uptake of olefin by the four-co-ordinate complexes, the equilibrium constant (Kf) of the formation reaction [MX2(dmphen)]+ olefin ⇌[M(η2-olefin)X2(dmphen)], and the activation energy for olefin rotation in the five-co-ordinate complexes have been measured for different olefins and halogen ions. The uptake, in chloroform, takes place in a one-step process and the reaction rate increases by a factor of 104 going from the chloro 1 to the iodo 4 complex. The equilibrium constant for the formation reaction decreases by a factor of 103 going from ethylene to trans-but-2-ene and increases by a factor of 102 going from the chloro 1 to the iodo 4 species. The activation energy for olefin rotation (ethylene and propene)(ca. 20.5 kcal mol–1) is higher than any reported value for platinum complexes and does not appear to depend upon the nature of the halogen atoms.

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