Treatment of the neutral trimethyl compounds Cp*MMe 3 (M ) Ti, Zr, Hf) with the highly electrophilic borane B(C 6 F 5 ) 3 in methylene chloride in the presence of various arenes results in methyl carbanion abstraction and coordination of the arene to form complexes of the type [Cp*MMe 2 (η 6 -arene)][MeB(C 6 F 5 ) 3 ] (M ) Ti, Zr, Hf; arene ) benzene, toluene, m-and p-xylene, anisole, styrene, mesitylene). Indications of relative metal-arene affinities have been gleaned from a variety of low-temperature 1 H NMR experiments, and it seems that both steric and electronic factors play significant roles in determining stabilities. The crystal and molecular structures of [Cp*HfMe 2 (η 6 -toluene)][MeB(C 6 F 5 ) 3 ] have been determined; as anticipated, there are no significant contacts between anion and cation, and the latter assumes a bent-metallocene type of structure.

A kinetic study of hydrogen-deuterium (H/D) exchange in Pt(II)-1-methylimidazole complexes has been performed in D 2 O/NaOD solution, at 60°C, by means of 1 H NMR spectroscopy. Isotopic exchange has been observed at C(2)-H, C(4)-H, and C(5)-H of the imidazole moiety. Kinetic data analysis of H/D exchange in the complexes cis-[Pt(NH 3 ) 2 (MeIm) 2 ]Cl 2 (3), trans-[Pt(NH 3 ) 2 (MeIm) 2 ]Cl 2 (4), [Pt(en)(MeIm) 2 ]Cl 2 (5), and [Pt(MeIm) 4 ](ClO 4 ) 2 (6) revealed that Pt(II) enhances C(2)-H exchange in the coordinated 1-methylimidazole (MeIm) by ca. 10 2 to 10 3 , relative to the neutral substrate. However, it is ca. 10 4 -10 5 times less effective compared to H + and CH 3 + . In complex 5, C(5)-H exchange is ca. 6 times faster than C(4)-H exchange, in contrast with expectations based on an inductive/field effect by Pt(II). The observation of the relative reactivity order, C(5)-H exchange > C(4)-H exchange in 5, is discussed by consideration of contributing resonance structures of the intermediates formed upon abstraction of C(4)-H and C(5)-H, which would place the positive charge at the more favorable N(1)-CH 3 position. An X-ray crystal structure determination of 6 was also performed. The complex crystallizes in the triclinic space group P1 h, with a ) 8.219 (1) Å, b ) 9.424(1) Å, c ) 9.139(3) Å; R ) 107.68(2)°, ) 83.72(2)°, γ ) 114.87(1)°; and Z ) 1. The complex has a center of symmetry with the 1-methylimidazole ligands coordinated in a square-planar arrangement around the Pt(II) atom.

Reductive carbonylation of TaCl(5) in the presence of 1,2-bis(diphenylphosphino)ethane (dppe) under the appropriate conditions results in the formation of TaCl(CO)(2)(dppe)(2) (1), as the major product, and the possibly cyclic oligomer [TaCl(CO)(2)(dppe)(2)](2)(x)() (2, 2x >/= 4) as a minor product. Carbonylation of 1 (1 atm) results in the rapid but reversible formation of TaCl(CO)(4)(dppe) (3). Solutions of all three compounds exhibit low levels of paramagnetism, possibly attributable to thermal population of low-lying triplet excited states. Crystal data for the toluene solvate of 1, C(68)H(64)ClO(2)P(4)Ta: triclinic, P&onemacr; (No. 2), a = 13.937(12) Å, b = 14.811(7) Å, c = 14.929(9) Å, alpha = 102.30(5) degrees, beta = 95.60(7) degrees, gamma = 98.41(5) degrees, Z = 2.