Christin N. Carlson

Classical versus bridged allyl ligands in magnesium complexes: the role of solvent.

Magnesium allyl complexes are regularly isolated with classical, sigma-bonded ligands, and this has been thought to be their preferred mode of bonding. Density functional theory calculations confirm that such bonding is the most stable mode when coordinated bases are present, but in their absence, pi-bonded forms are expected to be lower in energy. The isolation of the unsolvated [Mg{C(3)(SiMe(3))(2)H(3)}(2)](2) complex supports this prediction, as it is a dinuclear species in which two allyl ligands bridge the metals and display cation-pi interactions with them.

Monomeric f-element chemistry with sterically encumbered allyl ligands

AbstractA new class of allyl-lanthanide salts of the type [K(thf) 4 ][(C 3 H 3 (SiMe 3 ) 2 ) 3 LnI] (Ln / Ce, Pr, Nd, Gd, Tb, Dy, Er) have beenprepared and isolated by reaction of three equivalents of the 1,3-bis(trimethylsilyl)allyl anion with LnI 3 . The neutral complex[C 3 H 3 (SiMe 3 ) 2 ] 3 Nd(thf) has been isolated from the reaction of the triflate complex Nd(O 3 SCF 3 ) 3 with three equivalents of the 1,3-bis(trimethylsilyl)allyl anion. These complexes have been structurally characterized using single crystal X-ray diffraction.# 2003 Elsevier B.V. All rights reserved. Keywords: Allyl; Lanthanide; Tetrahydrofuran; Complex; Crystal structure; Bulley ligands 1. IntroductionSeveral examples of homoleptic and pseudo-homo-leptic allyl-lanthanide complexes have recently beenprepared and shown to be catalytically active [1 / 10].For instance, Nd(allyl) 3 (solv) X has been employed as acatalyst for stereospecific butadiene polymerization [4].However, the allyl moiety undergoes insertion, becom-ing incorporated into the growing chain, and the fate ofthe catalyst thereafter is not well understood. Our hopeis to gain better control of the activity of thesecomplexes by protecting the allyl moiety using bulkyend groups that will hinder insertion. Extremely bulkycyclopentadienyl ligands have been used effectively toimprove the stability of base-free metallocenes of thedivalent lanthanides [11

Nature of bonding in complexes containing "supershort" metal-metal bonds. raman and theoretical study of M2(dmp)4 [M = Cr (natural abundance Cr, 50Cr, and 54Cr) and Mo; dmp = 2,6-dimethoxyphenyl].

We report an investigation of complexes of the type M(2)(dmp)(4) (M = Mo, Cr; dmp = 2,6-dimethoxyphenyl) using resonance Raman (RR) spectroscopy, Cr isotopic substitution, and density functional theory (DFT) calculations. Assignment of the Mo-Mo stretching vibration in the Mo(2) species is straightforward, as evidenced by a single resonance-enhanced band at 424 cm(-1), consistent with an essentially unmixed metal-metal stretch, and overtones of this vibration. On the other hand, the Cr(2) congener has no obvious metal-metal stretching mode near 650-700 cm(-1), where empirical predictions based on the Cr-Cr distance as well as DFT calculations suggest that this vibration should appear if unmixed. Instead, three bands are observed at 345, 363, and 387 cm(-1) that (a) have relative RR intensities that are sensitive to the Raman excitation frequency, (b) exhibit overtones and combinations in the RR spectra, and (c) shift in frequency upon isotopic substitution ((50)Cr and (54)Cr). DFT calculations are used to model the vibrational data for the Mo(2) and Cr(2) systems. Both the DFT results and empirical predictions are in good agreement with experimental observations in the Mo(2) complex, but both, while mutually consistent, differ radically from experiment in the Cr(2) complex. Our experimental and theoretical results, especially the Cr isotope shifts, clearly demonstrate that the potential energy of the Cr-Cr stretching coordinate is distributed among several normal modes having both Cr-Cr and Cr-ligand character. The general significance of these results in interpreting spectroscopic observations in terms of the nature of metal-metal multiple bonding is discussed.