Quimica02-lantánidos.pdf

Solo disponible en BuenasTareas
  • Páginas : 40 (9879 palabras )
  • Descarga(s) : 0
  • Publicado : 9 de septiembre de 2012
Leer documento completo
Vista previa del texto
ACS Symposiu m Series 885, Activation and Fun ctionalization of C-H Bonds,
Karen I. Goldb erg and A lan S. Goldman, eds. 2004.

Chapter 7

Lanthanide complexes: electronic structure
and H-H, C-H and Si-H bond activation from a
DFT perspective
Lionel Perrin1, Laurent Maro n*2, Odile Eisens tein*1
1

LSDSMS (UMR 5636)
Universit é Montpellier 2, 34095 Montpellier C edex 5, Fra nce
2Laboratoire de Physique Quantique, (UMR 5626), IRSAMC,
Universit é Paul Sabatier,
118 Route de Narbonne, 31062 Toulouse C edex 4 , France

This paper discusses so me relationships between the electron ic
structure and th e reactivity of lan thanid e comp lex es. The
electronic structures of some represen tativ e lan than ide
complexes are d escr ibed. The 4 f electrons do not participate inthe Ln-X bond and a co mparison betw een lanthan ide and d0
transition metal co mplex es from Groups 3 and 4 highlights the
dominant ionic character in th e bonding to lan thanid e cen ter s.
However some covalent character cannot be excluded and th is
covalent character rationalizes the g eometry of th e comp lex es
such as th e non-planar structure of LaX3 (X = H, Me, F). The
consequencesof th e nature of th e bonding on the β Si- C
agostic interaction in La{CH( SiMe3)2}3 is presented . The
unusual O-bonding mode of CO to Cp*2Yb is briefly
summar ized. Th e reactivity of X2Ln-Z (X = Cp, H, Cl,
effectiv e group potential; Z = H , Me) with simple mo lecules
like H2, CH4, and SiH4 is compar ed. It is shown that th e stro ng
ionic char acter of the lanthan ide bonding is key torationalizing the selectivity of the σ-bond metathesis with
alkan e and to the lack of it in the case of silan e. In par ticular,
the position β to the metal center in the diamond shape 4 c-4etransition state is not allowed for a methy l group (transition
state of very high energy) whereas it is permitted to a silyl
group. This is shown to be related to the relativ e ab ility of
carbon andsilicon to be hyperv alent.

© 2004 American Ch emical Society

116

117

The activation of in ert bonds has long been a k ey concern of organometallic
chemists. Ox idative addition has been recognized as an eff icient route for in ert
bond activation and considerab le effort has been spen t analyzing the reactiv ity
with a wid e variety of transition metal frag men ts in wh ich themetal can b e
oxidized.(1) Sigma-bond metathesis, which occurs without change of oxidation
state at the metal cen ter, has also b een used especially in relation to efficient
catalysts for polymer ization.(2) These two types of reactions have both attracted
the atten tion of compu tational chemists.(3) It is not our intention to pr esen t an
overview of this very rich f ield. W e focus on thetheoretical approach of th e
reactiv ity of lanth anocen e der ivativ es w ith H2, CH4 and SiH4 .
Bond activation by lan thanid e co mplexes h as attracted less attention desp ite
the k ey discovery by Watson of CH4 activation by Cp*2 Lu-CH3 ( Cp* = η5C5Me5).(4) Organometallic chemistry of lanth anide co mplex es is the concern of
a relatively small number of groups in the world desp ite th ewell known ability
of lanthan ide co mplexes to be involved in important catalytic processes
includ ing polymerization.(2,5) In addition, on ly a f ew members of the ser ies of
lanth anid e elements have b een exper imentally stud ied b ecau se of stability
problems.
Computational studies of lan than ide sp ecies are still f ew (6-37) (a noncomprehensive list is giv en) in co mparison to theincr easing flow of calcu lations
on d transition metals. Only recent reviews on th e theo retical treatmen t of
reactiv ity are cited here.(3) A number of studies h ave b een carried out on
relatively small systems like th e halide co mplex es.(14-17) Some stud ies on 4f
elemen t comp lex es have been focused on the coordination of lan thanid e
ions(10,11) in connection to ex traction and...
tracking img