金属有机化学

Bridging Alkyls and Related Ligands

Alkyls can bridge two metal centers, something that is well known from aluminum-alkyl chemistry. For example, consider the condensed phase structure of these Al-alkyls (see Oliver et. al. Organometallics 1982, 1, 1307):
H O
-p y A l M e 3
C p 2Z r C
R R
In this structure, the bridging Me group is nearly planar

Another type of unusual alkyl group involves agostic interactions in which part of the alkyl substituent coordinates to the metal in addition to the M-C bond.
M e2 C l
P Ti P Cl H
M e2 Cl
P Ti P Cl H
M e2 C l α - a g o s t ic in t e r a c t io n
M e2 Cl β - a g o s t ic in t e r a c t io n
Alkylidenes, Alkylidynes, and Carbides
ZnEt2, Grignard Reagents(1900) Pope and Peachey’s Me3PtI(1909)


No development during the 1920s through 1940s.
It was originally thought that such species were inherently unstable due to weak metal-carbon bonds. In fact, the issue is not their thermodynamic stability (M-C bond dissociation energies are typically 40-60 kcal/mol with 20-70 kcal/mol being a practical range), but their kinetic stability.

Beware of reactions where the hard-soft interactions are not so clear as these can represent equilibria instead of complete reactions

MCl + AlR3 M-R + AlR2Cl.
Alkylidynes: CR can bridge to three, or to two metals or
act as a terminal group with an M-C triple bond.
R
R
M
C M M M
CR
C M M
Carbides:
a carbon atom bridge four metals.
coworkers examined the above reaction using a stererochemical probe to show that this reaction proceeds with complete inversion of stereochemistry, consistent with an SN2 mechanism. See J. Am. Chem. Soc. 1974 , 96, 2814. However, the nature of the nucleophile is important, and under certain circumstances this can occur through the SET mechanism.
Metal-centered nucleophiles (i.e. using R+ as a reagent)
Typical examples are a metal anion and alkyl halide (or pseudohalogen). for example:
N aa F p + R X N Fp + RX
Lesson 2 Decomposition Modes of Metal Alkyls
Topic1 Empirical order of stabilities Topic2 β-elimination, α-elimination Topic3 Related σ bonded ligands
Lesson 3 Metal Hydrides
Topic1 The important role of the M Topic2 Characterization: Tpoic3 Synthesis H
Metal alkyls and aryls
Reagents:RMgX, the first source nucleophilic alkyl group (alcohols)
Chapter 3
Metal Alkyls, Aryls, Hydrides and Related σ-Bonded Ligands
Lesson 1 Metal Alkyls and Aryls
Topic1 Alkylidenes, Alkylidynes, and Carbides Topic2 Metalacycles Topic3 Synthesis of Metal Alkyl Complexes
CH2 CH 2 LnM CH 2 CH 2
L nM CH2
CH2
H 2C CH 2 CH2 L nM CH 2 L nM CH2 H 2C CH2 CH2

Aryl, vinyl and acyl ligands can form stronger M-C bond , since they have empty π* orbitals that can accept electron density from the metal. Such as pentahalophenyl ligands. Vinyl and acyl also have an alternative η2-bonding mode:
Oxidative Addition.
This requires a covalently unsaturated, low-valent complex (16 e- or less). A classic example:
(C O )3 Ru (C O )3 (C O )3 Ru Ru
C
Ru Ru (C O )3
R u (C O )3
(C O )3
H gO A c
C H gO A c AcO H g H gO A c
Metalacycles

Cyclic dialkyls are examples of metalacycles.
Synthesis of Metal Alkyl Complexes
Metathetical exchange using a carbon nucleophile (R-).
Common reagents are RLi, RMgX (or R2Mg), ZnR2,AlR3,BR3, and PbR4.
Structure and Bonding

Simple alkyls are simple sigma donors, that can be considered to donate one or two electrons to the metal center depending on which electron counting formalism you use.
ZnM e2
N b M e 2 C l3
Z n C l2

notice that some of the homoleptic alkyls are rather unstable. This is because they have low d-counts, are susceptible to alpha- and/or beta-hydride elimination, and lack good pidonating ligands to stabilize their high oxidation states:
This
spans a range from strong to very weak as strong nucleophiles can sometimes result in undesired reduction.
Much
of this alkylation chemistry can be understood with Pearson's "hard-soft" principles. Here are just a few examples of nucleophilic routes;
F p FR -+ N+ X a [ F p [ F C p ( C O )(2C e ] ) F e ] - p R a N X = p = Cp FO 2
Mபைடு நூலகம்n (C O )5
-
M eI
M n (C O )5