ca01 only detailed ToC 1..24

2.6.7 Complexes with Doubly Bonded Heteroelement Ligands 125
For this reason, bridged dinuclear compounds are also included in this section, although
emphasis is placed on the terminally bonded derivatives. The mononuclear structure is
favored by a sterically encumbering coordination sphere, whereas electronic configurations
that allow the formation of metal-metal bonds lead preferentially to dinuclear
structures. First-row heteroelement-containing ligands (oxo, imido) are found terminally
bonded more frequently than their heavier congeners because of their superior ð-bonding
ability.
As seen for the triply bonded heteroelement derivatives, the most common synthetic
method for the present subclass consists of the introduction of the organic fragment into
an inorganic substrate that already contains the desired doubly bonded heteroelement ligand.
This is especially true for the oxo compounds, as metal oxides or oxometalate precursors
are readily available and inexpensive starting materials. The methods discussed
in this section are those leading to the assembly of the metal-heteroatom double bond
starting from substrates that already contain hydrocarbyl ligands.
Scheme 37 Monomer–Dimer Dichotomy for Doubly Bonded Heteroelement Ligands
2 [M] E [M]
E
E
[M]
Synthesis of Product Subclass 7
2.6.7.1 Method 1:
From Complexes Containing Singly Bonded Heteroelement Ligands
Singly bonded heteroelement ligands that contain a hydrogen substituent may be deprotonated
by either an internal or an external base and transformed into doubly bonded ligands.
In many cases the singly bonded hydrogen-bearing ligand is formed in situ by
ligand exchange from a halide or alkoxide precursor. This is the case for the reaction between
dichlorobis(ç 5 -pentamethylcyclopentadienyl)tungsten(IV) and potassium hydroxide,
leading to the oxo derivative 83 by spontaneous elimination of water; see Scheme
38. [102] In this case the proton scavenger is a coordinated base (OH) and the conjugate
acid is expelled. In high oxidation state systems, halides may be sufficiently good bases
leading to the expulsion of the hydrogen halide, as in the hydrolysis of tetrabromo(ç 5 -cyclopentadienyl)molybdenum(V).
[151] An intramolecular hydrogen transfer to a carbyne ligand
furnishes the oxo–alkyl derivative 66 (Scheme 27).
For the synthesis of 84, aminolysis of tungsten-methyl bonds yields an imido and an
amido ligand in a first step. An external base, however, is necessary to produce the second
imido ligand, as neither the residual methyl ligand nor excess aniline is sufficiently basic
to carry out the last deprotonation. [152] Imido derivatives have also been obtained from trimethylsilylamido
derivatives, the elimination of the trimethylsilyl group (a proton equivalent)
being favored by the presence of chloro, alkoxo, or oxo ligands. An external base
may also serve as a catalyst for the intramolecular proton transfer to another ligand, as
shown in the triethylamine-catalyzed isomerization of the amido–carbyne complexes 20
to the imido–carbene complexes 21 (Scheme 7). [8,17,29]
A reverse strategy involves rearrangement from a precursor complex that contains
the proton on the metal center and the base on the heteroatom ligand, as in the synthesis
of the phosphinidene complex 85. [153] Ligand exchange from halide precursors with lithium
disulfide has provided access to sulfido derivatives, occasionally involving metal oxidation,
as in the formation of (ç 5 -pentamethylcyclopentadienyl)trisulfidotungstate(VI)
86. [154] The same transformation can also be performed, although in lower yield, using hydrogen
sulfide in the presence of triethylamine. [155]
for references see p 135