Language of the Computer: Instruction Set and Architecture
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Alkenes are typically relatively weakly coordinating ligands. They are also extremely
important substrates for catalytic reactions. The strongest alkene-metal bonds occur
with
third row metals
(as with almost all ligands) and when one can get more
-
backbonding to occur.
The amount of
-backbonding depends strongly on how
electron-rich
the metal
center is and whether or not there are
electron-withdrawing groups
on the alkene to
make it a better acceptor ligand.
If the metal is electron-rich enough and/or if there are electron-withdrawing groups
on the alkene, one can actually get a formal
oxidation
of the metal via the transfer of
2e- to the alkene to form a dianionic
metallocyclopropane
ligand that is now
coordinated via two anionic alkyl
-bonds (thus the assignment of Pt(+2)).
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:
1)
Electron-withdrawing
groups on the alkene generally increase the strength of the
metal-alkene bonding, while
electron-donating
groups generally decrease the
stability.
Exception??
2)
In cases where
cis
-trans
isomerism is possible, the more stable complex is
almost always formed by the
cis
-alkene (steric factors).
3)
Metal complexes of ring-strained cycloalkenes (e.g., cyclopropene) display higher
than expected stability. The ring strain raises the energy of the cycloalkene ring
system making it a better donor to the metal center (better orbital energy
matching).
4)
Chelating dienes show the expected stabilization from the
chelate effect
. The
most common examples are
norbornadiene
and
cyclooctadiene
shown below.
5
)
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A triumph of the early days of organometallic chemistry was the successful synthesis
of (
4
-C
4
H
4
)
2
Ni
2
(
-Cl)
2
Cl
2
, a stable metal-coordinated cyclobutadiene molecule, by
Criegee
in 1959. This was actually predicted theoretically by
Longuet-Higgins
and
Orgel
in 1956 using an early form of molecular orbital theory.
A simpler route was discovered shortly after involving the cyclodimerization of
diphenyl acetylene by Fe(CO)
5
:
A
l
k
y
n
e
s
Note how the bridging alkyne is drawn. This indicates
a perpendicular bridging mode and that both carbons
are interacting equally with both metals (the alkyne is
donating 2e- to each metal). It dos NOT indicate that
each carbon has 6 bonds to it !!
Alkynes are essentially like alkenes, only with another perpendicular pair of
-
electrons. Thus they can act as neutral 2 or 4 e- donors, depending on the needs of
the metal center. They are also much better bridging ligands because of this second
set of
-electrons.
When alkynes bridge, they almost always do so perpendicular to the M-M axis, the
parallel bridging mode is known, but is quite rare:
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The computer's language is defined by its instruction set and architecture. This language, hidden from users, serves as the bridge between hardware and software. Explore the nuances of different instruction sets, such as RISC and CISC, and understand the role of instruction set architectures like MIPS in modern computing.
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Presentation Transcript
Alkenes/Alkynes C C Dewar-Chatt- Duncanson bonding model (1953) M M C C empty alkene -back donation via the -donation via the filled alkene -system -system Alkenes are typically relatively weakly coordinating ligands. They are also extremely important substrates for catalytic reactions. The strongest alkene-metal bonds occur with third row metals (as with almost all ligands) and when one can get more - backbonding to occur. The amount of -backbonding depends strongly on how electron-rich the metal center is and whether or not there are electron-withdrawing groups on the alkene to make it a better acceptor ligand.
NC H H HH NC PR3 PR3 Cl Pt Pt Pt Cl Cl PR3 PR3 H NC H H H NC Pt(2+) Pt(0) Pt(+2) C=C = 1.37 Zeiss's Salt C=C = 1.43 C--C = 1.49 metallocyclopropane If the metal is electron-rich enough and/or if there are electron-withdrawing groups on the alkene, one can actually get a formal oxidation of the metal via the transfer of 2e- to the alkene to form a dianionic metallocyclopropane ligand that is now coordinated via two anionic alkyl -bonds (thus the assignment of Pt(+2)).
F C=C = 1.40 Rh-C = 2.02 F The electron-withdrawing fluorine groups on the F2C=CF2 alkene makes it a better -acceptor ligand. This weakens the C=C bond, but strengthens the alkene-metal bond. F F Rh HH C=C = 1.35 Rh-C = 2.16 H H
1.46 1.46 1.45 1.40 Fe Zr 1.45 C C 1.36 O C O O Zr Zr is in a very low oxidation state (+2, but really wants to be +4) and is, therefore, extremely electron-rich. So electron-rich that it transfers two electrons to the butadiene via the - backdonation and generates a metallo- cyclopentene resonance structure
Electronic Effects C=C (cm 1) 1623 Ethylene Complex Free Ethylene [Ag(H2C=CH2)2]+ 1584 Fe(CO)4(H2C=CH2) [Re(CO)4(H2C=CH2)2]+ [CpFe(CO)2(H2C=CH2)]+ 1551 1539 1527 1525 Pd2Cl4(H2C=CH2)2 [PtCl3(H2C=CH2)] CpMn(CO)2(H2C=CH2) 1516 1508 1506 Pt2Cl4(H2C=CH2)2 1493 CpRh(H2C=CH2)2
The thermodynamic stability of metal-alkene complexes is strongly affected by the nature of the alkene (and metal): 1) Electron-withdrawing groups on the alkene generally increase the strength of the metal-alkene bonding, while electron-donating groups generally decrease the stability. Exception?? 2) In cases where cis-trans isomerism is possible, the more stable complex is almost always formed by the cis-alkene (steric factors). 3) Metal complexes of ring-strained cycloalkenes (e.g., cyclopropene) display higher than expected stability. The ring strain raises the energy of the cycloalkene ring system making it a better donor to the metal center (better orbital energy matching). 4) Chelating dienes show the expected stabilization from the chelate effect. The most common examples are norbornadiene and cyclooctadiene shown below. 5) Third-row metals form the strongest bonds and most stable complexes (as with most ligands). M M norbornadiene complex cyclooctadiene complex
Problem:To which of the following (each with a single open coordination site) will trifluoroethylene bond to the most strongly? Why? a) b) ? ? Me2 P CO OC CO W Cr P Me2 PMe2 OC CO CO Me2P c) F F O ? Ti F H
Cyclobutadiene A triumph of the early days of organometallic chemistry was the successful synthesis of ( 4-C4H4)2Ni2( -Cl)2Cl2, a stable metal-coordinated cyclobutadiene molecule, by Criegee in 1959. This was actually predicted theoretically by Longuet-Higgins and Orgel in 1956 using an early form of molecular orbital theory. Me Me Me Me Cl Me Me Cl + Ni(CO)4 Cl Ni Ni Cl Cl Cl Me Me Me Me Me Me A simpler route was discovered shortly after involving the cyclodimerization of diphenyl acetylene by Fe(CO)5: O O O C C C Fe + Fe(CO)5 Ph Ph 2 Ph Ph Ph Ph
Fe Fe Fe metal d orbitals non-bonding Fe cyclobutadiene , non-bonding, and * orbitals Fe Fe Fe Fe Fe
Alkynes Alkynes are essentially like alkenes, only with another perpendicular pair of - electrons. Thus they can act as neutral 2 or 4 e- donors, depending on the needs of the metal center. They are also much better bridging ligands because of this second set of -electrons. R R C R C R C C Note how the bridging alkyne is drawn. This indicates a perpendicular bridging mode and that both carbons are interacting equally with both metals (the alkyne is donating 2e- to each metal). It dos NOT indicate that each carbon has 6 bonds to it !! M M M When alkynes bridge, they almost always do so perpendicular to the M-M axis, the parallel bridging mode is known, but is quite rare:
Problem:The Cp2Rh2[ -(CF3CCCF3)](CO)(CN-R) complex shown below has a Rh-Rh bond distance of 2.67 , strongly indicating a covalent bond between the rhodium atoms. How would you electron count this complex to accommodate a Rh-Rh covalent bond?
Problem:Which of the following ligands will coordinate the most strongly to a generic metal center (not too electron-rich or deficient, with enough open coordination sites)? a) b) d) c)
