Lecture 20 Dinuclear metal complexes with multiple M-M bonds 1) Transition metal-metal bonding: MO...
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Lecture 20 Dinuclear metal complexes with multiple M-M bonds 1) Transition metal-metal bonding: MO diagram • Compared to main group elements, transition metals (TM’s) have more options for bonding with one another. • In addition to one - and up to two -bonds common for organic compounds (acetylenes, cumulenes) and some other main group elements two more bonds of - symmetry are possible for TM’s (five bonds in total). • If the metal d x2-y2 orbitals are involved in -bonding with ligands, four metal atomic orbitals remain only available to form M-M bonds. • Species with TM-TM bond orders 1-4 are well known. • Multiple M-M bonds are most typical for such metals as V- Ta, Cr-W, Re, Ru, Os, Rh. M M d z2 d z2 z d xz ,d yz d xz ,d yz d xy ,d x2-y2 d xy ,d x2-y2 g g g g g g
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Transcript of Lecture 20 Dinuclear metal complexes with multiple M-M bonds 1) Transition metal-metal bonding: MO...
Lecture 20
Dinuclear metal complexes with multiple M-M bonds 1) Transition metal-metal bonding: MO diagram
• Compared to main group elements, transition metals (TM’s) have more options for bonding with one another.
• In addition to one - and up to two -bonds common for organic compounds (acetylenes, cumulenes) and some other main group elements two more bonds of -symmetry are possible for TM’s (five bonds in total).
• If the metal dx2-y2 orbitals are involved in -bonding with ligands, four metal atomic orbitals remain only available to form M-M bonds.
• Species with TM-TM bond orders 1-4 are well known.
• Multiple M-M bonds are most typical for such metals as V-Ta, Cr-W, Re, Ru, Os, Rh.
M M
dz2 dz2
z
dxz, dyz dxz, dyz
dxy, dx2-y2 dxy, dx2-y2
g
g
g
g
g g
2) Evidences for M-M double, triple and quadruple bonding
• One of the arguments in favor of multiple bonds is M-M bond length (Å):
• Look now at the solid state structures of salts derived from [Re2Me8]2- anion which shows the eclipsed conformation of the methyl groups.
• This unfavorable conformation is possible if stabilized by dxy-dxy -bonding between two rhenium atoms (l(ReRe)=2.18 Å).
• The dxy-dxy orbital overlap is impossible in the staggered conformation. • The barrier of rotation about M-M quadruple bond was estimated in a dinuclear
MoII porphyrin complex to be of 10 kcal/mol.
N
N N
NMo
R
R
R
R
R
R
R
R
N
N N
N
R
R
R
R
R
R
R
R
MoMe
Me
MeMe Me
Me
Me
Me
Re
Re
Me
Me Me
Me
MeMe
Me Me
2-Me
MeMe
Me
Me
Me
Me
Me
not
2.72
W W
MeO
OMe
MeO
MeO
MeO
OMe
ClCl
Cl Cl
l(W-W)=
W W
HOMe
OMe
MeO
2.48
MeO
OMe
ClCl
Cl Cl
MeOH
W W
MeO
OMeMeO
OMe
MeO
OMe
2.30
W
O
O OO O
O OW
O
2.21
3) More ways to form multiple M-M bonds
• The M/M bond order is a function of transition metal electron configuration. The Table below is valid for the case when metal dx2-y2 orbital is used for -bonding with ligands.
Cr
O
O O
O O
O OCr
O
l(Cr-Cr)=2.36
OH2H2O
d1 d2 d3 d4 d5 d6 d7 d8
1 2 3 4 3 2 1 0
• Some examples are given on the previous page and on the right. A very week quadruple Cr/Cr bond is believed to exist in Cr2(OAc)4(OH2)2 paddlewheel complex above.
• Another way to form M-M’ multiple bonds is through donor-acceptor mechanism.
• Ti center is roughly tetrahedral (Rh-Ti-O = 109o; sp3 hybridization) while RhP3 fragment is almost planar (d2s hybridization). Rh atom has two filled dxz and dyz orbitals suitable for formation of two -bonds with Ti atom. The latter uses the empty dxz and dyz orbitals.
l(Ti-Rh) = 2.21
Ti
O
OO
Rh
PPh2
PPh2
PPh2
Ti Rh
