Chapter 15 Parallels Between Main Group and Organometallic Chemistry
15-4 Cluster compounds
15-3 Metal-metal bonds
15-2 The isolobal analogy
15-1 Main group parallels with binary carbonyl complexes
15-1 Main group parallels with binary carbonyl complexes
Consider several parallels between main group and organometallic compounds.
7 electrons 17 electronsElectronically equivalent
15-1 Main group parallels with binary carbonyl complexes
15-1 Main group parallels with binary carbonyl complexes
15-1 Main group parallels with binary carbonyl complexes
Tetrahedral tetramers
15-2 The isolobal analogy
Ronal Hoffmann in his 1981 Nobel lecture;
Hoffmann defined molecular fragments to be isolobal
if the number, symmetry properties, approximate energy and shape of the frontier orbitals and the number of electrons in them are similar-not identical, but similar.
15-2 The isolobal analogy
Orbitals of octahedral and tetrahedral fragments
15-2 The isolobal analogy
15-2 The isolobal analogy
Cyclic trimers
15-2 The isolobal analogy
The isolobal species Ir(CO)3, Co(CO)3, CR, and P
Structures resulting from combinations of Co(CO)3 and CR
15-2-1 Extensions of the analogy
1. The isolobal definition may be extended to isoelectronic fragments having the same coordination number.
2. Gain or loss of electrons from two isolobal fragments yields isolobal fragments.
Mn(CO)5 CH3, Re(CO)5 CH3[Fe(CO)5]+[Cr(CO)5]-
Mn(CO)5 CH3, Cr(CO)5 CH3+
Mo(CO)5W(CO)5
Fe(CO)5 CH3-
Ru(CO)5Os(CO)5
15-2-1 Extensions of the analogy
3. Other 2-electron donors are treated similarly to CO
4. Ligands η5-C5H5 and η6-C6H6 are considered to occupy three coordination sites and to be 6-electron donors
Mn(CO)5 Mn(PR3)5 [MnCl5]5- Mn(NCR)5 CH3
Mn(CO)5 CH3,
(η5-C5H5)Fe(CO)2
(η6-C6H6)Mn(CO)2
[Fe(CO)5]+ CH3,
15-2-1 Extensions of the analogy
5. Octahedral fragments of formula MLn (where M has a dx
configuration) are isolobal with square-planar fragments of formula MLn-2 (where M has a dx+2 configuration and L is a 2-electron donor).
Cr(CO)5d6
Fe(CO)4d8
[PtCl3]-d8
Pt(PR3)2d10
MLn MLn-2
Octahedral Fragments Square-planar Fragments
15-2-1 Extensions of the analogy5. Octahedral fragments of formula MLn (where M has a dx
configuration) are isolobal with square-planar fragments of formula MLn-2 (where M has a dx+2 configuration and L is a 2-electron donor).
Comparison of square-planar fragments with octahedral fragments
15-2-1 Extensions of the analogy
6 5 4 3 2
5 4 3
6 5 4 3
n-1
n-2n-3
15-2-2 Examples of applications of the analogy
The 5-electron fragment CH is isolobal with P and other Group 15 atoms.A potential application of this relationship is to seek phosphorous-containing analogues to organometallic complexes containing cyclic ligands such as C5H5 and C6H6
15-3 Metal-Metal Bonds
Single, double, triple, and quadruple bonds
We need to consider how metal atoms can bond to each other
15-3-1 Multiple Metal-Metal Bonds
Main group –highest possible bond order –--- 3 How about transition metal?
Bonding interactions between metal d orbitals
15-3-1 Multiple Metal-Metal Bonds
Relative energies of orbitals
Main group –highest possible bond order –--- 3 How about transition metal?
15-3-1 Multiple Metal-Metal Bonds
Re(III) – 4 d electrons
Occupied by ligand electrons
Main group –highest possible bond order –--- 3 How about transition metal?
15-3-1 Multiple Metal-Metal Bonds
Structures ?
staggered
eclipsed
Relative energies of orbitals
15-3-1 Multiple Metal-Metal Bonds
Bond order and electron count
15-3-1 Multiple Metal-Metal Bonds
The effect of population of δ and δ* orbitals on bond distances can sometimes be surprisingly small.
15-3-1 Multiple Metal-Metal Bonds
Quintuple bonds
Formal shortness ration = 0.774- Trans-bent geometry and apparent interactions
Influence of bridging ligandsNonlinearityMore complex interactions
Science, 2005 JACS, 2007 Angew. Chem. Int. Ed., 2008
Chromium(I) complexes with extremely short metal-metal bonds
15-4 Cluster compounds
Some of boron compounds exhibit similarities in their bonding and structures to transition metal clusters.
15-4-1 Boranes
13 bonding orbitals (= 2n + 1)
7 bonding orbitals (= n + 1) 6 bonding orbitals (= n) – boron-hydrogen
bonding11 nonbonding or antibonding orbitals
Each boron has four valence orbitals---- total 24 boron orbitals
6 of the hybrids
7 among remaining 6 of the hybrids and 12 of the unhybrides
s and 3p
15-4-1 Boranes
15-4-1 Boranes
Closo, nido, and arachno borane structure
Structures of closo, nido, and arachno boranes having 6 to 12 borons
15-4-1 Boranes
15-4-2 Heteroboranes
Isoelectronic species such as the carboranes (carbaboranes)CH+ unit is isoelectronic with BH.
15-4-2 Heteroboranes
Formally each C should be converted to BH
C2B8H10 B10H12
B10H12 - 2H+ = B10H102-
Therefore closo Many derivatives of boranes containing other main group atoms
15-4-3 Metallaboranes and metallacarboranes
Fe(CO)3 unit is isolobal with BH.
C should be converted to BH
Organometallic derivatives of B5H9
Orbitals of isolobal fragments BH and Fe(CO)3
15-4-3 Metallaboranes and metallacarboranes
15-4-3 Metallaboranes and metallacarboranes
As ligand
Carborane analogs of ferrocene.
Comparison of C2B9H112- with C5H5
-.
15-4-3 Metallaboranes and metallacarboranes
The approach used to describe bonding in boranes may also be applicable to bonding in carbonly clusters and other clusters.
Total # of valence electrons in cluster
# of electrons involved in framework bonding
# of electrons involved in metal-ligand bonding= +
Related to the classification of the structure as closo, nido……
Closo-B6H62-
26 valence electrons= 14 + 12
86 valence electrons= 14 + 72
Nine valence orbitalsFour valence orbitals
Five extra orbitals→ 10 more electrons per framework atom
15-4-4 Carbonly clusters
15-4-4 Carbonly clusters
n designates the number of framework atoms
15-4-4 Carbonly clusters
15-4-4 Carbonly clusters
Metal cores for clusters containing seven skeletal bond pairs.
Seven metal-metal framework bonding pairs are the most common
15-4-4 Carbonly clusters
The predictions of structures of transition metal-carbonly complexes, using Wade’s rules are often, but not always, accurate.
Co4(CO)12 60 valence electrons
nido
A trigonal bipyramid (the parent structure) with one position vacant
However, tetrahedral metal cores
15-4-4 Carbonly clusters
Ionic clusters of main group elements (Zintl ions).
15-4-4 Carbonly clusters
An extension of Wade’s rules has been described for electron counting
mno rule – states that for a closed cluster structure to be stable, there must be m + n + o skeletal electron pairs, where
m = # of condensed (linked) polyhedran = total # of verticeso = # of single-atom bridges between two polyhedra
A fourth term
p = # of missing vertices (e.g. p =1 for nido, p = 2 for arachno)
15-4-4 Carbonly clusters
B12H122-
m = # of condensed (linked) polyhedran = total # of verticeso = # of single-atom bridges between two polyhedrap = # of missing vertices (e.g. p =1 for nido, p = 2 for arachno)
m = 1n = 12o = 0p = 0
13 pairs
m = 2n = 23o = 1p = 0
26 pairs
15-4-5 Carbide clusters
Carbon exhibits unusual coordination numbers and geometries not found in classic organic structures
Carbide clusters
Encapsulated carbon contributes its 4 valence electrons.
86 electrons
15-4-5 Carbide clusters
The formation of four C-Ru bonding orbitals
The octahedral Ru6 core has framework bonding orbitals of the same symmetry as in B6H6
2-.
Bonding interactions between central carbon and octahedral Ru6.
15-4-5 Carbide clusters
Examples of large clusters.
Encapsulated iron
Homework
Exercise 15-1~15-9
Problem 1, 2, 5, 6, 11, 21, 23, 25.
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