1. Chapter 15 Main Group/Organometallic Parallels (pp. 590-607)
I. Main Group Parallels with Binary Carbonyl Complexes
Previously discussed organic/main group parallels
Benzene and Borazine
Alkanes vs. Silanes
a) Silanes = molecules with Si—Si bonds
Unstable because Si—Si (340 kJ/mol) weaker than C—C (368 kJ/mol)
Si less electronegative than H (1.90 vs. 2.20) so attacked by Nu
Si larger than C, which also makes it more easily attacked by Nu
Empty d-orbitals act as e-pair acceptors
b) Decomposition of Silanes

2. Parallels between Main Group and Binary Carbonyl Organometallic Species
Electron count in relation to filled valence shells
Chemistry can be explained by species achieving filled valence shells

3. 3) Group 5 Elements and 15 Electron Complexes: P vs. Ir(CO)3

4. II. The Isolobal Analogy
Limitations of parallels
No organometallic analogue for expanded valence shell of S, P, Cl, etc…
No analogues of IF7 or XeF4
Many organometallic complexes with ligands weaker than CO don’t follow the 18 electron rule, thus parallels to the main group fail
Loss of CO is much easier than loss of outer atoms from main group element
II. The Isolobal Analogy
Molecular Fragments are called “Isolobal” if the number, symmetry, energy, and shape of the frontier orbitals and the number of electrons in them are similar
Roald Hoffman, Nobel Prize 1982
Metal complexes can have similar reactivity/properties to main group molecules or fragments with which they are isolobal: Methane fragments vs Oh complex

6. Molecular Fragments can be combined in to molecules, even between groups
All of the compounds below are known and stable
These parallels aren’t always so perfect
H2C=CH2 is, of course, stable, but (CO)4Fe=F(CO)4 is not
Yet, both fragments form stable 3-membered rings, (but with extra CO’s)

7. Extending the Analogy
Charged species can be included
Mn(CO)5  CH3  [Fe(CO)5]+  [Cr(CO)5]-
Coordination number should be the same
Gain or loss of electrons from two isolobal fragments yields isolobal fragments
CH3  [Fe(CO)5]+  [Cr(CO)5]- (7e- or 17e- species)
CH3+  [Fe(CO)5]2+  [Cr(CO)5]o (6e- or 16e- species)
CH3-  [Fe(CO)5]o  [Cr(CO)5]2- (8e- or 18e- species)
Other ligands besides CO can be used
Mn(CO)5  CH3  Mn(PR3)5  [Mn(Cl)5]5-
Cp and Benzene ligands are taken to occupy 3 sites and be 6e- doners
Mn(CO)5  CH3  Mn(Cp)(CO)2  Mn(C6H6)(CO)2
Octahedral MLn fragments are isolobal with Square-Planar MLn-2 fragments
Cro, d6, 16e-
Lobe = LUMO
Pt2+, d8, 14e-
Lobe = LUMO
Pto, d10, 16e-
Lobes = non-bonding
Feo, d8, 18e-
Lobes = non-bonding

9. Applications of the Isolobal Analogy
Seek new molecules made from isolobal fragments
P analogue of Cp
5e- CH unit of Cp is isolobal to P atom
Combine 5 P atoms just like 5 CH units to produce a P5 ring like Cp
Linear Au versions of Mn(CO)5
Can combine these fragments similarly with CH3, each other, etc…

10. III. Metal-Metal Multiple Bonds (p. 601-607)
A. Parallel to Organic Bonds
1) Organic bonding: s, p orbitals
give 1 s, and 2 p bonds
2) Metals: s, p, d orbitals
give 1 s, 2 p, and 2 d
Discovery
1) discovery of very close
metal ions, closer than in solid,
pure metal (240 vs. 275 pm)
2) Cotton discovers M-M bonds
in 1963 crystal structures

11. Bonding
Quadruple bonding is routine in M-M interactions
d-orbital interactions with other d-orbitals are most important for M-M bonding

12. b) Ligand e- go into new bonding MO
Bonding in [Re2Cl8]2-
a) Re3+, d4 gives 8 d e-
b) Ligand e- go into new bonding MO
c) 8 d-electrons go into s, p, p, d
d) Result is BO = 4
e) d bond is week, but prevents rotation
f) d-d* distance matches visible light
[Re2Cl8]2- is blue
[Mo2Cl8]4- is red
g) Multiply bonded main group
elements are colorless (N2, CO)
because p-p* distance is UV
h) [Os2Cl8]2- has Os3+ d5, 10 d e-
d* populated, BO = 3, staggered
Cl8 group orbitals
Matching dx2-y2
symmetry
dx2-y2 used for bonding to ligand
This modifies the available M-M MO’s
Re2Cl82- d4 x 2 = 8 d e-, BO = 4 (d bond, eclipsed)
Os2Cl82- d5 x 2 = 10 d e-, BO = 3 (no d bond, staggered)
Cl8 group orbital interacts only with dx2-y2 group

13. Additional examples
Formal Shortness Ratio = multiple bond distance/single bond distance
a) Shows the effect of multiple bonds on bond distance

14. Quintuple Bonds
Cr-Cr and Mo-Mo complexes with quintuple (maximum) bonding
Cr-Cr Bond length of 174.0pm and formal shortening ratio of smallest reported