Chapter 15 Main Group/Organometallic Parallels (pp. 590-607) I. Main Group Parallels with Binary Carbonyl Complexes

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. 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) 1935 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. 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 0.733 smallest reported