Isolobal Analogy 41i4j

Chapter 15 Parallels Between Main Group and Organometallic Chemistry

15-1 Main group parallels with binary carbonyl complexes 15-2 The isolobal analogy 15-3 Metal-metal bonds 15-4 Cluster compounds

15-1 Main group parallels with binary carbonyl complexes Consider several parallels between main group and organometallic compounds.

7 electrons

17 electrons

Electronically 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.

Mn(CO)5

CH3,

Re(CO)5 [Fe(CO)5]+ [Cr(CO)5]-

CH3

2. Gain or loss of electrons from two isolobal fragments yields isolobal fragments.

Mn(CO)5

CH3,

Cr(CO)5 Mo(CO)5 W(CO)5

CH3+

Fe(CO)5 Ru(CO)5 Os(CO)5

CH3-

15-2-1 Extensions of the analogy 3. Other 2-electron donors are treated similarly to CO

Mn(CO)5

Mn(PR3)5

[MnCl5]5-

Mn(NCR)5

4. Ligands η5-C5H5 and η6-C6H6 are considered to occupy three coordination sites and to be 6-electron donors

(η6-C6H6)Mn(CO)2

Mn(CO)5

(η5-C5H5)Fe(CO)2

[Fe(CO)5]+

CH3, CH3,

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 2electron donor). Octahedral Fragments

MLn

Square-planar Fragments

MLn-2

Cr(CO)5 d6

[PtCl3]d8

Fe(CO)4 d8

Pt(PR3)2 d10

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 2electron donor).

Comparison of square-planar fragments with octahedral fragments

15-2-1 Extensions of the analogy

6

5

n-1 n-2 n-3

5

4

4

3

6

5

3

2

4

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 phosphorouscontaining 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 Main group –highest possible bond order –--- 3 How about transition metal?

Relative energies of orbitals

15-3-1 Multiple Metal-Metal Bonds Main group –highest possible bond order –--- 3 How about transition metal?

Re(III) – 4 d electrons Occupied by ligand electrons

15-3-1 Multiple Metal-Metal Bonds

eclipsed

staggered

Structures ? 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 Chromium(I) complexes with extremely short metal-metal bonds

Formal shortness ration = 0.774 - Trans-bent geometry and apparent interactions Science, 2005

Influence of bridging ligands Nonlinearity More complex interactions JACS, 2007

Angew. Chem. Int. Ed., 2008

15-4 Cluster compounds Some of boron compounds exhibit similarities in their bonding and structures to transition metal clusters.

15-4-1 Boranes

Each boron has four valence orbitals ---- total 24 boron orbitals s and 3p

13 bonding orbitals (= 2n + 1) 7 bonding orbitals (= n + 1) 6 bonding orbitals (= n) – boron-hydrogen bonding 11 nonbonding or antibonding orbitals

6 of the hybrids 7 among remaining 6 of the hybrids and 12 of the unhybrides

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 - 2H+

B10H12 =

B10H102-

Therefore closo

Many derivatives of boranes containing other main group atoms

15-4-3 Metallaboranes and metallacarboranes C should be converted to BH Fe(CO)3 unit is isolobal with BH.

Orbitals of isolobal fragments BH and Fe(CO)3

Organometallic derivatives of B5H9

15-4-3 Metallaboranes and metallacarboranes

15-4-3 Metallaboranes and metallacarboranes

Comparison of C2B9H112- with C5H5-.

As ligand

Carborane analogs of ferrocene.

15-4-3 Metallaboranes and metallacarboranes

15-4-4 Carbonly clusters 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…… Four valence orbitals

Nine valence orbitals Five extra orbitals → 10 more electrons per framework atom

Closo-B6H6226 valence electrons = 14 + 12

86 valence electrons = 14 + 72

15-4-4 Carbonly clusters

n designates the number of framework atoms

15-4-4 Carbonly clusters

15-4-4 Carbonly clusters

Seven metal-metal framework bonding pairs are the most common Metal cores for clusters containing seven skeletal bond pairs.

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

A trigonal bipyramid (the parent structure) with one position vacant

However, tetrahedral metal cores

nido

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) polyhedra n = total # of vertices o = # 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 m = # of condensed (linked) polyhedra n = total # of vertices o = # of single-atom bridges between two polyhedra p = # of missing vertices (e.g. p =1 for nido, p = 2 for arachno)

B12H122m=1 n = 12 o=0 p=0

m=2 n = 23 o=1 p=0

13 pairs

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 octahedral Ru6 core has framework bonding orbitals of the same symmetry as in B6H62-. The formation of four C-Ru bonding orbitals

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.