In chemistry the polyhedral skeletal electron pair theory provides electron counting rules useful for predicting the structures of clusters such as borane and carborane clusters. The electron counting rules were originally formulated by Kenneth Wade^{[1]} and were further developed by D. M. P. Mingos^{[2]} and others; they are sometimes known as Wade's rules or the Wade/Mingos rules.^{[3]} The rules are based on a molecular orbital treatment of the bonding.^{[4]}^{[5]}^{[6]}^{[7]} These rules have been extended and unified in the form of the Jemmis mno rules.^{[8]}^{[9]}
Predicting structures of cluster compounds
Different rules (4n, 5n, or 6n) are invoked depending on the number of electrons per vertex.
The 4n rules are reasonably accurate in predicting the structures of clusters having about 4 electrons per vertex, as is the case for many boranes and carboranes. For such clusters, the structures are based on deltahedra, which are polyhedra in which every face is triangular. The 4n clusters are classified as closo, nido, arachno or hypho, based on whether they represent a complete (closo) deltahedron, or a deltahedron that is missing one (nido), two (arachno) or three (hypho) vertices.
However, hypho clusters are relatively uncommon due to the fact that the electron count is high enough to start to fill antibonding orbitals and destabilize the 4n structure. If the electron count is close to 5 electrons per vertex, the structure often changes to one governed by the 5n rules, which are based on 3connected polyhedra.
As the electron count increases further, the structures of clusters with 5n electron counts become unstable, so the 6n rules can be implemented. The 6n clusters have structures that are based on rings.
A molecular orbital treatment can be used to rationalize the bonding of cluster compounds of the 4n, 5n, and 6n types.
The structure of the
butterfly cluster [Re
_{4}(CO)
_{12}]
^{2} conforms to the predictions of PSEPT.
4n rules
The following polyhedra are closo polyhedra, and are the basis for the 4n rules; each of these have triangular faces.^{[10]} The number of vertices in the cluster determines what polyhedron the structure is based on.
Using the electron count, the predicted structure can be found. n is the number of vertices in the cluster. The 4n rules are enumerated in the following table.
Electron count

Name

Predicted structure

4n − 2

Bicapped closo

n − 2 vertex closo polyhedron with 2 capped (augmented) faces

4n

Capped closo

n − 1 vertex closo polyhedron with 1 face capped

4n + 2

Closo

Closo polyhedron with n vertices

4n + 4

Nido

n + 1 vertex closo polyhedron with 1 missing vertex

4n + 6

Arachno

n + 2 vertex closo polyhedron with 2 missing vertices

4n + 8

Hypho

n + 3 vertex closo polyhderon with 3 missing vertices

Pb_{10}^{2−}
When counting electrons for each cluster, the number of valence electrons is enumerated. For each transition metal present, 10 electrons are subtracted from the total electron count. For example, in Rh_{6}(CO)_{16} the total number of electrons would be 6(9) + 16(2)  6(10) = 86 – 6(10) = 26. Therefore, the cluster is a closo polyhedron because n = 6, with 4n + 2 = 26.
S_{4}^{2+}
Other rules may be considered when predicting the structure of clusters:

For clusters consisting mostly of transition metals, any main group elements present are often best counted as ligands or interstitial atoms, rather than vertices.

Larger and more electropositive atoms tend to occupy vertices of high connectivity and smaller more electronegative atoms tend to occupy vertices of low connectivity.

In the special case of boron hydride clusters, each boron connected to 3 or more vertices has one terminal hydride, while a boron connected to 2 other vertices has 2 terminal hydrogens. If more hydrogens are present, they are placed in open face positions to even out the coordination number of the vertices.

For the special case of transition metal clusters, ligands are added to the metal centers to give the metals reasonable coordination numbers, and if any hydrogen atoms are present they are placed in bridging positions to even out the coordination numbers of the vertices.
In general, closo structures with n vertices are nvertex polyhedra.
To predict the structure of a nido cluster, the closo cluster with n + 1 vertices is used as a starting point; if the cluster is composed of small atoms a high connectivity vertex is removed, while if the cluster is composed of large atoms a low connectivity vertex is removed.
To predict the structure of an arachno cluster, the closo polyhedron with n + 2 vertices is used as the starting point, and the n+1 vertex nido complex is generated by following the rule above; a second vertex adjacent to the first is removed if the cluster is composed of mostly small atoms, a second vertex not adjacent to the first is removed if the cluster is composed mostly of large atoms.
Os_{6}(CO)_{18}, carbonyls omitted
Example: Pb_{10}^{2−}

Electron count: 10(Pb) + 2 (for the negative charge) = 10(4) + 2 = 42 electrons.

Since n = 10, 4n + 2 = 42, so the cluster is a closo bicapped square antiprism.
Example: S_{4}^{2+}

Electron count: 4(S) – 2 (for the positive charge) = 4(6) – 2 = 22 electrons.

Since n = 4, 4n + 6 = 22, so the cluster is arachno.

Starting from an octahedron, a vertex of high connectivity is removed, and then a nonadjacent vertex is removed.
Example: Os_{6}(CO)_{18}

Electron count: 6(Os) + 18(CO) – 60 (for 6 osmium atoms) = 6(8) + 18(2) – 60 = 24

Since n = 6, 4n = 24, so the cluster is capped closo.

Starting from a trigonal bipyramid, a face is capped. The carbonyls have been omitted for clarity.
B_{5}H_{5}^{4−}, hydrogens omitted
Example:^{[11]} B_{5}H_{5}^{4−}

Electron count: 5(B) + 5(H) + 4 (for the negative charge) = 5(3) + 5(1) + 4 = 24

Since n = 5, 4n + 4 = 24, so the cluster is nido.

Starting from an octahedron, one of the vertices is removed.
The rules are useful in also predicting the structure of carboranes. Example: C_{2}B_{7}H_{13}

Electron count = 2(C) + 7(B) + 13(H) = 2(4) + 3(7) + 13(1) = 42

Since n in this case is 9, 4n + 6 = 42, the cluster is arachno
The bookkeeping for deltahedral clusters is sometimes carried out by counting skeletal electrons instead of the total number of electrons. The skeletal orbital (electron pair) and skeletal electron counts for the four types of deltahedral clusters are:

nvertex closo: (n+1) skeletal orbitals, (2n+2) skeletal electrons

nvertex nido: (n+2) skeletal orbitals, (2n+4) skeletal electrons

nvertex arachno: (n+3) skeletal orbitals, (2n+6) skeletal electrons

nvertex hypho: (n+4) skeletal orbitals, (2n+8) skeletal electrons
The skeletal electron counts are determined by summing the total of the following number of electrons:

2 from each BH unit

3 from each CH unit

1 from each additional hydrogen atom (over and above the ones on the BH and CH units)

the anionic charge electrons
5n rules
As discussed previously, the 4n rule mainly deals with clusters with electron counts of 4n+k, in which approximately 4 electrons are on each vertex. As more electrons are added per vertex, the number of the electrons per vertex approaches 5. Rather than adopting structures based on deltahedra, the 5ntype clusters have structures based on a different series of polyhedra known as the 3connected polyhedra, in which each vertex is connected to 3 other vertices. The 3connected polyhedra are the duals of the deltahedra. The common types of 3connected polyhedra are listed below.
5n cluster: P_{4}
5n+3 cluster: P_{4}S_{3}
5n+6 cluster: P_{4}O_{6}
The 5n rules are as follows.
Total electron count

Predicted structure

5n

nvertex 3connected polyhedron

5n+1

n–1 vertex 3connected polyhedron with one vertex inserted into an edge

5n+2

n–2 vertex 3connected polyhedron with two vertices inserted into edges

5n+k

nk vertex 3connected polyhedron with k vertices inserted into edges

Example: P_{4}

Electron count: 4(P)= 4(5)= 20

It is 5n structure with n=4, so it is tetrahedral
Example: P_{4}S_{3}

Electron count 4(P)+ 3(S)= 4(5)+3(6)= 38

It is 5n+3 structure with n=7. Three vertices are inserted into edges
Example: P_{4}O_{6}

Electron count 4(P)+ 6(O)= 4(5)+ 6(6)= 56

It is 5n+6 structure with n=10. Six vertices are inserted into edges
6n rules
As more electrons are added to a 5n cluster, the number of electrons per vertex approaches 6. Instead of adopting structures based on 4n or 5n rules, the clusters tend to have structures governed by the 6n rules, which are based on rings. The rules for the 6n structures are as follows.
S_{8} crown
Total electron count

Predicted structure

6n–k

nmembered ring with k/2 transannular bonds

6n–4

nmembered ring with 2 transannular bonds

6n–2

nmembered ring with 1 transannular bond

6n

nmembered ring

6n+2

nmembered chain (nmembered ring with 1 broken bond)

Example: S_{8}

Electron count = 8(S) = 8(6) = 48 electrons.

Since n = 8, 6n = 48, so the cluster is an 8 membered ring.
6n+2 cluster: hexane
Hexane (C_{6}H_{14})

Electron count = 6(C)+ 14(H) = 6(4) + 14(1) = 38

Since n=6, 6n=36 and 6n+2=38 so the cluster is a 6 membered chain.
Isolobal vertex units
Provided a vertex unit is isolobal with BH then it can, in principle at least, be substituted for a BH unit, even though that BH and CH are not isoelectronic. The CH^{+} unit is isolobal, hence the reason why the rules are applicable to carboranes.
This can be explained due to a frontier orbital treatment.^{[10]} Additionally there are isolobal transition metal units. For example Fe(CO)_{3} provides 2 electrons. The derivation of this is briefly as follows:

Fe has 8 valence electrons.

Each carbonyl group is a net 2 electron donor after the internal σ and π bonding are taken into account making 14 electrons.

3 pairs are considered to be involved in Fe – CO σbonding and 3 pairs are involved in π back bonding from Fe to CO reducing the 14 to 2.
Bonding in cluster compounds
Polyhedron
B_{2}H_{6}
Molecularorbital (MO) diagram of B_{2}H_{6}. Atoms and their corresponding orbitals are colored the same. Green MOs symbolize bonding, while red symbolize antibonding.
The bonding in diborane is best described by treating each B as sp
^{3} hybridized. Two sp
^{3} hybrid orbitals on each boron form the bonds to the terminal hydrogens. The remaining sp
^{3} orbitals create the bonds with the bridging hydrogens. Because the angles in the diborane structure are not tetrahedral the orbitals also likely contain some sp
^{2} character.
ClosoB_{6}H_{6}^{2−}
MO diagram of B_{6}H_{6}^{2−} showing the orbitals responsible for forming the cluster. Pictorial representations of the orbitals are shown; the MO sets of T and E symmetry will each have two or one additional pictorial representation, respectively, that are not shown here.
The boron atoms lie on each vertex of the octahedron and are sp hybridized.
^{[11]} One sp hybrid radiates away from the structure forming the bond with the hydrogen atom. The other sp hybrid radiates into the center of the structure forming a large bonding molecular orbital at the center of the cluster. The remaining two unhybridized orbitals lie along the tangent of the sphere like structure creating more bonding and antibonding orbitals between the boron vertices.
^{[8]}
The orbital diagram breaks down as follows
The 18 framework molecular orbitals, (MOs), derived from the 18 boron atomic orbitals are:

1 bonding MO at the center of the cluster and 5 antibonding MOs from the 6 sp radial hybrid orbitals

6 bonding MOs and 6 antibonding MOs from the 12 tangential p orbitals.
The total skeletal bonding orbitals is therefore 7, i.e. (n+1).
Main group atom clusters The bonding in other main group cluster compounds follow similar rules as those described for the boron cluster bonding. The atoms at the vertex hybridize in a way which allows the lowest energy structure to form.
The 18 framework molecular orbitals, (MOs), derived from the 18 boron atomic orbitals are:

1 bonding MO at the center of the cluster and 5 antibonding MOs from the 6 sp radial hybrid orbitals

6 bonding MOs and 6 antibonding MOs from the 12 tangential p orbitals.
The total skeletal bonding orbitals is therefore 7, i.e. (n+1).
Transition metal clusters
Transition metal clusters use the d orbitals for bonding so have up to nine bonding orbitals, instead of only the four present in boron and main group clusters.^{[12]} There is also more bonding flexibility in transition metal clusters depending on whether vertex metal electron pairs are involved in cluster bonding or appear as lone pairs. The cluster chlorides and carbonyls of transition metals will be briefly discussed here as they represent opposite ends of the spectrochemical series and show important features of the differences between transition metal clusters with different ligands.^{[13]} In chloride clusters the energy splitting of the valence d orbitals increases upon formation of the cluster. The number and symmetry of these orbitals are dependent upon the type and structure of each individual cluster complex.^{[13]} Conversely in the carbonyl clusters the energy splitting of the valence d orbitals is greater before the formation of the cluster.^{[13]}

MO diagram clusters metal chlorides and metal carbonyls

General MO diagram of metal chloride structures. Green MOs represent bonding orbitals while red represent antibonding orbitals. The labeling on the MOs is as follows: ssigma, ppi, ddelta bonding, with * denoting antibonding interactions.

General MO diagram for metal carbonyl clusters. Green MOs represent bonding orbitals while red represent antibonding orbitals. The labeling on the MOs is as follows:ssigma, ppi, ddelta bonding with * denoting antibonding interactions.
References

^ The structural significance of the number of skeletal bonding electronpairs in carboranes, the higher boranes and borane anions, and various transitionmetal carbonyl cluster compounds K. Wade J. Chem. Soc. D, 1971, 792793 doi:10.1039/C29710000792

^ A General Theory for Cluster and Ring Compounds of the Main Group and Transition Elements D. M. P. MINGOS Nature Physical Science 236, 99102 doi:10.1038/physci236099a0

^ The significance and impact of Wade's rules Alan J. Welch Chem. Commun., 2013,49, 36153616 doi:10.1039/C3CC00069A

^

^ These notes contained original material that served as the basis of the sections on the 4n, 5n, and 6n rules.

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^ ^{a} ^{b} ^{c}
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