Hexaphosphabenzene

Hexaphosphabenzene is a valence isoelectronic analogue of benzene and is expected to have a similar planar structure due to resonance stabilization and its sp2 nature.

Preliminary ab initio calculations on the trimerisation of P2 leading to the formation of the cyclic P6 were performed, and it was predicted that hexaphosphabenzene would decompose to free P2 with an energy barrier of 13−15.4 kcal mol−1,[1] and would therefore not be observed in the uncomplexed state under normal experimental conditions.

The presence of an added solvent, such as ethanol, might lead to the formation of intermolecular hydrogen bonds which may block the destabilizing interaction between phosphorus lone pairs and consequently stabilize P6.

Isolation of hexaphosphabenzene was first achieved within a triple-decker sandwich complex in 1985 by Scherer et al. Amber coloured, air-stable crystals of [{(η5-Me5C5)Mo}2(μ,η6-P6)] are formed by reaction of [CpMo(CO)2/3]2 with excess P4 in dimethylbenzene, albeit with a yield of approximately 1%.

This was achieved by increasing the reaction temperature of the thermolysis of [CpMo(CO)2/3]2 with P4 to approximately 205 °C in boiling diisopropylbenzene, thus favouring the formation of [{(η5-Me5C5)Mo}2(μ,η6-P6)] as the thermodynamic product.

These include P6 triple‐decker complexes for Ti, V, Nb, and W, whereby the synthetic method is still based on the originally reported thermolysis of [CpM(CO)2/3]2 with P4.

In most triple-decker complexes with an electron count ranging from 26 to 34, the structure of the middle ring is planar ([{(η5-Cp)M}2(μ,η6-P6)] with M = Mo, Sc, Y, Zr, Hf, V, Nb, Ta, Cr, and W).

[7] Calculations have concluded that completely filled 2a*and 2b* orbitals in 28 valence electron complexes lead to a planar symmetrical P6 middle ring.

In 26 valence electron complexes, the occupancy of either 2a*or 2b* results in in-plane or bisallylic distortions and an asymmetric planar middle ring.

The magnetic moment of the dark teal crystals determined by the Evans NMR method is equal to 1.67 μB, which is consistent with one unpaired electron.

Density functional theorem (DFT) calculations confirm that this distortion is due to depopulation of the P bonding orbitals upon oxidation of the triple-decker sandwich complex.

[31] Adaptive Natural Density Partitioning (AdNDP) is a theoretical tool developed by Alexander Boldyrev that is based on the concept of the electron pair as the main element of chemical bonding models.

Both the D6h benzene-like structure, as well as the D2 isomer of P6 is similar to the reported AdNDP bonding pattern of the C6H6 benzene molecule:[32] 2c–2e σ-bond and lone pairs, as well as delocalized 6c-2e π-bonds.

Upon sandwich complex formation the PJT effect is suppressed due to filling of the unoccupied molecular orbitals involved in vibronic coupling in P6 with electron pairs of Mo atoms.

Structure of [{(η 5 - Me 5 C 5 ) Mo } 2 (μ,η 6 -P 6 )
The dominant MOs responsible for ligand metal interactions in the triple-decker sandwich complexes, imposed on a qualitative energy diagram for [{(η 5 -Cp)Mo} 2 (μ,η 6 -P 6 )]
Geometry of the middle P 6 ring in triple-decker sandwich complexes with 28, 26, and 24 valence electron counts
Bisallylic distorted P 6 ligand within the molecular structure of the [[{(η 5 - Me 5 C 5 )Mo} 2 (μ,η 6 -P 6 )]] + cation
Reactivity of [{(η 5 - Me 5 C 5 )Mo} 2 (μ,η 6 -P 6 )] towards the cations Cu + , Ag + , and Tl +
Crystal packing of a) [Ag([{(η 5 - Me 5 C 5 )Mo} 2 (μ,η 6 -P 6 )]) 2 ] + and b) [Tl([{(η 5 - Me 5 C 5 )Mo} 2 (μ,η 6 -P 6 )]) 2 ] + showing the alternation of anionic and cationic layers along the a axis. Tl + positions are half‐occupied.
Representative structures of P 6 . Included are point group symmetries and relative energies.
Interaction of the pairs of occupied and unoccupied molecular orbitals of P 6 responsible for the distortion of the planar D 6h structure toward the distorted D 2 structure
Chemical bonding picture of g). AdNDP analysis performed by Galeev and Boldyrev.
Suppression of the pseudo Jahn–Teller effect in P 6 upon complexation in a sandwich compound
Correspondence of unoccupied molecular orbitals of P 6 to those of [{(η 5 - Me 5 C 5 )Mo} 2 (μ,η 6 -P 6 )]. Occupation in the latter results in suppression of the PJT effect .