# Phosphinidene

Phosphinidenes (IUPAC: phosphanylidenes, formerly phosphinediyls) are low-valent phosphorus compounds analogous to carbenes and nitrenes, having the general structure RP. The parent phosphinidine has the formula PH. More common are the organic analogues where R = alkyl or aryl. In these compounds phosphorus has only 6 electrons in its valence level. Most phosphinidenes are highly reactive and short-lived, thereby complicating empirical studies on their chemical properties.
A variety of strategies have been employed to stabilize phosphinidenes (e.g. π-donation, steric protection, transition metal complexation), Furthermore reagents and systems have been developed that can generate and transfer phosphinidenes as intermediates in the synthesis of various organophosphorus compounds.

## Electronic structure

Like carbenes, phosphinidenes can exist in either a singlet state or triplet state, with the triplet state typically being more stable. The stability of these states and their relative energy difference (the singlet-triplet energy gap) depends on the substituents. The ground state in the parent phosphinidene (PH) is a triplet that is 22 kcal/mol more stable than the lowest singlet state. This singlet-triplet energy gap is considerably larger than that of the simplest carbene methylene (9 kcal/mol).
Ab initio calculations from Nguyen et al. found that alkyl- and silyl-substituted phosphinidenes have triplet ground states, possibly in-part due to a negative hyperconjugation. Substituents containing lone pairs (e.g. -NX$_2$, -OX, -PX$_2$ ,-SX) stabilize the singlet state, presumably by π-donation into an empty phosphorus 3p orbital; in most of these cases, the energies of the lowest singlet and triplet states were close to degenerate. A singlet ground state could be induced in amino- and phosphino-phosphinidenes by introducing bulky β-substituents, which are thought to destabilize the triplet state by distorting the pyramidal geometry through increased nuclear repulsion.

## Case studies
### Dibenzo-7-phosphanorbornadiene derivatives

One way to generate phosphinidines employs the decyclization of phosphaanthracene complexes.
Treatment of a bulky phosphine chloride (RPCl$_2$) with magnesium anthracene affords a dibenzo-7-phosphanorbornadiene compound (RPA). Under thermal conditions, the RPA compound (R = NiPr$_2$) decomposes to yield anthracene; kinetic experiments found this decomposition to be first-order. It was hypothesized that the amino-phosphinidene iPr$_2$NP is formed as a transient intermediate species, and this was corroborated by an experiment where 1,3-cyclohexadiene was used as a trapping agent, forming anti-iPr$_2$NP(C$_6$H$_8$).
Molecular beam mass spectrometry has enabled the detection of the evolution of amino-phosphinidene fragments from a number of alkylamide derivatives (e.g. Me$_2$NP+ and Me$_2$NPH+ from Me$_2$NPA) in the gas-phase at elevated temperatures.

### Phosphino-phosphinidene

The first singlet phosphino-phosphinidene has been prepared using extremely bulky substituents. The authors prepared a chlorodiazaphospholidine with bulky (2,6-bis[(4-tert-butylphenyl)methyl]-4-methylphenyl) groups, and then synthesized the corresponding phosphaketene. Subsequent photolytic decarbonylation of the phosphaketene produced the phosphino-phosphinidene product as a yellow-orange solid that is stable at room temperature but decomposes immediately in the presence of air and moisture. $^{31}$P NMR spectroscopy shows assigned product peaks at 80.2 and -200.4 ppm, with a J-coupling constant of J$_{PP}$ = 883.7 Hz. The very high P-P coupling constant is indicative of P-P multiple bond character. The air/water sensitivity and high solubility of this compound prevented characterization by X-ray crystallography.
Density functional theory and Natural bond orbital (NBO) calculations were used to gain insight into the structure and bonding of these phosphino-phosphinidenes. DFT calculations at the M06-2X/Def2-SVP level of theory on the phospino-phosphinidene with bulky 2,6-bis[4-tert-butylphenyl)methyl]-4-methylphenyl groups suggest that the tri-coordinated phosphorus atom exists in a planar environment. Calculations at the M06-2X/def2-TZVPP//M06-2X/def2-SVP level of theory were applied to a simplified model compound with diisopropylphenyl (Dipp) groups so as to reduce the computational cost for detailed NBO analysis. Inspection of the outputted wavefunctions shows that the HOMO and HOMO-1 are P-P π-bonding orbitals and the LUMO is a P-P π*-antibonding orbital. Further evidence of multiple bond character between the phosphorus atoms was provided by natural resonance theory and a large Wiberg bond index (P$_1$-P$_2$: 2.34). Natural population analysis assigned a negative partial charge to the terminal phosphorus atom (-0.34 q) and a positive charge to the tri-coordinated phosphorus atom (1.16 q).
Despite the negative charge on the terminal phosphorus atom, subsequent studies have shown that this particular phosphinidene is electrophilic at the phosphinidene center. This phosphino-phosphinidene reacts with a number of nucleophiles (CO, isocyanides, carbenes, phosphines, etc.) to form phosphinidene-nucleophile adducts Upon nucleophilic addition, the tri-coordinated phosphorus atom becomes non-planar, and it is postulated that the driving force of the reaction is provided by the instability of the phosphinidene's planar geometry.

### Phospha-Wittig fragmentation

In 1989, Fritz et al. synthesized the phospha-Wittig species shown to the right. Phospha-Wittig compounds can be viewed as a phosphinidene stabilized by a phosphine. These compounds have been given the label of "phospha-Wittig" as they have two dominant resonance structures (a neutral form and a zwitterionic form) that are analogous to those of the phosphonium ylides that are used in the Wittig reaction.
Fritz et al. found that this particular phospha-Wittig reagent thermally decomposes at 20 °C to give $^t$Bu$_2$PBr, LiBr, and cyclophosphanes. The authors proposed that the singlet phosphino-phosphinidene $^t$Bu$_2$PP was formed as an intermediate in this reaction. Further evidence for this was provided by trapping experiments, where the thermal decomposition of the phospha-Wittig reagent in the presence of 3,4,-dimethyl-1,3-butadiene and cyclohexene gave rise to the products shown in the figure below.

### Metal complexes
#### Terminal phosphinidine complexes

Terminal transition-metal-complexed phosphinidenes L$_n$M=P-R are phosphorus analogs of transition metal carbene complexes. The first "metal-phosphinidine" was reported by Marinetti et al. They generated the transient species [(OC)$_5$M=P-Ph] by fragmentation of 7-phosphanorbornadiene molybdenum and tungsten complexes inside a mass spectrometer. Soon after, they discovered that these 7-phosphanorbornadiene complexes could be used to transfer the phosphinidene complex [(OC)$_5$M=P-R] to various unsaturated substrates.
Donor-stabilized terminal phosphinidene complexes are also known, which could release free phosphinidene complexes L$_n$M=P-R at mild conditions by P-donor dissociation reactions. The phosphinidene complexes decomposed to white phosphorus if no unsaturated substrates were provided.
Terminal phosphinidene complexes of the type Cp$_2$M=P-R (M = Mo, W) can be obtained by combining aryl-dichlorophosphines RPCl$_2$ with [Cp$_2$MHLi]$_4$.

#### Phosphinidine-based clusters

Metal clusters containing RP substituents are numerous. They typically arise by the reaction of metal carbonyls with primary phosphines (compounds with the formula RPH$_2$). A partucularly well-studied case is Fe$_3$(PC$_6$H$_5$)$_2$(CO)$_9$, which forms from iron pentacarbonyl and phenylphosphine according to the following idealized equation:
    3 Fe(CO)$_5$ + 2 C$_6$H$_5$PH$_2$ → Fe$_3$(PC$_6$H$_5$)$_2$(CO)$_9$ + 2 H$_2$ + 6 CO
A related example is the tert-butylphosphinidene complex (t-BuP)Fe$_3$(CO)$_{10}$.