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 Phosphorus Monoxide  is a compound with molecular formula PO. PO is a colorless gas theorized to be the precursor of most phosphorous-containing compounds found in solar systems, and it has recently been identified as a useful ligand for metal-centered catalysts.

Discovery and Early Use
Phosphorus Monoxide (PO) is believed to be the most abundant phosphorus-containing molecule found in interstellar clouds. Phosphorous was identified as a cosmically abundant element in 1998 after researchers found a cosmic ratio of Phosphorus to Hydrogen (P/H) of about 3x10-7. Even with the prevalence of phosphorus in interstellar clouds, very few phosphorus bearing molecules had been identified and found in very few sources, Phosphorus Nitride, PN, and the free radical CP were found in a carbon envelope of IRC +10215 in 1987. This reasoned that more phosphorus containing molecules had to be found in interstellar space. While examining the oxygen- rich shell of the supergiant star VY Canis Majoris (VY CMa) the presence of PO was detected. VY CMa was studied using the Submillimeter Telescope (SMT) of the Arizona Radio Observatory (ARO) and was able to observe the rotational frequencies of PO. ARO’s 10 m SMT was able to measure the rotational transitions of PO showing J=5.5→4.5 at 240 GHz and J=6.5→5.5 at 284 GHz toward the evolved star, each consisting of well-defined lambda-doublets. Since the detection of PO towards the envelope of the VY CMa supergiant in 2001, PO has been found in many more interstellar clouds and is found in abundance around oxygen-rich shells.

Structure and Properties
Phosphorus Monoxide is a colorless gas. It is a free radical with Phosphorus double bonded to Oxygen with Phosphorus having an unpaired valence electron. PO is a planar molecule with a molecular weight of 46.96 g/mol. The bond length of the PO double bond is 1.476 Å and free PO shows a consistent vibrational frequency of 1220 cm-1 around the stretching of the bond. The free radical nature of PO makes it highly reactive and unstable compared to other phosphorus oxides that have been further oxidized.

In space
Phosphorus Monoxide is made readily in space from oxygen-rich spheres, such as VY Canis Majoris. The winds that flow out from VY CMa have multiple different outflows for molecules to form, behaving similarly to cosmic rays. VY CMa shows three main types of outflow winds a spherical-type wind, a diffuse redshifted flow wind, and a collimated blueshifted expansion. These different outflow regions have chemical selectivity for different compounds. The line profiles of PO that were measured when discovered toward VY CMa indicate that PO is primarily synthesized in the spherical-type wind flow coming from the Oxygen-rich sphere. For PO to form the region needs to be very oxygen-rich since the spherical flow also forms H2O, CO, and SiO in abundance. If not enough free oxygen is available in the spherical flow then the phosphorus with react with nitrogen forming PN, this is why low-oxygen rich shells with phosphorus molecules show abundance of PN and very little PO. This delayed PO’s discovery for some time. After the PO is formed in the outflow it will condense and disperse out as dust grains. This allows the phosphorus to act as a refractory element in space.

On Earth
Due to its reactivity as a free radical, Phosphorus Monoxide is highly unstable and does not form naturally on Earth. A characteristic synthesis of PO as a complex ligand was performed within a nickel complex. The PO ligand is formed by first reacting diphospha-dinickela-tetrahedrane with [W(CO)5(THF)]. The reaction then creates a tetragonal-pyramidally structured complex called Ni2WP2- complex. Using bis(trimethylsilyl) peroxide, this complex is oxidized to form a complex with PO ligands.

Current Study
Phosphorus monoxide is being studied as a potentially useful π-acceptor ligand. While electronically similar to nitrogen monoxide (NO) and carbon monoxide (CO), PO has vastly different bonding properties (Synthesis, structure, bonding, and reactivity in clusters of the lower phosphorus oxides). PO is being studied as a potential replacement for NO and CO ligands since it is less rapidly displaced from metals than NO or CO. Synthesized PO complexes follow Brönsted acid-base chemistry, but not much else is yet known about their reactivity.