why PH5 does not exist? why molecules are so unstable? Pls. give in detail info...?

I think you mean the chemical formula PH5, not pH5.

Its not anything to do with the octet rule, since phosphorus can hold more than 8 electrons. For example, both PCl3 and PCl5 are known.

Infact, PH5 does exist, it is called phosphoran. However, its unstable and bursts into flames on contact with air. So the only place youre likely to find some is in a university lab, under an argon atmosphere.

When I was studying for my PhD, I used phosphoran as a precurser (intermediate) in the synthesis of certain organometallic catalysts. Scary stuff.

To the answerer who requested a citation:

Here is a citation to some ab initio calculations carried out way back in 1972. There are plenty of others involving use of this compound, but I felt that a JACS paper would be most convincing.

"Electronic structure of PH5 and intramolecular ligand exchange in phosphoranes. Model studies"

Arvi Rauk, Leland C. Allen, Kurt Mislow

J. Am. Chem. Soc., 1972, 94 (9), pp 3035–3040

I note that Greenwood (2nd ed) states that PH5 has neither been isolated nor detected. The 'numerous attempts' they mention were not directly cited. The most recent citation in the phosphoran citation section of the book is dated 1987, nearly a quarter of a Century ago!

So much for text books.

Consider PH3 + H2 → PH5

there is the energy needed to promote an electron to a 3d orbital necessary for five sp3d AOs on P to form 5 σ bonds. There is the energy gained by forming two extra P-H bonds and although the energy of P-H bonds are slightly lower than for P-Cl bonds the difference is negligible (P-H = ~322 kJ mol-1; P-Cl = ~331 kJ mol-1); as we know PCl5 exists. What we are forgetting is the energy needed to break the H-H bond that is huge (~435 kJ mol-1: one of the strongest single bonds known). On the other hand, the F-F bond is abnormally weak ( ~153 kJ mol-1) and the Cl-Cl bond is not far behind (~242). I therefore think that the H-H bond strength is a major reason why PH5 does not exist: the formation of two extra P-H bonds does not compensate for the energy needed to break the H-H bond and hence the equation on the first line is thermodynamically uphill as written.

Note that PHF4 is known and although we are breaking the very strong H-F bond (~574 kJ mol-1) we presumably get it back by forming the extra P-F bond (couldn't find P-F bond strength but Si-F is ~565 kJ mol-1).

I did a SciFinder search for PH5 and only turned up the theoretical studies like the JACS ref given. A search of phosphoran yielded one hit: a strange PtPR2Pt cmpd.

I am surprised to learn that PH5 does exist; but phosphoranes (compounds of general formula PRnH(5-n) certainly do. I hope RPM can give us a literature citation

The only hydrides I can find in Greenwood are PH3, P2H4, and higher analogues.

The reason PH5 is either non-existent or very reactive is, using the usual valence bond story, that PX5 involves sp3d hybridization, and the d orbital is too high in energy to be brought into play unless the P is attached to sufficiently electronegative groups, thereby inducing a more positive (or less negative) charge on P and lowering the 3d energy.

So your answer would be the low electronegativity of H.

You need to provide some background to your questions:

1) There are many occasions when solutions can have a pH = 5.00. There is nothing special about this pH. I have no answer to your question, because it is not correct.

2) The vast majority of molecules are very stable, and some have been in existance for millions, if not billions of years.

Again, sorry, your question does not represent reality. Possibly you are misinterpreting something that you have read.

Ph5 Chemical Name