[skp524]

I can only interpret this fact by considering that the steric hindrance between the bulky phosphines somewhat destabilize this molecule. Can anyone suggest some idea about the electronic factor ?

[cheese (MSW)]

Research the classic work of Tolman on P ligands. steric: cone angle;
electronic (σ-donor π-acceptor) properties: Tolman electronic parameter based
on ν(CO), but also subsequently on C-13 δs.  See what QALE brings up on the web
(is the Giering site still there?).
What did Tolaman conclude from his studies on NiP4 cmplxs?
Tolman, incidentally, carried out his work at DuPont.   

[cheese (MSW)]

I must apologize for misreading your question the first time round: you want ideas for an electronic effect contributing to the instability of PtP4.  The kinetic instability of PtP4 can be the result of ground-state destabilization or transition-state stabilization, or a combination of both. PPh3 has a large (Tolman) cone angle: how would this contribute to the instability of PtPt4?  PPh3 is however a reasonably strong    
σ-donor but  a weak π-acceptor ligand (Tolman electronic parameter).  There is a build up of electron density on the Pt atom as each P is added: how would this affect P:→PtP3 bond strength?
To distinguish the two effects one needs to carry out a “QALE” analysis:
http://www.bu.edu/qale/  (see tutorial)
Tolman found for Ni(CO)4  +  xs PR3 ⇋ Ni(CO)4-x(PR3)x  that the size of x (range ~0.9 to ~2.9) depended almost entirely on the cone angle of PR3 (i.e., no electronic effect).