How to obtain a transition state (TS) for a SN2 reaction involving phosphorus as the center of nucleophilic attack?

I would like to know how can I obtain a transition state (TS) for SN2 reaction involving phosphorus as the center of nucleophilic attack?

Every time that I try to opt the TS in g09 or g03 as a saddle point in the PES, the convergence criteria generate the minimun point.

My input is like this:
opt=(ts,calcfc,noeigentest) b3lyp/gen

The problem consists in an intramolecullar nucleophilic attack, this may be contributing to difficult the TS optimization.
The theoretical mechanism is similar to that found in the figure 6 of the article

J. Mol. Biol (1996) 259, 737-748 (Mechanism of Fe(III)-Zn(II) Purple Acid Phosphatase Based on Crystal Structures) attached file