Trigonal bipyramids live-coordinate compounds

A number of compounds of the p-block elements containing two bidentate ligands can be considered to be live-coordinate and trigonal bipyramidal if a non-bonding pair of electrons occupies one of the equatorial sites, for example [Sn(S2CNR2)2(lone pair)] and [Pb(RCOCH-COR)2(lone pair)]. The greater repulsion observed from this line pair implies R(lone pair/bidentate) values of 0.6 for sulfur(IV) compounds, decreasing to 0.4 for seleniiun(IV) and 0.2 for tellurium(IV), antimony(III), tin(II) and leadfll). ... 

---