Group 15 forms binary halides with the elements in two oxidation states: tri-halides with the oxidation state of +3, and penta-halides with the oxidation state of +5.
Tri-Halides
All MX3 are formed, and they are all volatile and easily hydrolyzed by water. They are generally formed by direct reaction of the elements.
Structures:
The gaseous molecules have a pyramidal structure (cf. ammonia).
AsI3, SbI3 and BiI3 have layered lattice crystal structures.
BiF3 has an ionic lattice, and SbF3 contains SbIII in very distorted octahedral environments.
Reactions:
PF3 forms complexes with transition metals, analogous to those with carbon monoxide, eg. Ni(PF3)4 is the analogue of Ni(CO)4. Here the empty P(dπ) orbital acts as the π-acceptor in formation of synergic bonds rather than the empty π*-orbital in CO.
Other tri-halides act as mild Lewis acids to bases like R3N (trialkylamines) and halide ions: simple compounds like AsCl4– and SbF52- are formed.
PCl3 is very reactive, and is used as the precursor in the manufacture of various organic compounds.
Penta-Halides
All MF5 are known.
PF5 has a trigonal bipyramidal structure in the gas phase, which reacts readily with F– to form an octahedral complex. The gaseous molecule is fluxional, that is the axial and equatorial F atoms exchange positions. This occurs on the NMR timescale, and is known as the Berry pseudorotation:
Two of the original equatorial Cle atoms swing round to give a square planar intermediate, and then the two original axial Cla atoms move down to regenerate the trigonal bipyramidal structure and are now in the equatorial positions. The horizontal plane of symmetry therefore moves from perpendicular to the plane of the page to parallel to the plane of the page. AsF5 has the same structure as PF5, but SbF5 polymerizes in the gas phase to give the associated (SbF5)4 molecule, in which each Sb atom is octahedrally coordinated by F atoms.
SbF5 polymerises to give the (SbF5)4 molecule with a Sb4 square.
Other MX5 also show the tendency to form octahedrally coordinated complexes, and also tetrahedral complexes in the condensed phases.
Complex formation:
Reactions of PCl5:
AsCl5, however, decomposes above -50oC to give AsCl3 and Cl2. This instability relative to PCl5 and SbCl5 is due to the stabilisation of the 4s2 electrons by the contraction of the 3d orbitals after the lanthanide series: this is known as the inert pair effect (there is a lone pair of electrons on As in AsCl3).
In general, MX5 species interact with electron pair donor ligands (Lewis bases), especially X–, to form octahedral MX5L species.