Robert R. Holmes Bell Telephone Loborotories, Inc. Murray Hill, New Jersey
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Ionic and Molecular Halides of +he Phosphorus Family
At the present time sufficient information has appeared in the literature regarding ionic and molecular halides of the Group V elements (P, As, Sh and Bi) to warrant a summary discussion to ascertain whether some semblance of correlation exists. If so, perhaps possible directions for future research may suggest themselves. Many of the investigations, by necessity, have been concerned with the synthesis and identification of ionic and molecular halides and a study of their simple chemistry. There also have been a smaller number of mor? detailed investigations concerned with structural aspects of the halides. Little thermodynamic data is available and studies of reaction mechanisms are almost nil. As will be seen, one reason for the lack of investigations concerned with more elegant experimentation, aside from air and moisture sensitivity, is the existence of a multitude of structural states for these halides and the complexity of their interconversions. The Pentahalides While all the trihalides of phosphorns, arsenic, antimony and bismuth are known, not all of the pentahalides have been prepared. The most recent preparation is that of BiF5 (1). The melting points and boiling points of the known pentahalides are listed in Table 1. I n addition the mixed pentahalides, Table 1. The Penlahalides: Melting and Boiling Points l0C): I
-FsP As
sb Bi
MP -93.7 -79.8 +7 151"
BP -84.5 -53.2 +15o 230
MP 167(subl.)
BP
+5
-i40
...
...
... ...
-Brs MP PCLF > PCL, toward pyridine (37) in line with electronegativity considerations is opposite to that of the boron halide series, BBr3 > BC13 > BF3 (40). I n the boron halide series variations in degree of pi honding have been advanced to explain the ordering. Apparently the pentahalides may coordinate with little disturbance to the degree of pi bonding present. As discussed elsewhere (37) the latter seems reasonable. I n the boron trihalide molecules, boron can enter into pi honding with its one p orbital. On coordination such stabilizing bonding is largely destroyed since the p orbital now enters into bonding with the incoming group. For the phosphorus pentahalide molecules, if phosphorus does enter into pi hondiug, it may do so with up to four of its d orbitals. On coordination, the degree of pi bonding need not be disturbed to as large a degree as it does in the boron halide series. There remain three d orbitals of phosphorus yet availahle for pi interaction in the coordinated molecule (Fig. 2). H a l i d e I o n Stability (Trivalent)
As with the pentavalent state, a variety of halide complexes are known for trivalent Group V elements lists the common types of ions that (7, IS). Table (i have been reported for each element. None are known for phosphorus. Here again no comparative
Table 6.
Holide Ions of Trivalent P, As, Sb and Bi
Table 7.
Donor"
Acceptorb
PFs,BHs PClr.G&Cb PC13.BBr8 PBrs .BBr. 2PCls.B1CIt (As Sb, Bi none&tent)
PC&.AsMea PF. .NMe3 PCI,.NMea PBr3.NMea (As, Sb, Bi, a variety)
Change in hybridization ond pi bonding on coordination for BXa
128 / Journal of Chemical Education
Complexc
(X,PhPtCI, Ni(PXdr Ni(CO)BbC18 Fe(COJG3hCIa).
All the known donor complexes. All known acceptor complexes of PXa. The list for As, Sh and Bi is extensive. =Seereference (7) and (13). a
Obviously this comparison is even more premature than in the case of the pentavalent state. I n fact the existence of simple M X P - ions has yet to be observed in the crystalline state. Thus, single crystal X-ray studies (43) have shown that I
