Phosphorus remains exciting!

Today, the search for new element modifications is basically characterized by work under extreme conditions, in particular, high pressures and very often high temperatures. This situation is not surprising since the discovery of new elements and their characterization under normal conditions is more a topic of chemical-historical literature. Despite this, surprises are still found in the area of the main-group elements, such as the discovery of the fullerenes for which Curl, Kroto, and Smalley were awarded the Nobel Prize in 1996. In addition to carbon, there is another element for which new modifications can be envisaged when kinetic effects are considered, and whose phase diagram still has unexplained features: phosphorus. Initially this notion appears surprising because the element has been known for about 350 years, but when the numerous possibilities for phosphorus atoms to bond with one another are taken into account, the idea seems quite plausible. In 1995 B-cker and H/ser investigated theoretically and systematically the energetic relationships between the many plausible bonding patterns of phosphorus. This work showed that, for example, a whole series of energetically equivalent possibilities exists for the formation of phosphorus polymers. Presumably this situation also explains the difficulties encountered clarifying the structures of different forms of the so-called red phosphorus. As well as the multiatom polymers, there is also tetrahedral P4, which is a very small phosphorus molecule that is familiar to every chemist. The discovery of phosphorus lies far in the past but now the time at last appears to be ripe for a whole series of new experimental findings. Thus Ruck et al. have reported the elucidation of the crystal structure of socalled fibrous phosphorus, a form that had been postulated by Thurn und Krebs as early as 1969. Apparently, depending upon the crystallization conditions, the polymeric phosphorus tubes with the familiar pentagonal cross-section are either linked crosswise with one another (Hittorf7s or violet phosphorus) or parallel so that double tubes are formed (fibrous phosphorus). According to the current results of Ruck et al., Hittorf7s phosphorus is formed from amorphous red phosphorus either from lead melts or by heating with small amounts of iodine. In contrast, the fibrous modification was not obtained from lead; crystals in detectable amounts are formed best in a temperature gradient of approximately 10K at about 580K. Ruck et al. conclude from their observations that crystallization of the tubes on the wall of the quartz glass ampoule supports the parallel orientation of the tubes. They were able to determine the crystal structure of the fibrous modification of phosphorus by X-ray diffraction on a single crystal (Figure 1) and demonstrate the clear difference from Hittorf7s phosphorus by high-resolution electron diffraction imaging. The extensive theoretical investigations in ref. [2] showed that unlike Hittorf7s phosphorus, in the fibrous form the P P bond between the strands has no p-bonding component. At this position the fibrous modification has a trans conformation, whereas the linkage in Hittorf7s phosphorus can be regarded as a gauche conformation. In this way the significantly different bond lengths at this position can be explained very well. In contrast to earlier assumptions the density of both forms is the same. It may be concluded from this result that the synthesis conditions are solely responsible for the formation of the one or the other modification, and not, for instance, different thermodynamic stabilities. This situation also explains the simultaneous formation of Hittorf7s and fibrous phosphorus from the gas phase. Calculations confirm the energetic equivalence of both allotropes. According to theoretical investigations by H/ser it can be assumed that in addition to these two differently linked tubes of phosphorus with pentagonal cross section, the polymers with the Figure 1. Linkage of the phosphorus strands in fibrous phosphorus (top; data from ref. [2]) and Hittorf’s phosphorus (bottom; data from ref. [3]).