In a similar fashion to when we discussed other molecules containing lots of fluorine, the presence of such an electronegative element (3.98) vs. phosphorus (2.19) dents to impact of the lone pair electron dipole since charge is a little more evenly spread throughout the molecule. As a result phosphorus trifluoride has a melting point of -152˚C and a boiling point of -102˚C. This makes the molecule a gas at standard temperature and pressure. The structure reacts relatively slowly with water (i.e. in terms of solubility) due to the lower dipole moment.
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| PF3 Ball and Stick Model. Created with MolView. |
Similar to when we discussed the polarity of Carbon Monoxide, we noted the biological properties of the molecule at hand. In that case, CO disrupted the hemoglobin function by binding to iron and therefore disrupting the flow of oxygen. This made the gas toxic at concentrations greater than 35 ppm. PF3 performs a similar kind of binding to iron within hemoglobin and therefore it also toxic to human health. In fact, PF3 has an affinity to bind to many of the different transition metals besides iron.
How do I know when I need to look at geometry rather than electronegativity? When I first determined the electronegativity of PH3, I declared it non-polar because the difference was 0.01. But the Lewis structure and the shape of the molecules show their polarity.
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