It should make sense that phosphorus has the positive charge,and gold itself should have the negative charge in gold phosphide(and for any other gold phosphides) because phosphorus has a lower electronegativity than gold (phosphorus is 2.19, gold is 2.54),but then, they can both just share electrons. Is this true or not?
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2$\begingroup$ FWIW, two other phosphides are also known: $\ce{Au2P3}$ and $\ce{Au3P4}$ $\endgroup$– Nilay GhoshMay 14, 2021 at 3:30
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1$\begingroup$ This source^ suggests it's alloy-like. Hard to tell what actual partial charges would be, but it's indeed possible that Au hard slight negative one. $\endgroup$– MithoronMay 14, 2021 at 14:12
1 Answer
Because their electronegativity difference is less than 0.4, I think the bond would have a lot of covalent character. I personally don't think it is a good idea to treat this substance as an ionic compound and assign a positively charged ion and a negatively charged.
According to a chemical supply company, $\ce{Au}$ is +3 and $\ce{P}$ is -3. We typically write the cation first, so from the formula $\ce{AuP}$, you should get another clue that the gold ion is positively charged (sort of if you have to assign a positively charged ion).
I don't think you can assign charge signs solely on electronegativities. Atomic radius and position on the periodic table also play roles in determining which atom is more positively charged.
For electronegativity, more reference here.