[tulip11]

I'm trying to draw the P³¹ NMR for diphosphine in terms of tree splitting diagrams and coupling constants etc but am getting confused as when I look at it it appears the two phosphorous are chemically equivalent but magnetically inequivalent. When I take phosphorous and compare it to a probe nuclei (circled H in image) one phosphorous displays ²J coupling and the other ³J. I thought that this meant they were inequivalent and so would produce a second order spectrum and that tree diagrams would not work but must be misunderstanding something. I would be very grateful if someone could explain this.

[Babcock_Hall]

https://pubs.acs.org/doi/pdf/10.1021/ja00959a010
I am not sure why you think that you must be misunderstanding something; the two phosphorus nuclei do not look magnetically equivalent to me.  In a paper by Mesmer and Carroll, "The kinetics and mechanism of the hydrolysis of pyrophosphite, J. Am. Chem. Soc., 1966, 88 (7), pp 1381–1387
DOI: 10.1021/ja00959a010, there is a discussion and a figure of a not entirely dissimilar system.