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Electronic Structure of Ylides
Francis A. Carey and Richard J. Sundberg
Advanced Organic Chemistry, Part B: Reactions and Synthesis, 3rd ed., 1990, 95.

(CH3)3P(+)-CH2(-) <---> (CH3)3P=CH2

... NMR spectroscopic studies, however, are more consistent with the dipolar ylide structure and suggest only a minor contribution from the ylene structure. Theoretical calculations support this view. ...

Reference is made to:
A. Streitwieser, A. Rajca, R. S. McDowell, and R. Glaser,
J. Am. Chem. Soc. 1987, 109, 4184.