Isolation and x-ray structure of bis[(tetrahydrofuran)(.mu.2-di-tert-butylphosphido)(.mu.3-di-tert-butylphosphido)dilithium] ([Li2(.mu.3-tert-Bu2P)(.mu.2-tert-Bu2P)(C4H8O)]2) containing a staggered, planar lithium (Li4) unit. The first structurally characterized alkali metal diorganophosphide