Interconverting edge-sharing and face-sharing bioctahedral dinuclear tungsten(III) phosphine complexes. Preparations, properties, and structures of W2Cl6L4, where L = PMe3, PEt3, and P(n-Bu)3, W2Cl6L3, where L = PEt3 and P(n-Bu)3, and [HPEt3]+[W2Cl7(PEt3)2]-