Unusual coordination mode for 1,3-diphosphete ligands towards a Cr-Cr quintuple bond complex.

The reaction of LCr5̲CrL (L = N2C25H29, 1) with the phosphaalkynes R-C≡P (R = tBu, Me, Ad) yields the neutral dimerisation compounds [L2Cr2(µ,η1:η1:η2:η2-P2C2R2)] (R = tBu (2), Me (3)) and the tetrahedrane complex [L2Cr2(µ,η2:η2-PCAd)] (4). The 1,3-diphosphete ligands in complexes 2 and 3 are the first to possess this structural feature spanned over a metal-metal multiple bond, while the slightly bigger adamantyl phosphaalkyne remains a monomer in 4 with a side-on coordination mode.