Frontiers in the Crystallographic Characterization of Extended Conjugated Polyynes: New Structures of Diplatinum Adducts and a Literature Survey

The crystal structures of the polyynes trans,trans ‐(C 6 F 5 )( p ‐tol 3 P) 2 Pt(C≡C) 14 Pt(P p ‐tol 3 ) 2 (C 6 F 5 ) ( PtC 28 Pt ) and trans,trans ‐(C 6 F 5 )(Me 3 P) 2 Pt(C≡C) 4 Pt(PMe 3 ) 2 (C 6 F 5 ) ( Pt*C 8 Pt* ) are described. The former is available by a reported oxidative homocoupling and the latter by a fourfold P p ‐tol 3 /PMe 3 substitution. The former represents the longest conjugated polyyne structurally characterized to date, and “near misses” involving higher homologs are detailed. The difference between the average C≡C and CC bond lengths (bond length alternation (BLA)) of PtC 28 Pt (1.217 and 1.338 Å;BLA(avg) 0.121 Å) is diminished relative to those of Pt*C 8 Pt* (1.240 and 1.370 Å; BLA(avg) 0.130 Å) or the P p ‐tol 3 analog (1.231 and 1.358 Å; BLA(avg) 0.127 Å). Implications for the electronic structure of the sp carbon allotrope carbyne are discussed. In both new structures, all platinum–platinum vectors run parallel, but there are two distinct sets, offset by ≈14 and ≈5 sp carbon atoms, respectively. Within a set, the chains are well separated, but PtC 28 Pt exhibits 3.47 Å van der Waals contacts between sets. Considering all literature data, still higher polyynes are viewed as more challenging to crystallographically characterize, but not impossible.