Ground-State and Excited-State C⋮C Stretching Frequencies of Dimetallapolyyne Redox Congeners. Direct Evidence for Metal→Alkynyl π-Back-Bonding

Paramagnetic [K(crypt-222)][Mo 2 (C⋮CSiMe 3 ) 4 (PMe 3 ) 4 ] ([K(crypt-222)] 1 ) is prepared by one-electron reduction of Mo 2 (C⋮CSiMe 3 ) 4 (PMe 3 ) 4 ( 1 ) by K(C 10 H 8 ) in the presence of crypt-222. The ν(C⋮C) frequencies for 1 and [K(crypt-222)] 1 in their ground states and δ → δ* electronic excited states are controlled by the occupancy of the δ(M 2 ) and δ*(M 2 ) orbitals, lying in the order 1 (δ 2 ) > 1* (δ 1 δ *1 ) > [K(crypt-222)] 1 (δ 2 δ *1 ) > [K(crypt-222)] 1* (δ 1 δ *2 ); this order indicates that M→C⋮CSiMe 3 π-back-bonding is of increasing importance across this series.