text: Acyl complexes are usually low-spin and spin-paired.
  Monometallic acyl complexes adopt one of two related structures, C-bonded and η-C-O-bonded.  These forms sometimes interconvert. For the purpose of electron-counting, C-bonded acyl ligands count as 1-electron ligands, akin to pseudohalides.  η-Acyl ligands count as 3-electron "L-X" ligands.  
  bridging acyl ligands are also well known, where the carbon bonds to one metal and the oxygen bonds to a second metal. One example is the bis(μ-acetyl) complex [(CO)Fe(C(O)CH)Fe(CO)].