M3L9(ligand) complexes

The frontier orbitals of an M3(C0)9 unit are constructed from three M(CO)3 pieces or by removing three ligands from a M3(CO),2. They consist of a relatively low-lying Walsh-like set la, + le and a higher cyclopropenium-like 2ai + 2e. The latter are empty for a d8 M. These orbitals are used to analyze the electronic and geometrical structure of M3Ly(li- gand) complexes where the ligand is CO, S, (CO)2, S2, CR, CCH>+, ethylene, acetylene, or acyclic and cyclic polyenes. In the case of (CO)~CO~CCH~+ the symmetrically topped structure is not a local energy minimum, but instead there are three equiv- alent unsymmetrical structures of a complexed ethylidene type The rich and beautiful chemistry of metal clusters is un- folding only now.I,* Perhaps its most highly developed aspect is the reactivity of trinuclear compounds. Both structural data and reactivity studies cover a wide range of such complexes including systems with both bridging and terminal carbonyls or hydrides or highly complicated molecules where the ligand no longer can be separated from the cluster unit.