Octacarbonyl Anion Complexes of Group Three Transition Metals [TM(CO)₈]⁻ (TM=Sc, Y, La) and the 18‐Electron Rule

Abstract

We report the gas-phase synthesis of stable 20-electron carbonyl anion complexes of group 3 transition metals, TM(CO) (TM=Sc, Y, La), which are studied by mass-selected infrared (IR) photodissociation spectroscopy. The experimentally observed species, which are the first octacarbonyl anionic complexes of a TM, are identified by comparison of the measured and calculated IR spectra. Quantum chemical calculations show that the molecules have a cubic (O ) equilibrium geometry and a singlet ( A ) electronic ground state. The 20-electron systems TM(CO) are energetically stable toward loss of one CO ligand, yielding the 18-electron complexes TM(CO) in the A electronic ground state; these exhibit a capped octahedral structure with C symmetry. Analysis of the electronic structure of TM(CO) reveals that there is one occupied valence molecular orbital with a symmetry, which is formed only by ligand orbitals without a contribution from the metal atomic orbitals. The adducts of TM(CO) fulfill the 18-electron rule when only those valence electrons that occupy metal-ligand bonding orbitals are considered.

Transition‐Metal Chemistry of Alkaline‐Earth Elements: The Trisbenzene Complexes M(Bz)3(M=Sr, Ba)

“…The latter molecules have two singly occupied degenerate orbitals and one lower lying ligand‐based molecular orbital (MO) with a 2u symmetry that has zero coefficient at the metal, because it has no valence AO having proper symmetry. We earlier reported the observation of the 20‐electron transition metal octa‐carbonyl anions [TM(CO) 8 ] − (TM=Sc, Y, La) . These complexes also have cubic symmetry but a singlet ( 1 A 1g ) ground state, where the degenerate HOMO is fully occupied.…”

Transition‐Metal Chemistry of Alkaline‐Earth Elements: The Trisbenzene Complexes M(Bz)3(M=Sr, Ba)

“…Thecomplexes M(Bz) 3 (M = Ca, Sr, Ba) are formally 20electron species possessing as inglet ( 1 A 1 )e lectronic ground state.O ur previously reported isoelectronic alkaline earth complexes M(CO) 8 [7] and M(N 2 ) 8 [11] are 18-electron adducts with cubic (O h )symmetry that possess atriplet ( 3 A 1g )ground state.T he latter molecules have two singly occupied degenerate orbitals and one lower lying ligand-based molecular orbital (MO) with a 2u symmetry that has zero coefficient at the metal, because it has no valence AO having proper symmetry.W ee arlier reported the observation of the 20electron transition metal octa-carbonyl anions [TM(CO) 8 ] À (TM = Sc,Y ,L a). [21] These complexes also have cubic symmetry but as inglet ( 1 A 1g )g round state,w here the degenerate HOMO is fully occupied. The1 8-electron rule first suggested by Langmuir [22] remains valid in the above complexes,i fo nly the metal-ligand bonding electrons are counted.…”

Observation of alkaline earth complexes M(CO) 8 (M = Ca, Sr, or Ba) that mimic transition metals

“…Because two valence electrons of the ligands in a cubic field are not available for donation to the central metal atom M in ML 8 , where L is a ligand, the complex must provide a total of 20 electrons to fully occupy the metal's valence shell. This scenario has recently been explored in the transition 2 of 5 (15). Only 18 electrons are available in M(CO) 8 (M = Ca, Sr, or Ba), so the degenerate e g MO is occupied by two electrons with the same spin, giving a triplet ( 3 A 1g ) electronic ground state.…”