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)8− (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 (Oh) equilibrium geometry and a singlet (1A1g) electronic ground state. The 20‐electron systems TM(CO)8− are energetically stable toward loss of one CO ligand, yielding the 18‐electron complexes TM(CO)7− in the 1A1 electronic ground state; these exhibit a capped octahedral structure with C3v symmetry. Analysis of the electronic structure of TM(CO)8− reveals that there is one occupied valence molecular orbital with a2u symmetry, which is formed only by ligand orbitals without a contribution from the metal atomic orbitals. The adducts of TM(CO)8− fulfill the 18‐electron rule when only those valence electrons that occupy metal–ligand bonding orbitals are considered.

Filling a Gap: The Coordinatively Saturated Group 4 Carbonyl Complexes TM(CO)8 (TM=Zr, Hf) and Ti(CO)7

“…Information,f or other isomers). [21] AllG roup 4o ctacarbonyl complexes are minima on the potential energy surface, but the titanium complex Ti(CO) 8 is thermodynamically unstable for the loss of one CO ligand.T he dissociation reaction TM(CO) 8 ! TM(CO) 7 + +CO is calculated to be exothermic for TM = Ti,w hereas it is endothermic for TM = Zr,H f( Figure 5).…”

“…À (TM = Sc, Y, La), whicha re isoelectronic with the neutral Group 4c omplexes TM(CO) 8 TM = Ti,Z r, Hf. [21] Given that the valence shell of the anions has al arger radius than that of the neutrals pecies, we were hopingt hat ah igher coordination number than six could be achieved. Our expectation was approved, buts urprisingly we observed the octacarbonyl complexes TM(CO) 8 À (TM = Sc, Y, La) as coordinatively saturatedc omplexes.…”

“…Our theoretical analysiss howed that the complexes possess cubic (O h )symmetry,inwhich one of the metalÀCO valence MOs with a 2u symmetry has no AO coefficient at the metal. [21] The remaining nine MOs have AO coefficients at the metal;t hey can be easily associated with the TM ! CO s donation and TM!CO p back-donation with the latter being the dominant term.…”

“…CO s donation and M!CO p backdonation, for which the metal AOs of the latter term come from the valence dA Os. [21] The heavy alkaline-earthe lements Ca, Sr,a nd Ba bind like transition metals in the octacarbonyl complexesM (CO) 8 .T he complexes are formally1 8-electron systems, but the occupied valence a 2u MO of the cubic (O h )s tructures has an ode at the metal atom,a nd thus, the complexes are actually 16-electron systems. The degenerate e g HOMO is only doubly occupied, and therefore, the complexes M(CO) 8 (M = Ca, Sr,Ba) have at riplet ground state.…”

“…The degenerate e g HOMO is only doubly occupied, and therefore, the complexes M(CO) 8 (M = Ca, Sr,Ba) have at riplet ground state. [21] With the above knowledge, we re-investigated the question of the stability of the coordinatively saturated Group 4c arbonyl complexes, which may be realized in octacarbonyl complexes TM(CO) 8 rather than heptacarbonyl complexes TM(CO) 7 (TM = Ti,Z r, Hf). Here we report ac ombined infrared spectroscopic and theoretical study on coordinatively saturated Group 4m etal carbonyl complexes prepared in the gas phase and/or solid neon matrix.…”