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

Abstract

Homoleptic Group 4 metal carbonyl cation and neutral complexes were prepared in the gas phase and/or in solid neon matrix. Infrared spectroscopy studies reveal that both zirconium and hafnium form eight‐coordinate carbonyl neutral and cation complexes. In contrast, titanium forms only the six‐coordinate Ti(CO)6+ and seven‐coordinate Ti(CO)7. Titanium octacarbonyl Ti(CO)8 is unstable as a result of steric repulsion between the CO ligands. The 20‐electron Zr(CO)8 and Hf(CO)8 complexes represent the first experimentally observed homoleptic octacarbonyl neutral complexes of transition metals. The molecules still fulfill the 18‐electron rule, because one doubly occupied valence orbital does not mix with any of the metal valence atomic orbitals. Zr(CO)8 and Hf(CO)8 are stable against the loss of one CO because the CO ligands encounter less steric repulsion than Zr(CO)7 and Hf(CO)7. The heptacarbonyl complexes have shorter metal−CO bonds than that of the octacarbonyl complexes due to stronger electrostatic and covalent bonding, but the significantly smaller repulsive Pauli term makes the octacarbonyl complexes stable.

Alkaline Earth Metals Activate N2 and CO in Cubic Complexes Just Like Transition Metals: A Conceptual Density Functional Theory and Energy Decomposition Analysis Study

“…[38][39][40] For Ti(N 2 ) 8 ,t he cubic singlet structure is only al ocal minimum, which is thermodynamically unstable for loss of one N 2 ligand leading to Ti(N 2 ) 7 as the coordinatively saturated stable complex, similar to the octa-coordinated CO complexes. [9] The M VI (N 2 ) 6 complexes are also singlet species. All neutral complexes have O h symmetry in their ground state, and all geometry optimizations were performed using the M06-2X-D3 functional and the def2-TZVPPb asis set for all atoms using the Gaussian [41] software conforming with recent work on octa-coordinated alkaline earth complexes.…”

“…The synthesis and characterization as cubic octa-carbonyl complexes of both the anionic group III complexesa nd the neutral group IV complexes was also recently reported by the same authors. [8,9] In the abovementioned heavy alkaline earth main-groupm etal complexes, the metal-CO bondingw as scrutinized, and was shown to be dominated by the M(dp)! (CO) 8 p back-donation of the degenerate (e g ) set of singly occupied( nÀ1)d atomico rbitals of the metal into the antibonding p*m olecular orbitals (MOs) of CO. [8] Further analysiss hows that the complexes obey the 18-electronr ule if only those electrons are countedt hat are engaged in the metal-ligand interactions.…”

“…[6] The so-called nucleophilic Fukuif unction f À (r)a nd the corresponding local softness s À (r)( as different systemsa re compared with each other rathert han differents ites on one complex [22b, 28] )w ere selected for this purpose. In particular,t he evolution of these quantities upon passing from transition metal complexes (the cases of group IV transition metals( Ti,Z r, Hf) also yieldingc ubic octacoordination, [9] and group VI transition metals(Cr,Mo, W) yielding octahedral coordination)t om ain-group metal complexes will be investigated in detail.T he fundamental question pertains to the similarity in activation of the ligand, increasing its reactivity,b etween main-group and transition metals.I nv iew of their particular relevance (vide supra) the N 2 resultsw ill be highlighted in more detail;t he overall tendencies will, however,b ec hecked for parallelism between the N 2 and CO complexes. Aw ord of caution should be given here.…”

Metal–CO Bonding in Mononuclear Transition Metal Carbonyl Complexes

“… 35 The recent work on the first observation of coordinatively saturated carbonyl complexes of group 4 reported the surprising finding of octacarbonyls M(CO) 8 for M = Zr, Hf instead of heptacarbonyls. 36 In the case of Ti only the heptacarbonyl Ti(CO) 7 was observed. Note that stable salts of the isoelectronic cations [M’(CO) 7 ] + (M’ = Nb, Ta) were recently synthesized.…”

“… 4 Theoretical calculations showed that the coordinatively saturated Ti(CO) 8 is a minimum on the potential energy surface, but it is thermodynamically unstable for the loss of one CO. 36 This is likely due to the release of interligand repulsion in the titanium octacarbonyl, which has significantly shorter metal–CO bonds than the heavier group 4 homologues. 36 The analysis of the electronic structure of the octacarbonyls M(CO) 8 , which are formally 20-electron species, showed that one (a 2u ) occupied valence orbital of the complexes having cubic ( O h ) symmetry has only coefficients at the CO ligands. The s/p/d valence AOs of the metals do not have proper symmetry to mix with the a 2u MO of (CO) 8 ( O h ).…”