Valence orbital energetics as a tool for organometallic synthesis: On the scarcity of two-legged piano-stool complexes

From an Easily Accessible Pentacarbonylcobalt(I) Salt to Piano‐Stool Cations [(arene)Co(CO)2]+

“…The geometry for all complexes have the Co(CO) 2 plane orthogonal to the plane of the arene ring, with the C CO ‐Co‐C CO angle equal to 90° (87.5–90.9°). This is in accordance with [(C 6 H 6 )Rh(CO) 2 ] + , and previous calculations by Bursten and Gatter for [(C 6 H 6 )Co(CO) 2 ] + . Cations 2 + and 4 + have nearly C s , while 3 + has C 2 v symmetry.…”

“…Al ikely example for the substitution chemistry of such an accessible[ Co(CO) 5 ] + [WCA] À salt would be the two-legged piano-stool complex [(C 6 H 6 )Co(CO) 2 ] + ;p redicted to be stable by DFT calculations in 1990. [23] To the best of our knowledge, the only known [(arene)M(CO) 2 ] + derivatives are those with M = Rh and with arene = C 6 H 6 (XRD), [11] and C 6 Me 6 (no XRD). [24] But again, the structurally characterized cation needed the very elaborate [1-Et-CB 11 F 11 ] À WCAt ob es tabilized, [11] probably [ 4 ]c an be used for the polymerization of iso-butylene, [26] while [(C 6 H 4 F 2 )Rh(PiBu 3 ) 2 ][BAr F 4 ]c atalyzes dehydrocoupling reactions of amine-boranes.…”

“…A likely example for the substitution chemistry of such an accessible [Co(CO) 5 ] + [WCA] − salt would be the two‐legged piano‐stool complex [(C 6 H 6 )Co(CO) 2 ] + ; predicted to be stable by DFT calculations in 1990 . To the best of our knowledge, the only known [(arene)M(CO) 2 ] + derivatives are those with M=Rh and with arene=C 6 H 6 (XRD), and C 6 Me 6 (no XRD) .…”

A Computational Study of Two-State Conformational Changes in 16-Electron [CpW(NO)(L)] Complexes (L=PH3, CO, CH2, HCCH, H2CCH2)

“…[46] It has been noted previously by Bursten and co-workers that the HOMO of 18-e [CpML 2 ] complexes increases in energy upon going from a late transition-metal CO complex to a more electropositive metal containing NO groups (e. g. from [CpCo(CO) 2 ] to [CpMn(NO) 2 ]), and that this highenergy HOMO can result in the complex with an earlier transition metal being relatively less stable. [47] Although alignment of the alkyne ligand with the W ± NO bond would result in an 18-electron, singlet [CpW(NO)(HCCH)] complex similar to the [Cp*Re(CO)-(MeCCMe)] compound, the high energy of the resultant metal-centered, nonbonding HOMO ( Figure 18) apparently destabilizes this complex with respect to the alternative alkyne conformation depicted in Figure 12. Thus, the M ± alkyne conformational variation can be ascribed to the higher energy orbitals of the tungsten(0 0) nitrosyl complex compared to the isolobal rhenium(i) carbonyl species.…”

A Computational Study of Two-State Conformational Changes in 16-Electron [CpW(NO)(L)] Complexes (L=PH3, CO, CH2, HCCH, H2CCH2)

“…This type of phenomenon was first recognized three decades ago for another series of W 0 alkyne complexes, namely [W(RCCR) 3 (L)], [44] and has since been extended to other p-loaded metal centers with three 1s,2p ligands. [47] Although alignment of the alkyne ligand with the W ± NO bond would result in an 18-electron, singlet [CpW(NO)(HCCH)] complex similar to the [Cp*Re(CO)-(MeCCMe)] compound, the high energy of the resultant metal-centered, nonbonding HOMO ( Figure 18) apparently destabilizes this complex with respect to the alternative alkyne conformation depicted in Figure 12. Such an orientation is in contrast to the isolobal, 18-electron [Cp*Re(-CO)(MeCCMe)] complex, which contains a 4-electron donor alkyne ligand aligned with the ReÀCO bond.…”