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Fluorocarbons have been shown experimentally by Baker and coworkers to combine with the cyclopentadienylcobalt (CpCo) moiety to form fluoroolefin and fluorocarbene complexes as well as fluorinated cobaltacyclic rings. In this connection density functional theory (DFT) studies on the cyclopentadienylcobalt fluorocarbon complexes CpCo(L)(C n F 2n ) (L = CO, PMe 3 ; n = 3 and 4) indicate structures with perfluoroolefin ligands to be the lowest energy structures followed by perfluorometallacycle structures and finally by structures with perfluorocarbene ligands. Thus, for the CpCo(L)(C 3 F 6 ) (L = CO, PMe 3 ) complexes, the perfluoropropene structure has the lowest energy, followed by the perfluorocobaltacyclobutane structure and the perfluoroisopropylidene structure less stable by 8 to 11 kcal mol −1 , and the highest energy perfluoropropylidene structure less stable by more than 12 kcal mol −1 . For the two metal carbene structures Cp(L)CoC(CF 3 ) 2 and Cp(L)CoCF(C 2 F 5 ), the former is more stable than the latter, even though the latter has Fischer carbene character. For the CpCo(L)(C 4 F 8 ) (L = CO, PMe 3 ) complexes, the perfluoroolefin complex structures have the lowest energies, followed by the perfluorometallacycle structures at 10 to 20 kcalmore »mol −1 , and the structures with perfluorocarbene ligands at yet higher energies more than 20 kcal mol −1 above the lowest energy structure. This is consistent with the experimentally observed isomerization of the perfluorinated cobaltacyclobutane complexes CpCo(PPh 2 Me)(–CFR–CF 2 –CF 2 –) (R = F, CF 3 ) to the perfluoroolefin complexes CpCo(PPh 2 Me)(RCFCF 2 ) in the presence of catalytic quantities of HN(SO 2 CF 3 ) 2 . Further refinement of the relative energies by the state-of-the-art DLPNO-CCSD(T) method gives results essentially consistent with the DFT results summarized above.« less

Density functional theory studies show that the lowest energy C 4 F 8 Fe(CO) 4 structure is not the very stable experimentally known ferracyclopentane isomer (CF 2 CF 2 CF 2 CF 2 )Fe(CO) 4 obtained from Fe(CO) 12 and tetrafluoroethylene. Instead isomeric (perfluoroolefin)Fe(CO) 4 structures derived from perfluoro-2-butene, perfluoro-1-butene, and perfluoro-2-methylpropene are significantly lower energy structures by up to ∼17 kcal mol −1 . However, the activation energies for the required fluorine shifts from one carbon to an adjacent carbon atom to form these (perfluoroolefin)Fe(CO) 4 complexes from tetrafluoroethylene are very high ( e.g. , ∼70 kcal mol −1 ). Therefore the ferracyclopentane isomer (CF 2 CF 2 CF 2 CF 2 )Fe(CO) 4 , which does not require a fluorine shift to form from Fe 3 (CO) 12 and tetrafluoroethylene, is the kinetically favored product. The lowest energy structures of the binuclear (C 4 F 8 ) 2 Fe 2 (CO) n ( n = 7, 6) derivatives have bridging perfluorocarbene ligands and terminal perfluoroolefin ligands.

The known sandwich compound [η 5 -(CH 2 ) 3 N 2 (BPh) 2 CMe] 2 Fe in which adjacent C 2 units are replaced by isoelectronic BN units can be considered as a boraza analogues of ferrocene similar to borazine, B 3 N 3 H 6 , considered as a boraza analogue of benzene. In this connection, the related bis(1,2,3,5-tetramethyl-1,2-diaza-3,5-diborolyl) derivatives (Me 4 B 2 N 2 CH) 2 M (M = Ti, V, Cr, Mn, Fe, Co, Ni) for all of the first row transition metals have been optimized using density functional theory for comparison with the isoelectronic tetramethylcyclopentadienyl derivatives (Me 4 C 5 H) 2 M. Low-energy sandwich structures having parallel B 2 N 2 C rings in a trans orientation are found for all seven metals. The 1,2-diaza-3,5-diborolyl ligand appears to be a weaker field ligand than the isoelectronic cyclopentadienyl ligand as indicated by higher spin ground states for some (η 5 -Me 4 B 2 N 2 CH) 2 M sandwich compounds relative to the corresponding metallocenes (η 5 -Me 4 C 5 H) 2 M. Thus (η 5 -Me 4 B 2 N 2 CH) 2 Cr has a quintet ground state in contrastmore »to the triplet ground state of (η 5 -Me 4 C 5 H) 2 Cr. Similarly, the sextet ground state of (η 5 -Me 4 B 2 N 2 CH) 2 Mn lies ∼18 kcal mol −1 below the quartet state in contrast to the doublet ground state of the isoelectronic (Me 4 C 5 H) 2 Mn. These sandwich compounds are potentially accessible by reaction of 1,2-diaza-3,5-diborolide anions with metal halides analogous to the synthesis of [η 5 -(CH 2 ) 3 N 2 (BPh) 2 CMe] 2 Fe.« less

Theoretical methods show that the lowest energy bis(butadiene)metal structures (C 4 H 6 ) 2 M (M = Ti to Ni) have a perpendicular relative orientation of the two butadiene ligands corresponding to a tetrahedral coordination of the central metal atom to the four CC double bonds of the butadiene ligands. Distribution of the metal d electrons in the resulting tetrahedral ligand field rationalizes the predicted spin states increasing monotonically from singlet to quartet from nickel to manganese and back from quartet to singlet from manganese to titanium.