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The bulky bis(carbene)borate ligand Ph2B(tBuIm)2– stabilizes low-coordinate and low-valent iron complexes. The four-coordinate iron(II) complex Ph2B(tBuIm)2FeCl(THF) is a precursor to the low-spin iron(I) complex Ph2B(tBuIm)2Fe(CO)3. Despite a reversible reduction wave in the cyclic voltammogram, access to the iron(0) complex [Ph2B(tBuIm)2Fe(CO)3]− requires reduction under a CO atmosphere. Without excess CO, KC8 reduction leads to formation of the dimer K2[Ph2B(tBuIm)2Fe(CO)2] along with loss of CO. Each iron center in this complex adopts a distorted-square-planar geometry. Spectroscopic analysis of these carbonyl complexes reveals the very high strength of the bis(carbene)borate ligand.

A bulky tris(carbene)borate ligand allows several low-valent iron carbonyl complexes to be isolated. One-electron reduction of the cationic iron(II) complex [PhB(MesIm)3Fe(CO)3][B(C6F5)4] (1) ([PhB(MesIm3)]− = phenyltris(1-mesitylimidazol-2-ylidene)borate) yields the low-spin (S = 1/2) iron(I) complex PhB(MesIm)3Fe(CO)2 (2), as determined by structural and spectroscopic methods. This complex can in turn be reduced to provide the anionic dicarbonyl complex [K][PhB(MesIm)3Fe(CO)2] (3), which crystallizes as a dimer in which the potassium cation coordinates in a side-on fashion to one CO ligand. Protonation of 3 yields the weakly acidic iron hydride PhB(MesIm)3Fe(CO)2H (4), which can also be isolated by treating the κ3-coordinated alkylborohydrido complex PhB(MesIm)3Fe(κ3-BH(CH2CH3)3) (5) with CO. The strong donor ability of the tris(carbene)borate ligand results in significant reduction of the CO bonds, as measured by IR spectroscopy.

The alkyne complexes [(PNP)Ti(η2-HC≡CH)(CH2tBu)] (2) and [(PNP)Ti(η2-HC≡CMe)(CH2tBu)] (3) have been prepared by treatment of [(PNP)Ti═CHtBu(OTf)] (1) with the Grignard reagents H2C═CHMgCl and MeHC═CHMgBr, respectively. Complex 3 can be also prepared using the Grignard H2C═C(Me)MgBr and 1. The 2-butyne complex [(PNP)Ti(η2-MeC≡CMe)(CH2tBu)] (4) can be similarly prepared from 1 and MeHC═C(Me)MgBr. Complexes 2 and 3 have been characterized with a battery of multidimensional and multinuclear (1H, 13C, and 31P) NMR spectroscopic experiments, including selectively 31P decoupled 1H{31P}, 1H–31P HMBC, 1H–31P HOESY, and 31P EXSY. Variable-temperature 1H and 31P{1H} NMR spectroscopy reveals that the acetylene ligand in 2 exhibits a rotational barrier of 11 kcal mol–1, and such a process has been corroborated by theoretical studies. Formation of the titanium alkyne ligand in complexes 2 and 3 proceeds via the vinyl intermediate [(PNP)Ti═CHtBu(CH═CHR)] followed by a concerted, metal-mediated β-hydrogen abstraction ste...