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A systematic, efficient approach to first complexes containing a triple bond between niobium and the elements silicon, germanium or tin is reported. The approach involves a metathetical exchange of the niobium-centered nucleophile (NMe 4 )[Nb(CO) 4 (κ 2 -tmps)] ( 1 ) (tmps = MeSi(CH 2 PMe 2 ) 3 ) with a suitable organotetrel(ii)halide. Compound 1 was obtained from (NMe 4 )[Nb(CO) 6 ] and the triphosphane tmps by photodecarbonylation. Reaction of 1 with the disilene E -Tbb(Br)Si[double bond, length as m-dash]Si(Br)Tbb in the presence of 4-dimethylaminopyridine afforded selectively the red-brown silylidyne complex [(κ 3 -tmps)(CO) 2 Nb[triple bond, length as m-dash]Si-Tbb] ( 2-Si , Tbb = 4- tert -butyl-2,6-bis(bis(trimethylsilyl)methyl)phenyl). Similarly, treatment of 1 with E(Ar Mes )Cl (E = Ge, Sn; Ar Mes = 2,6-mesitylphenyl) afforded after elimination of (NMe 4 )Cl and two CO ligands the deep magenta colored germylidyne complex [(κ 3 -tmps)(CO) 2 Nb[triple bond, length as m-dash]Ge-Ar Mes ] ( 3-Ge ), and the deep violet, light-sensitive stannylidyne complex [(κ 3 -tmps)(CO) 2 Nb[triple bond, length as m-dash]Sn-Ar Mes ] ( 3-Sn ), respectively. Formation of 3-Sn proceeds via the niobiastannylene [(κ 3 -tmps)(CO) 3 Nb-SnAr Mes ] ( 4-Sn ), which was detected by IR and NMR spectroscopy. The niobium tetrylidyne complexes 2-Si , 3-Ge and 3-Sn were fully characterized and their solid-state structures determined by single-crystal X-ray diffraction studies. All complexes feature an almost linear tetrel coordination and the shortest Nb-E bond lengths ( d (Nb-Si) = 232.7(2) pm; d (Nb-Ge) = 235.79(4) pm; d (Nb-Sn) = 253.3(1) pm) reported to date. Reaction of 3-Ge with a large excess of H 2 O afforded upon cleavage of the Nb-Ge triple bond the hydridogermanediol Ge(Ar Mes )H(OH) 2 . Photodecarbonylation of [CpNb(CO) 4 ] (Cp = η 5 -C 5 H 5 ) in the presence of Ge(Ar Mes )Cl afforded the red-orange chlorogermylidene complex [Cp(CO) 3 Nb[double bond, length as m-dash]Ge(Ar Mes )Cl] ( 5-Ge ). The molecular structure of 5-Ge features an upright conformation of the germylidene ligand, a trigonal-planar coordinated Ge atom, and a Nb-Ge double bond length of 251.78(6) pm, which lies in-between the Nb-Ge triple bond length of 3-Ge (235.79(4) pm) and a Nb-Ge single bond length (267.3 pm). Cyclic voltammetric studies of 2-Si , 3-Ge , and 3-Sn reveal several electron-transfer steps. One-electron oxidation and reduction of the germylidyne complex of 3-Ge in THF are electrochemically reversible suggesting that both the radical cation and radical anion of 3-Ge are accessible species in solution., (This journal is © The Royal Society of Chemistry 2017.)

Cyclic voltammetric studies of the hydridodisilicon(0,II) borate [(Idipp)(H)Si II [double bond, length as m-dash]Si 0 (Idipp)][B(Ar F ) 4 ] ( 1H [B(Ar F ) 4 ], Idipp = C[N(C 6 H 3 -2,6- i Pr 2 )CH] 2 , Ar F = C 6 H 3 -3,5-(CF 3 ) 2 ) reveal a reversible one-electron reduction at a low redox potential ( E 1/2 = -2.15 V vs. Fc + /Fc). Chemical reduction of 1H [B(Ar F ) 4 ] with KC 8 affords selectively the green, room-temperature stable mixed-valent disilicon(0,I) hydride Si 2 (H)(Idipp) 2 ( 1H ), in which the highly reactive Si 2 H molecule is trapped between two N-heterocyclic carbenes (NHCs). The molecular and electronic structure of 1H was investigated by a combination of experimental and theoretical methods and reveals the presence of a π-type radical featuring a terminal bonded H atom at a flattened trigonal pyramidal coordinated Si center, that is connected via a Si-Si bond to a bent two-coordinated Si center carrying a lone pair of electrons. The unpaired electron occupies the Si[double bond, length as m-dash]Si π* orbital leading to a formal Si-Si bond order of 1.5. Extensive delocalization of the spin density occurs via conjugation with the coplanar arranged NHC rings with the higher spin density lying on the site of the two-coordinated silicon atom., (This journal is © The Royal Society of Chemistry 2016.)

An efficient method for the synthesis of the NHC-stabilised Si(i) halides Si 2 X 2 (Idipp) 2 ( 2-X , X = Cl, Br, I; Idipp = C[N(C 6 H 3 -2,6- i Pr 2 )CH] 2 ) was developed, which involves the oxidation of Si 2 (Idipp) 2 ( 1 ) with 1,2-dihaloethanes. Halogenation of 1 is a diastereoselective reaction leading exclusively to a racemic mixture of the RR and SS enantiomers of 2-X . Compounds 2-Br and 2-I were characterised by single-crystal X-ray crystallography and multinuclear NMR spectroscopy, and their electronic structures were analysed by quantum chemical methods. Dynamic NMR spectroscopy unraveled a fluxional process of 2-Br and 2-I in solution, which involved a hindered rotation of the NHC groups about the Si-C NHC bonds. Iodide abstraction from 2-I by [Li(Et 2 O) 2.5 ][B(C 6 F 5 ) 4 ] selectively afforded the disilicon(i) salt [Si 2 (I)(Idipp) 2 ][B(C 6 F 5 ) 4 ] ( 3 ). X-ray crystallography and variable-temperature NMR spectroscopy of 3 in combination with quantum chemical calculations shed light on the ground-state geometric and electronic structure of the [Si 2 (I)(Idipp) 2 ] + ion, which features a Si[double bond, length as m-dash]Si bond between a trigonal planar coordinated Si II atom with a Si-I bond and a two-coordinate Si 0 center carrying a lone pair of electrons. The dynamics of the [Si 2 (I)(Idipp) 2 ] + ion were studied in solution by variable-temperature NMR spectroscopy and they involve a topomerisation, which proceeds according to quantum theory via a disilaiodonium intermediate ("π-bonded" isomer) and exchanges the two heterotopic Si sites., (This journal is © The Royal Society of Chemistry 2015.)