Structures and bonding of C<sub>2</sub>X<sub>2</sub>Y<i><sup>q</sup></i>(X=Si,Ge,Sn,Pb; Y=C,Si,Ge,Sn,Pb; <i>q</i> = +1,0,−1): π-type templates for planar tetracoordinate heavier group 14 atoms

<p>Contrasting the big family of the planar tetracoordinate carbon (ptC), species featuring the planar tetracoordinate heavier group element M (ptM) have been largely limited. Effective structural frameworks to accommodate such ptM centres are thus highly desired. In the present article, we report an extensive computational study on 60 pentatomic systems C<sub>2</sub>X<sub>2</sub>Y<i><sup>q</sup></i> (X=Si,Ge,Sn,Pb; Y=C,Si,Ge,Sn,Pb; <i>q</i> = +1,0,−1) covering both the low and high spin states. Up to 34 systems were shown to have the very low-lying singlet planar tetracoordinate heavier group 14 (ptM with M=Si,Ge,Sn,Pb) structures bearing the 19 (<i>q</i> = +1), 20 (<i>q</i> = 0) and 21 (<i>q</i> = −1) valence electrons (ve). Structural and bonding analysis confirmed the effectiveness of the inherent π-type ligand skeleton XCCX or XCCY that each have several sets of π-bonding orbitals to stabilise the ptM centre. The structural and bonding motifs of these ptMs differ greatly from the classic ptMs, which have the σ-type ligand skeleton, smaller number of valence electrons (≤18ve), and the centre → ligand π-delocalisation.</p>