Many of the simplest Z-ligands are simple Lewis acids with electron-deficient center atoms such as BX, BH, BR, AlX, etc. While these molecules typically have trigonal planar geometry, when bonded to a metal center, they become tetrahedral.
This geometry change can be stabilized by the addition of an L-ligand on the metal center. The electrons donated from the L-ligand stabilize the Lewis acid into a tetrahedral form. Therefore, these Z-ligands can attack at (a) the metal (even in 18 electron compounds), (b) the metal-ligand bond, or (c) the ligands.
<br>In addition to the simple Lewis acids, there are several complex molecules that can act as both L- and Z-ligands. These are referred to donor buttresses, and are typically formed when large boron-alkyl molecules complex with a metal center.
In addition to the geometry changes involved in the dative bonding from the metal to the Z-ligand complex, the bond itself can differ greatly depending on the type of buttresses involved. Typical boron-boron bonds are around 1.59 Å. However, due to the dative bond character, the metal-boron bond distance can vary greatly depending on the bonding motif, as well as the various ligands attached to the metal. The boride and borylene motifs tend to have the shortest bonds, typically from 2.00 to 2.15 Å. Boryl complexes have metal-boron bond distances from 2.45 to 2.52 Å, and borane complexes have the largest range of metal-boron bond distances, 2.07-2.91 Å. In addition, for the metal base-stabilized borane complexes, the L-ligand that donates to the metal center plays an important role in the metal-boron bond length. Typically, the donor buttresses with sulfur and nitrogen donor ligands have metal-boron bond lengths of 2.05-2.25 Å, and donor buttresses with phosphorus donor ligands have metal-boron bond lengths of 2.17-2.91 Å.