Coordination number and geometry

Transition metal complexes are cationic, neutral or anionic species where a transition metal is coordinated through ligands. A classical or Werner complex is one created through a metal in a positive oxidation state along with donor ligands like H₂O, NH₃ or halide ions.

The coordination numbers (CN) discovered in complexes range to nine (e.g. [ReH₉]^2- from two (e.g. [Ag(NH₃)₂]⁺);). The most common geometries for 3d ions are octahedral (CN=6, example [M(H₂O)₆]²⁺) and tetrahedral (CN=4, example [MCl₄]^2-). Like in solid compounds, higher coordination numbers are frequently found with the larger 4d and 5d ions. Other Coordination geometries might be dictated through bonding arrangements relies on the d electron number.

The comparative preference for octahedral or tetrahedral coordination is fairly steric, but ligand field influences can also play a vital role. Ions along with the d³ and low-spin d⁶ configurations (examples Cr³⁺ and Co³⁺, respectively) comprise large octahedral ligand field stabilization energy and are particularly resistant to creating tetrahedral complexes. Square-planar complexes would never be supposed in preference to tetrahedra on steric grounds alone. Though, they are generally found with 4d⁸ and 5d⁸ ions like Pd²⁺ and Pt²⁺ in which the pattern of ligand field splitting is favorable if its magnitude is sufficiently large for spin-pairing to happen. The subsequent 3d⁸ ion Ni²⁺ gives square-planar complexes just with strong-field ligands like CN^-; or else octahedral or occasionally tetrahedral coordination is found. Along with the d⁹ or high-spin d⁴ configuration a deformed octahedral geometry is frequently found with just four ligands strongly attached. This is general with Cu²⁺, like in [Cu(NH₃)₄]²⁺, in which two weakly bound water molecules are also exist.

Low coordination numbers are frequently found with post-transition metal ions that are having the d¹⁰ configuration. This is also right for the d¹⁰ ions Cu⁺, Ag⁺ and Au⁺, that form several linear complexes with CN=2 (example [AuCl₂]^-, isoelectronic to HgCl₂).

Polynuclear complexes have more than one metal atom. Occasionally these might be held through bridging ligands, like in [(RuCl₅)₂O]^4- (1). In another cases metal-metal bonds might be exist, like in [Re₂Cl₈]^2-. Metal-metal bonding is common within the 4d and 5d series than with 3d elements, even though binuclear compounds of Cr^II are well-known; for instance, [Cr₂(CH₃CO₂)₄] (2), that has bridging acetate groups (only one displayed explicitly) and a quadruple Cr-Cr bond created through all remaining valence electrons of the 3d⁴ ions.