Complexes of the Group 5 Elements

Abstract

23 2.1 General Remarks Paddlewheel compounds having two dimetal units, each with a square planar confi guration, and each group parallel to the other are relatively new in group 5. With other transition metal atoms, this type of atom arrangement is commonly found for M 2 n+ units where n = 4, 5 or 6. This means that the oxidation state for each metal atom is between 2 and 3. It has been generally thought that when the formal oxidation numbers are higher the atoms shrink so much that good orbital overlap necessary for metal-metal bond formation is not attained. Oxidation numbers of less than two are not common in inorganic compounds of the fi rst transition series , with the exception of Cu or when compounds are stabilized by-donors such as carbonyl groups which are not generally covered in this monograph. Therefore only a few examples of compounds with values of n outside the range 4-6 are known. For the group 5 elements, compounds in which the metal atom has an oxidation number of three are commonly found forming edge-sharing bioctahedra, not paddlewheel compounds. Lower oxidation numbers such as two give rise to only a few coordination complexes for vanadium; an even lesser number is known for niobium and tantalum. Therefore part of the challenge in synthesizing metal-metal bonded paddlewheel compounds of the group 5 elements is the development of appropriate synthetic procedures that produce precursors in low oxidation states. Most of the compounds of the paddlewheel type for vanadium and niobium have the metal atom in the divalent state in a d 3 electronic confi guration. The overlap of these d 3-d 3 atoms give triply bonded dimetal units with an expected electronic confi guration of 2 4 or a variation thereof. 2.2 Divanadium Compounds Theoretical calculations at the Fenske-Hall and Hartree-Fock level for the model system V 2 (O 2 CH) 4 , which had been carried out in the mid-eighties, indicate that multiple bonds between vanadium atoms should be stable. 1 The calculations clearly show the possible existence of paddlewheel molecules of the type 2.1 and predict a vanadium-to-vanadium triple bond length between 2.0 and 2.1 Å with a 2 4 electronic confi guration for the V 2 4+ unit. However, all efforts to synthesize V 2 (carboxylato) 4 compounds from the reaction of carboxylates and a few known V 2+ starting materials available 2 (e.g., V(H 2 O) 6 (CF 3 SO 3) 2 , [V 2 Cl 3 (THF) 6 ] 2 [Zn 2 Cl 6 ] or VCl 2 (py) 4) fail to produce dinuclear complexes. 3 These reactions give oxo-centered trinuclear 2