Antiferromagnetic complexes with a metal—metal bond

Abstract The influence of pivalic acid on the triangular metal cluster (Cp 2 Cr 2 SCMe 3 )(μ 3 -S) 2 Co(CO) 2 (II) and its unstable precursor with a probable structure (Cp 2 Cr 2 SCMe 3 ) 2 (μ 3 -S) 2 Co(CO) 3 (A) has been studied. An X-ray study has shown that A transforms into an antiferromagnetic ** pseudotetrahedral cluster Cp 3 Cr 3 (μ 3 -S) 3 (μ 3 -O)Co(CO) 3 · Me 3 CCOOH · 0.5C 6 H 6 (III). The base of the cluster is an isosceles metal triangle, Cp 3 Cr 3 , which is capped by the tridentate bridging sulphur atom (CrCr 2.692(7), 2.868(8) and 2.884(8) A, av. CrS 2.26(1) A). The short CrCr bond is bridged by an oxygen atom, and the long bonds have sulphide bridges which in their turn are weakly bonded to the Co(CO) 3 group (CoO 2.16(2), CoS 2.47(1) and 2.48(1) A). In addition, the μ 3 -O bridging atom forms a hydrogen bond with the HOOCCMe 3 molecule, while the Co atom is situated at weakly-bonding distances from the Cr III atoms (CoCr 3.216(6), 3.238(6) and 2.892(6) A). The transformation of cluster II in the reaction with Me 3 CCOOH leads to the antiferromagnetic cluster (Cp 2 Cr 2 SCMe 3 ) 2 (μ 3 -S) 4 Co (IV), which according to an X-ray structural study, has a distorted bow-tie framework Cr 4 Co with the central Co II atom surrounded also by four μ 3 -bridging sulphide atoms. The angle formed by the planes of the Cr 2 Co metal triangles is equal to 85.40°. The bond lengths between pairs of chromium atoms in different triangles are almost equal (2.679(2) and 2.665(2) A) and close to the length of the CrCr bond in the initial cluster II. At the same time, CoCr bonds in one of the triangles (2.776(2) and 2.774(2) A) are noticeably shorter than in the other (2.809(2) and 2.959(2); all these values exceed, however, the length of the ordinary CoCr bond (2.59 A) found in II. The CoS bonds are also elongated (up to 2.26 A). It is proposed that elongation of CoCr bonds in III and IV (as well as CoO in III) results from an excessive number of electrons in the metal framework as compared to the number of corresponding bonding orbitals.