Is it True That the Normal Valence‐Length Correlation Is Irrelevant for Metal–Metal Bonds?

Abstract

The most intriguing feature of metal-metal bonds in inorganic compounds is an apparent lack of correlation between the bond order and the bond length. In this study, we combine a variety of literature data obtained by quantum chemistry and our results based on the empirical bond valence model (BVM), to confirm for the first time the existence of a normal exponential correlation between the effective bond order (EBO) and the length of the metal-metal bonds. The difference between the EBO and the formal bond order is attributed to steric conflict between the (TM)n cluster (TM=transition metal) and its environment. This conflict, affected mainly by structural type, should cause high lattice strains, but electron redistribution around TM atoms, evident from the BVM calculations, results in a full or partial strain relaxation.

Cobalt-platinum heterometallic clusters containing N-heterocyclic carbene ligands

“…3, Co(1)-Pt(2) = 2.6359 (19), Co(2)-Pt(2) = 2.9791 (18), Co(3)-Pt(2) = 2.6322 (19), Co(4)-Pt(2) = 3.0428 (19), Co(2)-Pt(1) = 2.6389 (19), Co(3)-Pt(1) = 2.8972 (18), Co 4 3 The bond indices for the Co-Pt bonds can be divided into two sets, with values in the ranges 0.13-0.16 (Pt(1)-Co(1), Pt(1)-Co(3), Pt(2)-Co(4) and Pt(2)-Co(2)), and 0.21-0.24 (Pt(1)-Co(4), Pt(1)-Co(2), Pt(2)-Co(1) and Pt(2)-Co(2)). They correlate well with both the calculated and experimental bond lengths; those with shorter bond lengths correspond to higher bond indices, in accord with the trend reported for metal-metal bonds [12]. Although there are no reports of bond indices for Co-Pt bonds, it has been reported that homonuclear metal-metal bonds such as the single Ru-Ru bond has bond index ∼0.2 [13], and that for a single Fe-Fe bond has bond index ∼0.1 [14].…”

“…The Co-C carbide and Co-Co bond lengths (1.846(12) -1.869(12) and 2.518(3) -2.705(3) Å, respectively) in 5 are comparable to those in the homometallic, octahedral cluster [Co 6 ( 6 -C)(CO) 13 ] 2-(1.852(3) -1.880 and 2.4618(8) -2.926(1) Å, respectively)[11]. The Pt-C carbide and the Co-Pt bond…”

Do the basic crystal chemistry principles agree with a plethora of recent quantum chemistry data?

“…the effective number of shared electron pairs is very close to the formal one. As demonstrated below, the exception is the metal-metal bonds with a more complicated rule of local electroneutrality (Levi & Aurbach, 2011;Levi et al, 2013aLevi et al, ,b, 2014Singh et al, 2016).…”

“…It was shown that the striking difference between formal and effective BOs of the TM-TM and TM-L bonds in cluster compounds is caused by a steric conflict between unusually short metal-metal bonds and a rigid ligand environment (Levi et al, 2013b;Singh et al, 2016). The conflict results in lattice strain, while the redistribution of the ED between the TM-TM and TM-L bonds ensures their partial or full relaxation.…”

“…(Ponec et al, 2010). In this case, equation 2is irrelevant and only the data of quantum chemistry calculations should be used to establish the character of BO ij -R ij correlation (Levi & Aurbach, 2011;Levi et al, 2013aLevi et al, ,b, 2014Singh et al, 2016). Unfortunately, the data obtained for the same compounds by different quantum chemistry methods give high BO dispersions and, in many cases, show the absence of a clear BO ij -R ij relationship.…”