The synthesis of the mer-Mo(CO)H(NO)(PMe 3 ) 3 complex 3 is described, which is obtained from Mo(CO) 4 (NO)AlCl 4 via the isolable intermediates MoCl(CO)(NO)(PMe 3 ) 3 (1) and the borohydride complex Mo(η 1 -BH 4 )(CO)(NO)(PMe 3 ) 3 (2). The reactivity of 3 with respect to insertions has been probed. 3 thus reacts with benzaldehyde, benzophenone, acetophenone, and acetone to afford the corresponding alkoxide complexes Mo(CO)(NO)(PMe 3 ) 3 (OCHR′R′′) (R′ ) H, R′′ ) Ph (4a); R′, R′′ ) Ph (4b); R′ ) Me, R′′ ) Ph (4c), and R′, R′′ ) Me (4d)). A high propensity of 3 to undergo carbonyl insertion was also manifested in its transformations with CO 2 and metal carbonyl compounds. The conversion of 3 with CO 2 leads to the formato-O-complex Mo(CO)(NO)(PMe 3 ) 3 (OCHO) (5), while CO induces PMe 3 substitution with formation of the nonisolable Mo(CO) 2 (NO)(PMe 3 ) 2 H compound (6). Fe(CO) 5 readily inserts to yield the dinuclear formyl species (Me 3 P) 3 (ON)(OC)Mo(µ-OCH)Fe(CO) 4 (7a), and Re 2 (CO) 10 is transformed to the related µ-formyl complex (Me 3 P) 3 (ON)(OC)Mo(µ-OCH)Re 2 (CO) 9 (7b) in an equilibrium reaction lying far to the formyl side. Temperature-dependent NMR measurements allowed us to derive equilibrium constants and ∆H ) -46.5 ( 0.6 kJ/mol and ∆S ) -103 ( 2 eu. These values for 7b were approximately reproduced by accurate DFT calculations and investigated further via an incremental analysis of bond dissociation energies. Finally insertion reactions of 3 with various imines were studied. Only in the case of a bisdihydroproline ester did an insertion take place, however, across the ester carbonyl group. Concomitant elimination of a phosphine moiety was observed with formation of the Scheme 2

A Dy3 complex with the highest energy barrier of all reported triangular-shaped Ln3 single-molecule toroics.

High-nuclear lanthanide clusters are generally formed by the rapid accumulation of simple building units. Thus, tracking and observing the stepwise assembly process, which is vital for understanding the assembly mechanism, are extremely difficult. Herein, the decanuclear nanocluster [Dy 10 (L 1 ) 6 (μ 5 -NO 3 ) 2 (OAc) 10 (HOAc, and 3-amino-1,2-propanediol (L 3 ). The reaction process was further tracked by time-dependent high-resolution electrospray ionization mass spectrometry, and seven reaction intermediate fragments were screened. A stepwise assembly mechanism was observed based on these fragments, that is, L → Dy1 → Dy2 → Dy3 → Dy4 → Dy5 → Dy6 → Dy10. This study is the first to discover a stepwise assembly mechanism during the formation of high-nuclear lanthanide clusters (cluster nucleus > 3). Magnetic studies have shown the multiple relaxation behavior of Dy10.