New orthomanganated aryl ketones of type 1 (Figure 1) have been prepared, including some in which the donor ketone carbonyl group is incorporated into a fused ring. Analogous derivatives of benzoates and heteroaromatic carboxylic acid esters are also described, together with several orthorheniated compounds. Single crystal X-ray structure determinations of η²-5-(1,4-benzopyronyl)tetracarbonylmanganese (13) and η²-(4-acetyl-2,5-dimethylthien-3 yl) tetracarbonylrhenium (32) are reported. Reactions of orthomanganated aryl ketones with SO₂ and with the electronically and structurally related cumulenes, N-sulfinylaniline (PhN=S=O), N-sulfinylbenzenesulfonamide (PhS(O)₂N=S=O) and the disulfonylsulfodiimide [PhS(O)₂N]₂S have been investigated. SO₂ inserts efficiently into the Mn-Cₐᵣᵧₗ bond of a range of orthomanganated ketones, to give the corresponding S-sulfinato complex e.g. (43) (Figure 1). The X-ray crystal structure of (43) was determined and it is the first reported example of a six-membered metallocycle incorporating an O, Mn and S atom. In contrast, reaction of PhN=S=O with a number of orthomanganated acetophenones led to the corresponding orthomanganated imines e.g. (38) (Figure 1). The structure of η²-3-chloro-2-[1-(N-phenylimino)ethyl]-phenyl-tetracarbonylmanganese (44) was determined by single crystal X-ray diffraction. PhS(O)₂NSO and [PhS(O)₂N]₂S did not react with orthomanganated p-methoxyacetophenone (39) under similar conditions. The palladium- and thermally-promoted coupling reactions of orthomanganated aryl ketones and esters with alkenes have been explored in detail. In the presence of Pd(II), η²-(o-acetylaryl)tetracarbonylmanganese compounds undergo coupling reactions with methyl acrylate, methyl vinyl ketone, acrylonitrile, acrolein, vinyl acetate and allyl alcohol. The coupling reactions normally proceed in excellent yield and produce three main products, arylalkene, arylalkane and indene, the ratios of which vary with reaction conditions and substrate (Scheme 1; Path (I)). Orthomercurated aryl ketones obtained via transmetalation of orthomanganated ketones with mercuric chloride allowed a comparison of the product distribution for Pd(II)-promoted methyl acrylate coupling of an orthomercurated precursor 4-acetyl-2,5-dimethylthien-3-ylmercury(II) chloride (112) and the corresponding orthomanganated precursor η²-(4-acetyl-2,5-dimethylthien-3-yl)tetracarbonylmanganese (41) under identical conditions. Cyclisation to give the indene-type product methyl 1,3,6- trimethyl-4H-cyclopenta[c]thiophene-5- carboxylate (53) (whose crystal structure is reported) was found to be much more strongly favoured in the Mn than the Hg case precluding the possibility that the only role of both replaced metals is in transmetalation with palladium prior to alkene coupling. This conclusion is supported by similar reactions with phosphite or phosphine derivatives of cyclomanganated aryl ketones e.g. (145), (146) (Figure 2), and by reactions of orthorheniated substrates e.g. (32) (Figure 2). In the absence of Pd(II), alkenes were also found to insert efficiently into the Mn-Cₐᵣᵧₗ bond of orthomanganated ketones at ca 80 °C. In this case, the major products were found to be arylalkane, indene and a mixture of the diastereoisomeric indanols (Scheme 1; Path (II)). Arylalkene products were formed in very small or indetectable amounts. Possible reaction pathways for both the Pd(II) and the thermally initiated reactions are presented. Several sterically crowded 3-substituted orthomanganated esters and ketones were reacted with ICl to give the corresponding 2-iodo-3-substituted compounds (e.g. Equation 1). 3-Substituted 2-iodo esters formed via this route are of interest as potential precursors for the formation of 5-substituted 9-oxo-9H-xanthene-4 acetic acid (XAA) compounds, which exhibit antitumour activity.